What big teeth you have…

3 10 2007

Smildon

One of Charles R. Knight’s paintings of Smilodon fatalis, this one menacing a giant sloth stuck in tar (off panel).

There are few fossil mammals that are as impressive the saber-toothed cat Smilodon fatalis, but despite it’s fearsome dentition some recent new reports have suggested it was more of a pussycat when it came to bite strength. This seems to be counter-intuitive; how could such an impressive animal be associated with the term “weak”? Part of it has to do with word choice, but the larger issue has to do with the fact that the bite of Smilodon wasn’t as strong as that of some other carnivores (extinct and extant), as well as dentition and feeding ecology. This issue goes far beyond just one genus or species, however, as Smilodon was only one of many genera that bore massive canines. In fact, huge “sabers” have evolved over-and-over again in the mammalian lineage (see this post and also this post for information about the cat-like ones), including the famous fangs of the machairodontine felids (saber-toothed cats) and their look-alike nimravid relatives.

Tusks

Lateral, anterior, and dorsal views of the herbivore Uintatherium (Note the prominent canines). From Marsh, O.C. “The Fossil Mammals of the Order Dinocerata.” The American Naturalist, Vol. 7, No. 3. (Mar., 1873), pp. 146-153

Cope

Skull of another member of the Dinocerata; “Loxolophodon cornutus” (today known as Eobasileus cornutus). Again, note the prominent canine. From Cope, E.D. “The Amblypoda (Continued).” The American Naturalist, Vol. 19, No. 1. (Jan., 1885), pp. 40-55.

Although this post will primarily be concerned with the great “sabercats,” large, dagger-like canine teeth having been evolved multiple times by many different unrelated animals during the course of life on earth. In some herbivorous creatures, like the extinct Uintatherium and even in the extant Musk Deer, the fangs reflect sexual dimorphism and probably sexual selection, but the sharp teeth don’t seem to have a prominent function in mastication or processing of food. Likewise, large canine teeth are present in living baboons (Papio sp.), and the sexual dimorphism exhibited between the dental equipment of the males and the smaller canines of the females has long been noted (males often yawn to show off their canines, the size of their teeth being very intimidating indeed). Do the same considerations of sexual selection and dimorphism hold true for the saber-toothed cats, too? Unfortunately, fossil evidence does not always allow comparisons of the two sexes, but extant big cats and some death-trap sites have provided some information to work with. From Salesa, et al. (2006);

Among the Carnivora, sexual dimorphism is more marked in canine size than in other dental features or skull size, and these differences can be related to the breeding system. Species in which a male defends a group of females tend to be more dimorphic than those with monogamous pairs or groups of males and females. Felids are dimorphic animals, but mainly in reference to body size, with the mane of male lions being a unique example of morphological variation between sexes among the family.

This makes sense; if a male keeps a harem of females and has to defend it from other males, the species is more likely to exhibit sexual dimorphism than not. In cats, however, it seems to be more about body size (and possibly characters that wouldn’t fossilize in extinct species) than about tooth size (which would serve important other functions, so any sexual selection would be mitigated by natural selection), although we can’t be sure of this being that there are no living sabercats to study. Personally, I think there could be a sexual-selection component in some groups, but the saber-canine is so prominent in so many extinct felids and nimravids that it is extremely doubtful that all the lineages converged on similar tooth structures because of sexual selection/dimorphism, the functional advantage of larger teeth likely coming first. A lack of sexual dimorphism when considering morphology as a whole, however, may suggest a more solitary lifestyle where territories may or may not overlap are maintained and direct competition for females is not as fierce, especially since the females move through territories rather than living with a male. Such a strategy may have been employed by the late Micoene sabercat Paramachairodus ogygia. Salesa, et al. (2006), working with an assemblage made up of many of the more basal felids, have even been able to come up with a hypothesis about life history of the ancient animals based upon their finds in Spain;

[T]he probable territorial behaviour for Par. ogygia would be very similar to that of jaguars, in which males defend large, overlapping territories that include smaller territories of several females. This model is similar to that of the leopard, but in this species male territories never overlap, which could explain the different sexual dimorphism index of this species with respect to Par. ogygia and jaguar…

So, if Par. ogygia behaved more like jaguars and leopards than lions, the presence of juveniles in the trap would be highly improbable, as is the case. But in addition to the scarcity of juveniles, the sample from Batallones-1 has another interesting feature: it is mostly composed of young adults, that is, individuals with the complete permanent dentition, but without any trace of wear. These animals, which would have recently become independent of their mothers, would not as yet have had any territory, moving instead through the ranges of other adults and being more easily attracted by an easy meal, such as carrion. This age distribution therefore suggests that the sample of Par. ogygia trapped in Batallones-1 corresponds to that fraction of non-resident young individuals, both males and females, which were in a phase of dispersion. In the case of leopards, such individuals are more daring – or less cautious – than adults, and they have been seen crossing rivers in spate, whereas resident adults only cross at times of lower water. It has also been noticed that among these individuals, males are even more inclined to make these incursions than females, which remain longer with the mother, especially if there is good availability of food. If this pattern of dispersion behaviour applied to the young adults of Par. ogygia, it is likely that they were trapped in Batallones-1 more often than the resident adults.

Saber Tooth Diversity

Saber-Toothed Felid and Nimravid diversity (click for a larger image). From Emerson, S.B., and Radinsky, L. “Functional Analysis of Sabertooth Cranial Morphology.” Paleobiology, Vol. 6, No. 3. (Summer, 1980), pp. 295-312.

While the life histories of extinct mammalian carnivores are interesting in and of themselves, it is the teeth and terrifying bite of the sabercats that we are most concerned with here. Smilodon is the celebrity of saber-toothed cats, but the fossil record preserves a wide diversity of carnivores with large canine teeth, and even within the larger groupings there are even more subdivisions, the skulls of saber-toothed felids being widely variable. As discussed in the background material, nimravids are saber-tooth look-alikes that diverged from a common ancestral line earlier than the carnivores that would give rise to Smilodon, but the two lines are still closely related and have undergone parallel evolution. There is still some reshuffling of taxa going on and the true evolutionary history/affinities of many of the forms is still being worked out, but most forms you’re likely to see grouped together at a museum fall into either the nimravid or felid camps. The focus of this essay, however, will be on felids, and although they are often discussed along with their nimravid cousins the larger amount of work has been done on the felids and so we must leave the nimravids.

With the felids, then, there seem to be three kinds of sabercat that hint at differing predatory tactics, prey, and habitat. Indeed, evolution did not create carbon copies of the same creature, barring life from becoming adapted to varying circumstances; there is more variety than would be first assumed if we based all our research on the presence of prominent canines. Instead, there seem to be three “ways of being” a saber-toothed cat, as outlined by Martin, et al.;

Saber-toothed carnivores… have been divided into two groups: scimitar-toothed cats with shorter, coarsely serrated canines coupled with long legs for fast running, and dirk-toothed cats with more elongate, finely serrated canines coupled to short legs built for power rather than speed. In the Pleistocene of North America, as in Europe, the scimitar-cat was Homotherium; the North American dirk-tooth was Smilodon. We now describe a new sabercat from the Early Pleistocene of Florida [Xenosmilus], combining the scimitar-tooth canine with the short, massive limbs of a dirk-tooth predator. This presents a third way to construct a saber-toothed carnivore.

Three Kinds

Xenosmilus hodsonae, Homotherium cf. crenatidens, and Homotherium serum. From Martin, L.D., Babiarz, J.P., Naples, V.L., and Hearst, J. “Three Ways To Be a Saber-Toothed Cat.” Naturwissenschaften, Vol. 87, No. 1 (Jan. 2000), pp. 41-44

As Martin notes, there appears to be a number of adaptational “trade offs” that sabercats in North America and Europe were subject to; fast-moving gracile forms had shorter sabers, but stouter and more powerful forms had the longer, more laterally flattened canine teeth. The “third way” that combined characters from both groups was exemplified by Xenosmilus (which Martin, et al. say would have seemed more like a bear than a cat, despite actual evolutionary relationships to the contrary). Still, leaving the overall structure of the body aside for a moment, the arrangement and sizing of the teeth of the different groups can be very telling. Martin, et al. again lay out what the usefulness of the differing tooth arrangements;

When biting, the long sabers of dirk-toothed cats may have cut parallel slits for some distance before the relatively smaller incisors could be applied. In scimitar-toothed cats the shorter canines and longer incisors worked more as a unit, first cutting parallel slits with the canines, immediately followed by the incisor arc removing the strip of flesh. Such a large open wound would have bled profusely, traumatizing the victim. If the incisors and canines acted in unison, the torsional forces on individual teeth would have been reduced, resulting in fewer restrictions on bite placement. In felids the size of the sagittal crest is directly proportional to the forces exerted by the temporalis musculature. Scimitar-toothed cats have a sagittal crest that is generally less pronounced than that in their dirk-toothed contemporaries. In a modification of the typical scimitar-tooth condition, the new cat from Florida exhibits both an elongated sagittal crest and an enlarged temporalis muscle that would have permitted a stronger bite.

While such a passage might not seem significant at first, it shows that there is more going on in a sabercat’s skull that is important to biting than just the size or shape of the canines. The placement of the incisors, for instance, seem to make a difference in biting strategy and force, dirk-toothed cats like Smilodon exhibiting a condition where the incisors are out forward of the canines. When this is taken into account, as well as the length of the canines, it seems that the canines would slash for quite some distance before the incisors could be used at all in comparison to the scimitar-toothed sabercats, the placement of the incisors in scimitar-tooths seemingly strengthening the biting teeth at the front of the jaw. The sagittal crests of these creatures should also be taken into account, such structures giving students of paleontology an indication of how carnivores (or herbivores, in the case of gorillas) have been adapted to achieve higher bite forces. Such ridges atop the skull for muscle attachment are not unique to sabercats, however, and there are some animals that have taken the structure to even greater extremes;

Amphicyon

The extinct “bear dog” Amphicyon at the AMNH. Note the size of the sagittal crest, the reduction of the bony enclosure around the eyes, and the large holes on the side of the skull for jaw muscle attachment.

Hyaenodon

The extinct “saber-toothed” creodont Hyaenodon at the AMNH. Again, note the sagittal crest, reduction of bone enclosure around the eye, and the large canines.

Hoplophoneus

The skull of the nimravid Hoplophoneus on display at the AMNH. Note the size of the canines and sagittal crest in comparison with Hyaenodon and Amphicyon.

Smilodon

The skull of Smilodon on display at the AMNH.

Thylacoleo

The skull of the marsupial predator Thylacoleo at the AMNH. Note the large openings on either side of the skull for the jaw muscles.

Thylacoleo

Ventral view of the skull of Thylacoleo. From E.D. Cope’s “The Tertiary Marsupialia” in The American Naturalist, Vol. 18, No. 7. (Jul., 1884), pp. 686-697.

Looking at the various groups, all show adaptations that increase the amount of available muscle attachment to achieve more powerful bites, modifying the skull in two ways. First, a sagittal crest (as already discussed) is often present to some degree, often being greater in omnivores or bone-crushing carnivores as they require greater forces to crack hard foods (although recent research by Wroe, et al. suggest that bone crushers like Spotted Hyena might not have the highest bite forces). Likewise, the holes between the skull and cheek bones are often enlarged or widened (the extreme of this group being Thylacoleo), the more muscle that can pass from lower jaw to skull being directly correlated to bite strength. What is interesting about sabercats, when considering these factors, is that they seem to be in the middle. They don’t exhibit adaptations of the skull to the extreme as in Amphicyon or Thylacoleo, but they still exhibit changes allowing for powerful bites (strong enough to kill and consume prey, at least). The trend is obvious and has not been missed by reseachers, and Emerson says the following about it;

With enlargement of upper canines, skulls of paleofelid, neofelid, marsupial and, as far as the record shows, creodont sabertooths were remodeled in similar ways. This evolutionary convergence in cranial morphology is not surprising, since most of the modifications relate to allowing increased gape while retaining bite strength at the carnassial. Those are factors essential for all sabertooths, and the possible ways to achieve them, starting from a generalized mammalian cranial morphology, are limited…

Why did sabertooth specializations evolve so many times? Their multiple evolution, plus the fact that several species of sabertoothed felids existed for most of the history of the family (from about 35 Myr to about 15,000 yr BP) suggest that sabertooth canines provided an effective alternative to the modern carnivore mode of killing prey

Megantereon

The skull of the saber-toothed cat Megantereon. Like in Smilodon, not how the incisors jut out (as well as the overly large nasal opening in this genus).

The basic mechanics of the skull just discussed gives researchers clues as to how sabercats could have killed their prey, but reconstructing ancient predator/prey interactions with no exact modern equivalent is difficult. Indeed, debate has gone on for years as to how sabercats used their teeth to bring down prey (see Simpson’s paper), either by stabbing, cutting, slicing, or even (as silly as it may seem) by crushing. What does seem apparent today, however, is that the canines of the sabercats were relatively delicate, and it would be unwise to fully sink them into a struggling animal as they may easily be broken off. Even if such an attempt to deeply puncture a prey item was not undertaken, biting full-force into bone could have also easily damaged teeth (or even broken them off), making it unlikely that sabercats jumped onto the back of their prey and tried to sink their teeth into the back of the prey’s skull like some modern cats. Recent research has even shown that the skull of Smilodon was ill-suited to handle stresses associated with struggling prey when compared to the skull of a lion, and I wonder how often individual Smilodon perished because of stresses associated with taking down prey if the victim was not brought down and killed quickly. Indeed, it seems that the long teeth were better suited to slicing soft flesh, i.e. cutting open the belly of prey or slicing open the throat, rather than piercing rough hides and ramming through bone.

Saber Tooth

Skulls (mandibles not pictured) of 4 “saber-toothed” mammals from “The Function of Saber-Like Canines in Carnivorous Mammals” by G.G. Simpson, American Museum Novitiates, August 4, 1941. Pictured are A) Machairodus (felid), B) Hoplophoneus (nimravid), C) Smilodon (felid), and D) Thylacosmilus (marsupial).

As just discussed in terms of tooth and skull stressed, many factors of life history, behavior, and morphology of extant big cats and sabercats might be similar, but the massive canines of the extinct group seem to infer a different killing strategy, and there is no reason to assume that they were like modern big cats in every respect. Salesa, et al. sums it up this way;

Extant felids kill small animals by biting on the nape or directly on the skull, using their rounded-section canines, but if any sabre-toothed cat tried to do this they would have risked breaking the laterally flattened upper canines. For this reason, it is more probable that they developed some behavioural mechanism to minimize that risk, such as ignoring prey below a given size. It is likely that machairodontines developed this ethological trait early in their evolution, and so narrowed their prey size range in comparison with that of felines, which hunt both large and small animals. This high specialization has been pointed out as one of the possible reasons for the gradual decline and final extinction of the sabre-toothed cats in the Pleistocene… The development of this strategy was probably the key reason for the sabre-tooted cats becoming the dominant predators in the land mammal faunas from the Late Miocene to Late Pleistocene.

It might not immediately make sense that felids with fragile teeth would specialize in eating large prey, but that is whale the fossil evidence (as we currently understand it) infers. While the smallest prey would pose no problems (outside of not being a fully satisfying meal), but medium sized prey with smaller areas of soft flesh (like the stomach and neck) would potentially be more dangerous and a more exact bite would be needed to prevent damage to the teeth and skull. Hence, it seems that the slashing and ripping of soft tissue in larger animals was the main method of killing prey (after it had been brought down or slowed by blood loss), taking hypercarnivory to an even more specialized extent.

Amur Leopard

An Amur Leopard yawns. Note the relatively small (but still fearsome) canines of the upper and lower jaw.

What, then, of a smaller living cat, the Clouded Leopard (Neofelis nebulosa and N. diardii), which has been heralded as a modern analog of sabercats? As Christiansen notes, Clouded Leopards are a bit bizarre, and it is incorrect to call them “small” big cats or modern sabercats, the genus showing a number of convergences with extinct forms while remaining distinct from the famed genus Panthera;

The skull morphology of the clouded leopard sets it apart from other extant felids, and in a number of respects it approaches the morphology of primitive sabertooths. This indicates convergence of several characters in machairodontine felids and the clouded leopard, mainly as adaptations for attaining a large gape. This raises doubts about the characters hitherto considered as distinguishing sabertoothed from nonsabertoothed predators…

Clearly, Neofelis and the sabertooths independently evolved a suite of the same specializations for the same overall purpose of attaining a large gape, a prerequisite for efficient jaw mechanics with large canines, but the reasons for evolving these characters need not have been similar. Based on analyses of lower jaw bending moments and inferred resistance to mechanical loadings, Therrien (2005) suggested that Neofelis could be at the beginning of a new sabertooth radiation. Such claims are difficult to test, however, since the extant sister taxon to Neofelis (Panthera) shares none of its sabertoothed characters, and the fossil record provides no clues of felids closer to Neofelis than Panthera. At present, however, there is little evidence to suggest that Neofelis can be regarded as an “extant sabertooth,” although it clearly shares a number of characters with them that are absent in other extant felids. On the other hand, it cannot be regarded as simply an intermediate between large and small felids, as normally assumed. The presence to some extent of characters normally ascribed to sabertooths in Neofelis raises doubts about their functional and evolutionary significance in primitive machairodonts such as Nimravides or Paramachairodus, hitherto the only reasonably well-known primitive machairodont. Such animals need not have shared the presumed functional skull morphology of later, more derived sabertooths and are perhaps not to be regarded as “sabertoothed” at all, if by sabertoothed is implied animals functionally significantly different from extant felids.

Again, this shows a convergence of functional morphology despite existing evolutionary relationships, many felids being adapted in similar ways. As stated previously, the large canines of saber-toothed predators required the animals to open their jaws wide but also narrowed their predatory niche to some extent. Likewise, various tests seem to show that the bite of sabercats like Smilodon was “weak,” with news reports often relating that the terrible felids were more like big housecats when compared to living big cats. This is a mistake (and it would be a grave one for anyone ever to cross a sabercat), born of a lack of recognition that bite forces exist on a continuum and are related to a number of factors and cannot simply be deemed “weak” or “strong” without further comment. Christiansen relates the bite force of Smilodon as such;

[A]lthough large sabertooths such as Smilodon and Homotherium had weaker bite forces than lions or tigers, their bite forces were broadly comparable to those of jaguars and large leopards, and, thus, cannot be claimed to have been “weak”. Lower bite forces at any given body size were probably evolutionarily possible owing to a marked contribution from the upper cervical musculature to the killing bite, which… was absent in Neofelis and primitive machairodonts such as Paramachairodus. Thus, bite force analysis may constitute a hitherto overlooked parameter in evaluating whether or not primitive machairodonts such as Paramachairodus or Nimravides really did employ a canine shear bite with a marked contribution from the cervical muscles to subdue prey, or killed in a manner similar to extant felids, which requires a stronger killing bite…

In many Plio-Pleistocene communities predator competition was more severe than today, and a sabertooth killing mode could be a way of ensuring faster kill rates, since a throat shear-bite most likely would kill prey faster than a throttling throat bite, common in extant pantherines. In lions, it can take up to 13 minutes to kill large prey, and in such cases the prey is frequently killed by disemboweling by other pride members. In the cheetah a suffocation bite can take even longer to kill prey. Carcass theft and feeding competition is very common among extant large, sympatric predators, and a faster kill mode could be a way of reducing the risk of carcass theft from competing predators. In many large predators with sympatric competitors, rapid consumption can be a way of reducing the risk of carcass theft, and this would most likely have been accentuated in past ecosystems with more intense large predator competition. Accordingly, the morphology and behavior of extant predators need not reflect the circumstances to which they became adapted when they evolved. More intense competition could accelerate the evolution of a sabertooth morphology…

This passage reflects the problems with reconstructing bite forces and predation techniques of extinct creatures; more is involved than just the opening and closing of the jaw. The neck muscles of many sabercats (except in some of the more basal members, as noted) likely contributed to the strength of the bite in a way that’s not directly testable today. Likewise, the killing technique of sabercats might not have required a bite as strong as a modern-day tiger, as in a land filled with other predators, it might simply take too long to try and suffocate a prey animal or bite through the back of their skull. Disemboweling or tearing out the throat of the prey item, by contrast, is a much quicker way to do large amounts of damage but it seems that it would require teamwork, solitary extant big cats often opting for a killing neck bite when the prey has been brought down. Even if this is eventually shown to be incorrect, it should be remembered that bite strength is not everything; despite its large size, the Great White Shark (Carcharadon carcharias) has a relatively weak bite, but it makes up for it with heavily serrated teeth, force of impact when attacking prey, and side-to-side head shaking to saw through its food. Crocodilians, by contrast, have very strong bite forces but they don’t saw through prey or chew, the emphasis being holding on to struggling prey and drowning it before ripping it apart. Such considerations bring us to another point mentioned above in our discussion of scimitar-tooths vs. dirk tooths in that the famous dirk-toothed cats like Smilodon were more powerfully built, seemingly focusing on bringing a large animal down to the ground and then delivering devastating bites once the stomach and neck were exposed (a process that would be made easier by groups working together, as seen in modern examples like lions bringing down giraffes or elephants).

A group of lions brings down a giraffe.

A group of lions brings down an elephant.

A new paper, just out in PNAS, does take the powerful neck muscles of Smilodon into account, however, and the information from the new models appear to corraborate the modern understanding of a felid that captured and killed prey in a way quite different from Panthera. From McHenry, et al.;

Our results demonstrate that bite force driven by jaw muscles was relatively weak in S. fatalis, one-third that of a lion (Panthera leo) of comparable size, and its skull was poorly optimized to resist the extrinsic loadings generated by struggling prey. Its skull is better optimized for bites on restrained prey where the bite is augmented by force from the cervical musculature. We conclude that prey were brought to ground and restrained before a killing bite, driven in large part by powerful cervical musculature. Because large prey is easier to restrain if its head is secured, the killing bite was most likely directed to the neck. We suggest that the more powerful jaw muscles of P. leo may be required for extended, asphyxiating bites and that the relatively low bite forces in S. fatalis might reflect its ability to kill large prey more quickly, avoiding the need for prolonged bites.

Hunting isn’t the only aspect of sabercat predation that seems to have differed from modern carnivores; they way they ate (and what they ate) is somewhat at variance with modern forms, as well. As is apparent at this point, the contact of the canines with bones would have been avoided, and it seems that the hard parts of the skeleton would have been avoided when a sabercat was consuming it. This could differ among different groups (perhaps some of the shorter-toothed forms not being so finicky about bone), but research into microwear patterns on teeth of Smilodon don’t seem to match with wear patterns of any living carnivores, suggesting a different dietary preference. It could be hypothesized, then, that creatures like Smilodon primarily consumed the soft parts of the carcass or what could be removed without too much damage to the teeth, and it should be remembered that living big cats often do not eat every part of the skeleton. Some, like cougars, have favored parts that they eat but end up leaving as much as 40% of the carcass behind. Other predators, especially bone-crushing ones, could take advantage of the leftovers, although the felids might have had to eat quickly as some of their osteophagus competitors may not have been patient (and, in fact, lions and hyenas often fight over kills and steal them from each other today).

Given all the prior considerations, it now seems that sabercats specialized in bringing down relatively large prey down quickly (some likely working in groups to do so), killing the victims by slashing open their stomachs or slicing through the blood vessels of the neck. This would be a much messier, but quicker, method than employed by living big cats, although the limitation of food sources likely caused in the eventual downfall of sabercats. Hypercarnivory can be a dangerous adaptive path to go down, and cats are clearly the most meat-dependant of the Carnivora, but it seems that extinct forms took their dental and dietary specialization above and beyond what is seen today. The price paid for such adaptations ended up being extinction, but given how many times they have shown up in the history of life on this planet, someday there may again be a saber-toothed predator stalking the shadows.

References;

Anyonge, W. “Microwear on Canines and Killing Behavior in Large Carnivores: Saber Function in Smilodon fatalisJournal of Mammalogy, Vol. 77, No. 4 (Nov., 1996), pp. 1059-1067

Christiansen, P. “Canine morphology in the larger Felidae: implications for feeding ecology.” Biological Journal of the Linnean Society. Vol. 91, No. 4 (Aug., 2007), pp. 573-592

Christiansen, P. “Sabertooth characters in the clouded leopard (Neofelis nebulosa Griffiths 1821).” Journal of Morphology, Vol. 267, No. 10 (Jul., 2006), pp. 1186 – 1198

Christiansen, P. and Wroe, S. “Bite Forces and Evolutionary Adaptations to Feeding Ecology in Carnivores.” Ecology, Vol. 88, No. 2 (Feb., 2007), pp. 347–358

Cope, E.D. “The Amblypoda (Continued).” The American Naturalist, Vol. 19, No. 1. (Jan., 1885), pp. 40-55.

Cope, E.D. “The Tertiary Marsupialia.” The American Naturalist, Vol. 18, No. 7. (Jul., 1884), pp. 686-697.

Emerson, S.B., and Radinsky, L. “Functional Analysis of Sabertooth Cranial Morphology.” Paleobiology, Vol. 6, No. 3. (Summer, 1980), pp. 295-312.

Leutenegger, W., and Kelly, J.T. “Relationship of sexual dimorphism in canine size and body size to social, behavioral, and ecological correlates in anthropoid primates.” Primates, Vol. 18, No. 1 (Jan., 1977), pp. 117-136

Lucas, P.W., Corlett, R.T., and Luke, D.A. “Sexual dimorphism of tooth size in anthropoids.” Human Evolution Vol. 1, No. 1 (Feb., 1986), pp. 23-39

Marsh, O.C. “The Fossil Mammals of the Order Dinocerata.” The American Naturalist, Vol. 7, No. 3. (Mar., 1873), pp. 146-153

Martin, L.D., Babiarz, J.P., Naples, V.L., and Hearst, J. “Three Ways To Be a Saber-Toothed Cat.” Naturwissenschaften, Vol. 87, No. 1 (Jan. 2000), pp. 41-44

McHenry, C.R., et al. “Supermodeled sabercat, predatory behavior in Smilodon fatalis revealed by high-resolution 3D computer simulation.” PNAS, Published online before print October 2, 2007

Salesa, M.J., et al. “Aspects of the functional morphology in the cranial and cervical skeleton of the sabre-toothed cat Paramachairodus ogygia (Kaup, 1832) (Felidae, Machairodontinae) from the Late Miocene of Spain: implications for the origins of the machairodont killing bite.” Zoological Journal of the Linnean Society, Vol. 144, No. 3, (Jul., 2005) pp. 363-377

Salesa, M.J., et al. “Inferred behaviour and ecology of the primitive sabre-toothed cat Paramachairodus ogygia (Felidae, Machairodontinae) from the Late Miocene of SpainJournal of Zoology, Vol. 268, No. 3 (Mar., 2006), pp. 243-254

Simpson, G.G. “The Function of Saber-Like Canines in Carnivorous Mammals.” American Museum Novitiates, August 4, 1941

Therrian, F. “Mandibular force profiles of extant carnivorans and implications for the feeding behaviour of extinct predators.” Journal of Zoology, Vol. 276, No. 3 (Nov., 2005), pp. 249-270

Therrian, F. “Feeding behaviour and bite force of sabretoothed predators.” Zoological Journal of the Linnean Society, Vol. 145, No. 3 (Nov., 2005), pp. 393-426

Van Valkenburgh, B., and Molnar, R.E. “Dinosaurian and mammalian predators compared.” Paleobiology, Vol. 28, No. 4 (Dec., 2002), pp. 527–543

Walker, Alan. “Mechanisms of honing in the male baboon canine.” American Journal of Physical Anthropology, Vol. 65, No. 1 (?, 1984), pp. 47 – 60

Wroe, S., McHenry, C., and Thomason, Jeffery. “Bite club: comparative bite force in big biting mammals and the prediction of predatory behaviour in fossil taxa.” Proceedings of the Royal Society B, Vol. 272, No. 1563 (Mar., 2005), pp. 619-625

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Philadelphia Zoo Photos, Pt. II

10 09 2007

As promised, here is the second set of the better photos from my trip to the Philadelphia Zoo. I probably should (at the reccomendation of several commentors) register with Flickr and upload the lot of them, but that will have to wait until tomorrow (I’ll also go back and do likewise for the pictures on this computer as time permits). Let’s pick up where we left off, with one of my most favorite of big cats, the Amur Leopard (Panthera pardus orientalis);

Amur Leopard

Amur Leopard

Amur Leopard

Amur Leopard
There is nothing quite so beautiful as the emerald, fiery stare of an Amur Leopard. The eyes of almost any big cat can be described as intense or as being as intricate as a precious stone, but there is something about the gaze of leopards that strikes me in an entirely different way than that of their cousins…

Amur Leopard
…yet even the most majestic and feline predators needs to make time for a brief tongue-bath every now and again.

Amur Leopard Yawn

Amur Leopard Yawn

Amur Leopard Drink

Amur Leopard
It’s amazing the amount of bravado an inch or so of glass can produce. The object of the leopard’s stare was a child that could not have been more than two years old, being held up to the glass by his parents to get a closer look at the “big kitty.”

Amur Leopard
At times the leopard seemed just as interested in what I was doing as I was in his activities.

Amur Leopard Brian Switek
It is sad enough that this leopard is among the last of his kind anywhere in the world, being the most endangered of all the big cats. Why he is left on display in isolation, with not even as much as a plaque explaining what species he is and the problem those still in the wild face, not to mention the (as far as I can ascertain) the lack of a breeding/conservation program, confuses and frustrates me.

Amur Leopard

Giant River Otter
The Giant River Otter (Pteronura brasiliensis) were also released just as my wife and I reached their enclosure. They certainly seemed excited to be out in their habitat, full of fish for them to snack on.

Giant Otter
At one point something apparently spooked the group, and they engaged in a “mobbing” behavior similar to that seen in the BBC’s Planet Earth series when a group of otters of another species faced a Mugger Crocodile. What the disturbance was, I couldn’t tell, but it seemed to come from the other side of their enclosure.

Unfortunately WordPress was a down for a little while last night so I didn’t get to upload the rest of the pictures, but I will do so during a break between my classes in a few hours. Snuggling Aardvarks, primates (from prosimians through apes), and mammalian herbivores of various description.





Photos from the Philadelphia Zoo, pt. I

9 09 2007

As promised, here are some of the better shots from yesterday’s visit to the Philadelphia Zoo. I’m sorry to say that I’m going to soon write up something about the Zoo’s shady dealings involving it’s African Elephants (visit Help Philly Zoo Elephants for a spoiler), but for now I’m going to focus on some of the better photos out of the 500+ I shot yesterday. And away we go…

Fountain
I absolutely love this fountain.

Highland Cattle
While not particularly exotic, Scottish Highland Cattle are still pretty neat.

Blue Eyed Lemur
A pair of rare Blue-Eyed Lemur, Eulemur macaco flavifrons. The black one is the male, the blonde the female, and they were very excited at the prospect of a snack (the mangabey next door was getting ded fed at the time)

Giant Elephant Shrew
One of my most favorite of all mammals, the Giant Elephant Shrew (Rhynchocyon petersi).

Mara
This, by far, was the thinnest Mara (Dolichotis sp.) I think I have ever seen.

Galapagos Tortoise
The Galapagos Tortoise (Geochelone nigra) were just beginning to stir when we arrived. They weren’t nearly as randy as they had been during our last visit (I thought I had heard it all until I hear the deep tones of tortoise-lovin’)

Petunia Elephant
An African Elephant (Loxodonta africana) that we were told was named “Petunia” was also up and about. The Philly elephants will soon be moved out of their rather meager accomodations, although it might not necessarily be for the better.

Amur Tiger Cub
This little male Amur Tiger (Panthera tigris altaica) really loved his tire. He wouldn’t let any of his brothers near it without showing his annoyance.

Amur Tiger Cubs

Amur Tiger Cubs

Amur Tiger Cub

White Lion
The strangely white female lions were relaxing in the early-morning shade. I know that their condition is a regional variation, although I forget the details at the moment.

White Lions

Male Lion

White Nosed Coati
Some of my most favorite Carnivores, White-Nosed Coati (Nasua narica) were scrounging for insects and other morsels when we passed by their enclosure.

White Nosed Coati

Red Panda
And, just for Jeremy, a Red Panda (Ailurus fulgens).

Caiman
We also came across the most evil-looking Caiman I had ever seen (there was no ID plaque, so I’m not sure what species it was).

Clouded Leopard
And the Clouded Leopard (Neofelis nebulosa), as ever, was asleep in it’s hammock. I have never seen this cat move a muscle in my four visits to the Philly Zoo thus far.

Amur Leopard
Just around the corner, however, was a much more active and curious cat; a male Amur Leopard (Panthera pardus orientalis). He is one of the most beautiful big cats I think I have ever seen, and it’s a shame that he’s essentially “locked up” in his enclosure, and as far as I know the zoo does not keep a female Amur Leopard to run a breeding program for this most critically endangered cat.

Amur Leopard

I still have at least 25 pictures to share, but you’ll just have to wait a little bit longer for them. Check back later tonight for more of our friend the Amur Leopard, some Giant River Otter, White-Handed Gibbons, and plenty more.





Mine!

8 09 2007

My trip to the Philadelphia Zoo this morning presented lots of great photo ops, especially in the morning. I’ll post more of the plethora (500+) of pictures I took tomorrow, but here’s a bit of a teaser. First, the three male Amur Tiger cubs born recently. The one in the middle really loved his tire;

Tigers

The male Amur Leopard also was very curious about what I was doing on the other side of the glass, being much more active than on previous occasions when I have visited (expect a larger post on Amur Leopards and their plight in the near future);

Amur Leopard





Convergence or Parallel Evolution?

6 09 2007

Many of the world’s great natural history museums devote at least one hall to creatures that no longer exist today. In the old tradition, in order to keep any young upstarts from getting any ideas about evolution, skeletons or parts of skeletons were grouped by the functions they performed, a visitor being likely to find the wing of a bat and the wing of the bird in the same display case even though the two animals extremely distantly related. Newer layouts, conversely, have largely ignored the end-function of one line or another to group animals together by homology and their shared characters, the most well-known example being the remodeled 4th Floor of the American Museum of Natural History in New York City which has attempted to arrange its fossil collections as a walk-through cladistic diagram.

Still, the generally discarded of grouping animals by their adaptations to general habitats or niches is not without it’s charms. Over and over again, evolution has produced forms that seem to converge on certain body plans, varying habitats making some traits advantageous and others a liability, helping to adapt different organisms to their local ecologies. Flight has independently evolved several times (and the ability to glide an even greater number of times), as well as adaptations to marine environments, saber-like canine teeth, immense sails along the spine, and slicing premolar teeth, although each time such familiar features seem to arise it shows that there is more than one way to solve an evolutionary problem from any given point in an organism’s natural history. Not everything can be chalked up to convergence of form in order to carry out particular functions, however. Parallel evolution, although sometimes difficult to determine, also allows relatively closely related forms to take the same evolutionary paths, showing many of the same anatomical characters even though they diverged from a common ancestor at some point in the past and occupy at least two different lines of descent. In fact, it is often these weird and wonderful creatures that are forgotten or overlooked, more people recognizing the term “saber-toothed cat” (or, loathe as I am to say it, “saber-toothed tiger”) or the genus Smilodon than the term “Nimravid” or the genus Dinictis. The following entry, therefore, will be an attempt to navigate through the somewhat “entangled bank” of evolutionary relationships among animals that appear to be shaped in similar ways by the environment but constrained by their species’ history, showing us that there is more than one way to make a saber-toothed cat.

Back into the pool: Of Ichthyosaurs, Sharks, and Cetaceans

Perhaps one of the most well-known (or at least widely cited) examples of evolutionary convergence has been that of the similar body shapes of sharks, ichthyosaurs, and cetaceans. It’s difficult to see these three distinct groups of creatures side by side and not recognize the similarities, but why are they similar in the first place? If they belong to groups that are distantly-related branches of the evolutionary “bush,” why should they have developed similar body forms?

Icthy Shark Porp
One of the most well-known examples of evolutionary convergence; (From Top to Bottom) An ichthyosaur Ophthalmosaurus icenicus, a Porpoise, and a Spiny Dogfish (Squalus acanthias)

Shark Icthy Porp
From the 1925 creationist book The Predicament of Evolution by George McReady Price.

Creationists have been quick to seize upon the idea of convergence as if it were one of evolution’s weak points. In 1926, George McCready Price wrote the following in one of the more well-known early American anti-evolution texts, The Predicament of Evolution;

For instance, we have the shark, the ichthyosaur (an extinct kind of fish-shaped reptile), and the dolphin (a true warmblooded mammal, and not a fish at all), all of which greatly resemble each other in external shape and general appearance. Each has the same long, sharp snout, the same powerful tail, the same general fishlike shape. And yet the first of these is a true fish, the second was just as true a reptile, while the third is a mam-mal, bringing forth its young alive and feeding them by milk, just as does a cow or a horse, though it lives in the sea.

Here the evolutionists have to say that this peculiar shape and general form has been evolved separately and independently in each of these three instances. Indeed, Henry Fairfield Osborn, President of the American Museum of Natural History, New York City, declares that a very similar shape and form has been independently evolved “at least twenty-four times.”—”Encyc. Brit.,” Vol. XX, p. 578…

From this large group of facts we become convinced that these many similar or identical structures, which must have been evolved quite independently (if evolved at all), make too great a draft on our credulity. At least, these hundreds of examples of “parallel evolution” greatly weaken our confidence in homology, or similarity of parts and organs, as a proof of blood relationship.

Such arguments have become traditional amongst creationist apologists, suggesting that if convergent evolution does occur then we must throw homology out the window as similar structures will only mislead us as to the true affinities of the creatures being studied. As we will later see with Cuvier’s Ptero-dactyl, this can be a danger for scientists who are unwary and wish to shoehorn creatures into existing taxonomic categories, but not for those who actually look beyond superficial appearances.

The reason why the shark, the ichthyosaur, and the porpoise should all look vaguely the same is because they live(d) in the same environment; the ocean. An organism that is suspended in a fluid that is much denser than air can be adapted in various ways to such an “alien” environment, but physics does dictate what shapes can be taken based upon life history. It is possible to be a floating filter feeder, exhibiting a round shape, but such a strategy is essentially out of the question for animals that need to move quickly and to hunt for food. What is required is not only a powerful propulsive organ to keep the organism moving forward, but also extra appendages to allow for the control of movement and a streamlined shape to reduce drag (and hence reduce energy costs for moving through the water).

Knight
One of Charles R. Knight’s renditions of an ichthyosaur.

In fact, sharks as a whole provide a good model for various forms of ichthyosaurs. While ichthyosaurs are generally presented as already being streamlined and possessing a large caudal fin with two equally long lobes, we would be loathe to forget that they too are products of evolution and many fossils show us that they were not always an Euryapsid (thank you, johannes) answer to modern-day Lamnid sharks. Early ichtyhosaurs actually had more of a “bump” towards the back of their tail rather than a full-blown caudal fin, their overall body shape and lack of a large propulsive surface keeping them from moving too quickly through the water. A similar tail type/form can be seen in many modern day sharks like the Nurse Shark, which generally live along the bottom feeding on crustaceans and inhabitants that can be sucked out of coral crevices. Being that ichthyosaurs lack gills, it is unlikely that their early representatives were bottom-dwellers, instead preferring shallow areas, which can be especially productive in terms of food.

Modification of the “tail kink” (which was at first thought to be a taphonomic feature, early reconstructions showing “amphibious” ichthyosaurs with straight tails) seen in early forms allowed for the eventual evolution of a crescent-moon shaped tail, as well as adaptations in the skull and of the limbs into fins (the addition of digits and the addition of bones in the digits being quite common in the latest forms). This more-familiar shape would allow ichthyosaurs maximum propulsion with their caudal fin (the spine going downwards instead of upwards, as in sharks) while they would be able to exert control over their motions with their pectoral fins and would be kept from rolling in the water by their dorsal fins. The evolution of large eyes and other features aside, the overall shape and basic skeletal structure of ichthyosaurs seems to be an optimal design for medium-to-large, fast-moving, oceanic predators (although mosasaurs, pliosaurs, and plesiosaurs took different evolutionary routes).

What allowed ichthyosaurs to develop an effective side-to-side motion of the tail would not work for cetaceans, however. Ichthyosaurs developed their mode of propulsion by side-to-side motions of the spine, perhaps swimming in a mode similar to eels or cat sharks at first, a common form of locomotion in modern reptiles. This sort of motion is usually accomplished on land via a sprawling gait, the limbs being held out to the sides and the animal exhibiting a bit of a side-to-side motion as it moves along.

Whether the ancestors of icthyosaurs were sprawlers (to a greater or lesser extent, predisposing them to side-to-side motions of the tail and body) or not, cetaceans evolved much more recently in evolutionary history, and developed from ancestors that carried their legs directly underneath their body. The plasticity of early archaeocetes and their artiodactyl ancestors was greatly diminished, their hip and spine structure adapted to up-and-down undulations rather than the side-to-side motion seen in the video of the salamander. This sort of constraint has not stopped mammals from becoming adapted to the water, however, and clues to the evolution of cetacean movement can be seen in living animals like Giant River Otters;

In the water, undulations of the spine accompanied with some propulsion from the limbs proves to be very effective, and it’s not hard to imagine an archaeocete like Ambulocetus, as my friend Neil so aptly described, as a “sexy otter.” Once undulation of the spine became established as a method of moving through the water, the eventual addition of a tail fluke would do for cetaceans what the crescent-shaped tail of tuna, sharks, and icthyosaurs acheived in terms of speed and power, the body being adapted towards a streamlined appearance with (again) the pectoral fins providing lift/control and the dorsal fin preventing rolling. Larger forms of whales, namely the Mysticetes or Baleen Whales, grew to immense size and gave up some of the features that seem to be convergent with sharks and the smaller ichthyosaurs (some, in fact, did acheive whale-size), but they are derived from more predatory designs and their niche as massive, far-ranging suspension feeders free them from some constrains while imposing some new ones.

Harder Ichthyosaur
A painting of leaping ichthyosaurs by Heinrich Harder (circa 1916)

Human engineering has recognized similar constraints for motion in the water and even in the air; planes and submarines most closely resemble sharks and dolphins in overall shape, the placement and size of the wings on a 747 having much the same function as the large pectoral fins of far-ranging pelagic fish like the Blue Shark. Life in the water adapted all three groups of animals towards the same shape because there does not seem to be any other way to be a fast-moving, medium-to-large sized marine predator; speed and some degree of maneuverability are paramount. Some other lines have diverged from this shape, as noted before, but the sharks, dolphins, and (I don’t think it’s too much of a stretch to say) ichthyosaurs all occupied essentially the same niche and therefore were adapted in a particular fashion.

Do not think, however, that the convergence of three lines towards one body plan gives credence to a kind of “orthogenesis” or progressive force driving evolution. There was no sort of supernatural or external force manipulating the genetic material of these groups with the shape of a dolphin or shark in mind. Rather, the environment and local ecology determined what form would be favored through time, and even though the three groups may look the same and have significant convergences, they also have many traits in common with their ancestors, allowing us to trace their evolutionary history (which is why no one is arguing that dolphins, sharks, and ichthyosaurs are closely related or form a small monophyletic grouping).

A marsupial you wouldn’t want to meet

Living members of the Carnivora (bears, cats, dogs, civets, weasels, etc.) have always caught my attention, but there was an entire group of carnivorous mammals, now extinct, that have left no living representatives. The last known member of this group was named Thylacoleo carnifex by Richard Owen, and it has some of the strangest dentition ever seen in a marsupial. Marsupial mammals are well-known in Australia, creatures like kangaroos, koalas, and wombats coming most immediately to mind out of living extant taxa. There was a much more diverse population of marsupials during the Pleistocene, however, and the “marsupial lion” was likely a formidable predator.

Thylacoleo
A skull of Thylacoleo on display at the AMNH.

In order to understand why Thylacoleo is relevant to our discussion of convergence we need to first understand what makes living placental Carnivores so special. Many carnivores, especially cats, have a rather specialized dentition, certain molars and premolars making up what is known as the “carnissal shear.” These teeth are pointed and act like scissors, easily cutting up flesh or crushing bone. The molars behind the shear are often reduced (some groups have retained their molars in order to incorporate a more generalized diet, like dogs and bears), the dental specialization perhaps being one of the keys to the success of this group. Earlier predators of now-extinct lines like Mesonychids lacked such specialized cutting teeth, and the teeth behind the canines of the large Andrewsarchus show that their oral tool-kit was a bit more blunted.

Andrewsarchus
The skull of Andrewsarchus, on display at the AMNH

Thylacoleo, a carnivorous marsupial not descended from the Miacids that gave rise to living carnivores, also developed something of a “carnissal shear” but in a different way. Rather than a battery of teeth that became sharpened, one of the upper and lower premolars of Thylacoleo became elongated and blade-like, and the cleaver-like teeth helped to sharpen each other as they moved past each other when opening or closing the jaw. Thylacoleo also had a terrible bite, the attachments for the muscles that opened and shut the jaw were massive, somewhat constricting the amount of space the brain could take up, but giving Thylacoleo what was perhaps the most powerful bite forces amongst mammalian predators, especially given it’s relatively small body size (it was only about four feet long and 220 pounds).

Thylacoleo is an odd marsupial in another respect; the claw on its thumb was retractable like that of a big cat. This sort of adaptation is especially useful in keeping claws sharp, and perhaps keeping the claws sharp would allow Thylacoleo to get a good hold on its prey before going to work on it with its teeth. At this point I should probably mention that some scholars in the past have thought that Thylacoleo was an herbivore, not unlike the extant marsupial Phalangers. I will leave the response to such an argument to Richard Owen;

These eminent authors received the support, in reference to objections to my conclusions, of the (then) Curator of the Australian Museum, Sydney, Mr. GERARD KREFFT, who, in his contribution to the ‘Annals and Magazine of Natural History,’ series 3, vol. 18, 1866, p. 148, records his opinion that “the famous marsupial Lion was not much more carnivorous than the Phalangers of the present time.”

The species of carnivorous Phalanger is not named. No evidence of such by fossil specimens has reached me, nor have I found such exceptional habit of an existing species of Phalangista elsewhere noted.

As my friend Zach has noted, however, calling Thylacoleo a “marsupial lion” is a bit misleading. Even though some lion-like aspects of the skull (the results of convergence on a hypercarnivorous lifestyle, and Thylacoleo means “pouched lion”) led the anatomist Richard Owen to name the creature on the basis of such resemblances, the ways in which Thylacoleo shows its marsupial affinities are much more important. Referring to this animal as the “marsupial lion” without qualifications (as well as calling the extinct Tasmanian Tiger the “marsupial wolf”) usually confuses more than illuminates, and creationists often take the names and superficial resemblances to mean that evolution didn’t occur. Instead, they propose that God made the beginning of a “kind” of carnivorous mammal which was preserved on Noah’s Ark and gave rise to all later forms, important reproductive habits deemed to be of little consequence.

Even so, Thylacoleo carnifex and its relatives represent a branch of marsupials that became fairly specialized predators, and given the plasticity of tooth structure, it’s not hard to see how sharp premolars could be adapted into a blade to cut flesh. While it may be easy to draw connections between this animal and living carnivores, however, perhaps we should be more measured in our descriptions; both groups met the same challenges in similar ways, but the differences are far more striking and important in this example of convergence on a particular niche.

On what day were the Ptero-Bats created?

Pterodactylus
An engraving of the creature now known as Pterodactylus antiquus, the very one described by Collini.

Before there were natural history museums, there were motley assortments of organic odds and ends known as curiosity cabinets, and in the cabinet of Karl Theodor there would eventually come to be a petrified treasure. Although it was probably collected around 1767, the first known pterosaur fossil was not described until 1784, when the appointed caretaker of the collection, Cosimo Alessandro Collini, attempted to determine the nature of the strange creature that came to him from the limestone of Bavaria (the same deposits that later yeilded Archaeopteryx). Although certain that he was the remains of an animal from an earlier time, Collini was agnostic about what kind of animal he had come to possess. Years later, the famed anatomist Georges Cuvier investigated Collini’s paper and illustrations, noting that the creature was certainly a reptile. Still, the fossil would remain without a proper name until Cuvier would write a more detailed analysis in 1809, dubbing the fossil “Ptero-dactyle.”

Not everyone agreed with the analysis of Cuvier, however, especially since Cuvier did not get to see the fossil himself and had to work from the drawings in Collini’s paper. Samuel Thomas von Soemmerring, of the Bavarian Academy of Science, thought that the pterosaur was some unknown type of bat, a view that would remain entrenched in the minds of some scientists for many years. Indeed, one restoration by Edward Newmann in 1843 (and “re-drawn” for Gosse’s work Omphalos, as shown below), depicted the two known types of pterodactyl known at that time as fuzzy bats, complete with cute little ears. It is clear from the drawing that pterosaurs do not make good bats, although this didn’t stop many German paleontologists from taking such a stance through the first half of the 19th century.

Ptero bats
Newmann’s “marsupial bats”, conspicuously missing their ears, from Gosse’s Omphalos. It’s likely that Gosse recognized the reptilian nature of these Pterodactyl by the time he wrote his book, so Newmann’s work was copied minus the more mammalian aspects.

But why was there such confusion? It is likely because there is something familiar about pterosaurs that had been seen in living bats; the extension of digits to hold a membraneous wing. While the first fossil, despite wonderful preservation, did not preserve a membrane impression, it is hard to look at it and not recognize the superficially similar structure of a bat’s wing, which also carries a membrane to enable flight. In fact, birds seem a bit unusual in developing feathers for flight; many varieties of gliding and flying creatures have taken to the air (regardless of whether they engage in powered flight or glide) by the use of membranes. Indeed, gliding may often precede powered flight, and once an animal has developed a membrane that can be stretched between its limbs to glide, the extension of the digits at the point(s) of attachment can help to expand the wing size. Such changes likely occur as a result of changes in development, natural selection favoring the invasion of a new niche based upon variations that exist in a population, although in the case of pterosaurs we can no longer test to see if this is correct.

As we just saw with Thylacoleo, however, the convergences of pterosaurs and bats are rather slight, overall. While both acheived flight on membraneous wings attached to extended digits (many more in the case of bats) and have relatively compressed bodies, pterosaurs had a much greater diversity in shape and size than modern bats. Likewise, they did not elongate the rest of their fingers, suggesting that there was some situation (be it climbing or hanging on to a perch) that the pterosaurs still needed their other fingers for (although bats can climb pretty well with their thumbs, and some have even evolved suction disks). Still, it can be said that both took to the air by similar means and had to deal with similar constraints, but their evolutionary paths are far more divergent than that of the aforementioned sharks, ichthyosaurs, and cetaceans.

It doesn’t look like much of a planet-eater to me

Gharial
A female Gharial at the National Zoo in Washington, D.C.

Perhaps one of the most unnecessarily confusing groups of extinct animals are the phytosaurs. Filling the niche now occupied by reptiles like the Saltwater Crocodile, the water-dwelling archosaurs have left no living descendants despite their past diversity. At first glance, the phytosaur Rutidon looks just like a modern-day Gharial, and even though it shares a common ancestor with the reptiles that now exist in tropical watery habitats all over the world, it is not otherwise related. The most prominent phytosaur feature is that their noses are over or just anterior to their eyes on their head, not at the end of their snouts. This would allow them to breathe while completely submerged, although their eyes might not have been above water when hiding in such a manner. Even beyond this feature, their jaws seem to be fairly simple, merely having a hinge at the back to open-and-close. Compare this arrangement, here represented by the giant Machaeroprosopus gregorii, with the more complex reconstruction of the true crocodilian Deinosuchus (although, admittedly, this reconstruction was heavily based upon the living Cuban Crocodile and may not be fully accurate. It still serves to show the differences between the groups, however).

phyto
Machaeroprosopus, currently on display at the AMNH

Deino
Deinosuchus reconstruction, formerly on display at the AMNH

The most notable difference are the complex bones at the back of the throat of Deinosuchus which are arranged to slide past each other as the jaw opened and closed. No such feature is seen in the giant phytosaur. Still, even after the phytosaurs died out, crocodilians did not return to the water until about the Cretaceous period, many forms being absolutely terrifying land predators that have also long been extinct. One of the early forms was Protosuchus, a small true crocodilian that represented a line that changed little during its tenure on the earth.

Protosuchus
Reconstruction of Protosuchus

Outside of walking relatively high off the ground, Protosuchus had a foreshortened snout which was lower than its eyes, quite different from the arrangement in living crocodilians. As seen in the Dwarf Caiman photograph, below, living crocodilians have their eye sockets on the top of their head, their eyes sticking out on the surface as well as the tip of their nose when they lie in wait for prey (or just rest, for those who would like a less sensationalist tone). Protosuchus, by contrast, has eyes to the sides of the head, even facing somewhat forward, showing that it was much more well-adapted to the land than any swamp or shallow pool. Crocodilans did eventually enter the water, however, and their fossils are among the most common of any vertebrates. Some, like New Jersey’s very own Thoracosaurus, even became marine species, and a few varieties evolved crescent-shaped caudal fins on the ends of their tails to help them swim. The common belief, however, is that crocodiles have always been crocodiles, “changing little since the time of the dinosaurs,” and such generalized half-truths do little justice to crocodilians or their distant phytosaur cousins.

Dwarf Caiman

Saber-toothed Nimravid doesn’t sound quite the same…

Many museums have cases devoted to the great saber-toothed cats of epochs long gone, but it would take someone with more than just a cursory understanding of paleontology to sort out what is really being displayed. Saber-teeth, or elongated canines, have evolved many times over in the course of mammalian history, showing up in herbivores like the living Musk Deer as well as extinct groups like the gorgonopsids. Animals as different as a Musk Deer and Inostrancevia are fairly easy to tell apart, even for the non-specialist, but what about nimravids and the “true” saber-toothed cats?

Nimravids
A diagram of the three ideas of Nimravid/Felid evolution.

James Whitcomb Riley is purported to have once written “When I see a bird that walks like a duck and swims like a duck and quacks like a duck, I call that bird a duck.” Unfortunately, this argument is quite popular (even being utilized by the likes of prominent Intelligent Design advocate Michael Behe) despite being very superficial and even vapid. Needless to say, it doesn’t apply to our discussion of Nimravids and true felid saber-toothed cats, but in decades past the two groups were lumped together.

<img src=”Skulls” alt=”Skulls” />

So, what makes a nimravid a nimravid? They look awfully like cats, so why aren’t they included in the Family Felidae? What makes such distinctions so difficult is that those investigating the skull of Smilodon and Eusmilus would have to be relatively well-versed in scientific jargon and anatomy in order to point out the most important differences. While some nimravids (like Eusmilus) had large canines, their teeth alone are not diagnostic, and the original factors used by E.D. Cope that differentiated these animals from “true” cats were the “alisphenoid canal, postglenoid foramen, carotid, posterior lacerate, and condyloid foramina, postparietal foramina” in the skull (Hunt, 1987). The various canals and foramina listed dictate the paths of various nerves and blood vessels in the skull, and the arrangement in nimravid skulls seem to be more primitive compared with true felids. Likewise, nimravids lack a two-chambered auditory bulla, which is a rounded bit of bone associated with the ear which true cats posess.

There are a few more obvious giveaways when dealing with some nimravids, however. Nimravids equipped with long canines often have more cone-shaped canines than those of saber-toothed cats (which are flatter in cross-section), and many have bony “sheaths” extending from the lower jaw into which the massive teeth fit. Perhaps the most famous example of this kind of arrangement is the genus Barbourofelis, an animal that has actually been assigned to its own family as it is likely more closely related to true cats than nimravids (Barbourofelis was previously classified as a nimravid). Because of this (and the fact that another cat-like offshoot, the marsupial Thylacosmilus) the tooth-sheath shouldn’t be considered diagnostic of nimravids only, but it does give you a substantial clue that you’re probably not dealing with an actual saber-toothed felid.

Despite these differences, it has often been difficult to differentiate the groups (and debate still continues). The diagram above, based upon one in Robert Hunt’s 1987 paper “Evolution of the aeluroid Carnivora. Significance of auditory structure in the nimravid cat Dinictis,” offers three simplified versions of the hypotheses about the relationships of nimravids and felids. Initially it was thought that there was a progressive evolution from ancestor to descendant in a straight line, the nimravids being the direct ancestors to the saber-toothed cats. This view does not represent how evolution truly works, however, and was found to be incorrect. In its place came a view that nimravids and saber-toothed cats diverged from a common ancestor at about the same time, going off in separate directions. This is better, and is more consonant with the data, but again it suggests that the line representing the common ancestor went extinct, either in becoming nimravids or saber-toothed cats. What seems to be the case based upon current data is that the nimravids split off from a common ancestor somewhat before the saber-toothed cats, the line containing their common ancestor continuing its own evolution as both groups evolved. Such a branching pattern is not unusual, and should even be expected, especially since there are living primates like tarsiers and lemurs that represent the overall kind of animal our ancestors once were, but still quite different and undergoing their own evolution alongside our own lineage.

Thylacosmilus
The skull of Thylacosmilus, the marsupial answer to the saber-toothed cat, on display at the American Museum of Natural History in New York City. Note how far back in the skull the roots of its massive canines extend.

Saber
The skull of Megantereon on display at the AMNH. It was one of the “true” saber-toothed cats.

To complicate things even further, the skull or skeleton of the marsupial Thylacosmilus is also often thrown into the mix. Although totally unrelated to nimravids or felid saber-toothed cats, the South-American Thylacosmilus converges closely on the appearance of the placental predators, although there are some important differences. As can be seen from the above photographs, the eye of Thylacosmilus is entirely enclosed by bone on the side of the head, while in many felids and nimravids the eye socket is not entirely ringed-in by bone as if someone had bored a hole in the skull (compare the skull of Thylacosmilus with that of Thylacoleo, above). Further, the teeth of Thylacosmilus have very deep roots, going back in the skull almost over the eye. Originally it was thought that the teeth faced outward, but this was based upon a distorted skull and later finds showed the true position of the long canines.

Nimravid
Hoplophoneus

Now that we have elimated Thylacosmilus from the running as another case of marsupial convergence, we must ask why the nimravids and felid saber-toothed cats are so close to each other in appearance. While many of the instances I’ve discussed previously have been instances of convergence, be it throughout the entire body or merely certain aspects of it, the nimravid-felid connection is a wonerful example of parallel evolution. W.E. le Gros Clark provides an excellent summation of understanding the difference in his 1959 book The Antecedents of Man;

From what has already been said, it is clear that, in assessing degrees of phylogenetic affinity, it is always necessary to take into account the factors of parallelism and convergence in the evolutionary development of related or unrelated groups. These processes can lead to structural similarities, which, taken by themselves, may be misleading. The term convergence is applied to the occasion in general proportions or in the development of analogous adaptations in response to similar functional needs. But such similarities are superficial and easily distinguishable by a detailed comparative study of the animal as a whole. For example, the resemblance in general appearance, even in a number of morphological features, of the Tasmanian wolf to a dog does not obscure the fact that in fundamental details of their anatomical construction they belong to quite different mammalian groups. On the other hand, the potentialities of parallelism seem often to have been much overestimated by some anatomists, for this phenomenon has sometimes been invoked in support of extreme claims for independant evolution of groups which are almost certainly quite closely related. We can agree with G.G. Simpson that the whole basis of parallelism depends on an initial similarity of structure and the inheritance of a common potentiality for reproducing homologous mutations, and that, this being so, the initial similarity and the homology of mutations themselves imply an evolutionary relationship. Expressed in another way, it may be said that convergence increases resemblances (which are, however, no more than superficial), while parallelism does not so much increase resemblances as maintain and perpetuate (by development ‘in parallel’ so to speak) similarities which have already existed ab initio in the genetic make-up of related types. Thus, ‘closeness of parallelism tends to be proportional to closeness of affinity.’

There are a few problems with this reasoning, namely that it seems to give credence to an almost pre-determined genetic course for the lines to evolve in parallel, although le Gros Clark makes it clear in the work that he does not support in any way the notion of orthogenesis. Still, the passage makes the important distinction that in order to undergo parallel evolution groups need to be somewhat closely related and already bear similar structures, evolution preserving many of the similar traits instead of working to the same end from two disparate points. In the case of the nimravids and the felids it seems that they evolved from a common ancestor which was probably taken to carnivory. Nimravids branched off earlier, being more “primitive,” while the felids came off the same line (or a very similar one) after it had accumulated a few more evolutionary changes. Indeed, even if form seems to be static or change little, it’s hard for me to believe that designs are not slightly adapted this way or that as if the creature was an already perfect creation not influenced by changing ecological circumstances. Still, it seems that the nimravids and felids were adapted in similar ways, their ancestral lines probably possessing at least semi-retractable claws, long and sharp canines (although not long to the extreme like its descendants), a shortened face, and a developed carnissal shear. It is really not that difficult to change a civet-like creature (or in the case of our hypothetical common ancestor, a creature a bit closer to a cat) into a saber-toothed Smilodon, the changes being modifications of existing structures more than the creation of something entirely new out of nowhere. In fact, the vertebrate tetrapod skeleton has proven to be quite versatile, and most of the major bones in any vertebrate skeleton can be found to correspond with those in another vertebrate, allowing us to compare rhinos with ceratopsians, dromeosaurs with birds, cats with dogs, ichthyosaurs with cetaceans, and humans with primates.

Of constraints and convergence

I hope that is has become clear why convergence is such a strong theme in the evolution of vertebrates. At this point in the history of evolution, vertebrates have had a chance to fill nearly every niche imaginable in a large variety of habitats over millions of years, and so common themes are bound to arise. When groups return to the ocean, the environmental constraints shape them in ways peculiar to their new way of life that would not be advantageous in other situtations (i.e. being such a large aquatic animal that you’d be crushed by your own weight if you came onto land). When mammals become adapted to be predators, their dentition and morphology must be altered if they are to be successful hunters, carnivores past and present showing some suprising similarities despite being only distantly related. Even when taking to the air, laws of physics still apply, and natural selection often works through physical and chemical constraints to produce new forms.

It is of little doubt that the tetrapod design is a versatile one, retaining its overall character through the various changes that it has endured. Indeed, even when a lineage dies out and may seem gone forever, there is no law that says a similar situtation in the future will not produce forms that may be strikingly familiar, even if such organisms are not directly related to the last group that filled their new niche. Evolution has produced “endless forms most beautiful and most wonderful” and will continue to do so long after I am gone, but random mutation/natural selection do not work in isolation from the rest of the natural world. Evolution has produced so many amazing creatures precisely because ecology, physics, and chemistry have offered up both opportunities and challenges, and I only regret that I will not be able to witness the familiar and unfamiliar about what is swimming in the seas, flying in the air, or stalking the land 500 million years from now.





Thylacoleo carnifex, ancient Australia’s marsupial lion

31 08 2007

My home state of New Jersey is the epitome of suburban sprawl, McMansions and cul de sacs being about as common as the White-Tailed Deer that take advantage of the grass and brush on the side of the Garden State Parkway year round. There is seemingly no place you can go in the state where the rumble and roar of traffic cannot be heard, although the sprawling network of impervious surface does allow for easy travel to almost anywhere in the “Garden State.” On these roads, usually on warm summer nights, you’re likely to see what appears to be a large white rat shuffling across the lanes. While there are no hackneyed jokes that I know of about the Virginia Opossum crossing the road (“To eat your garbage” would be the most realistic answer), the critters turn up as roadkill quite often, not a very dignified end for the only marsupial mammal to live north of the Rio Grande in North America. While the scruffy Virginia Opossum represents the whole of marsupial mammals in the U.S., it has many close relatives throughout South America (Order Didelphimorphia), and is a bit more distantly related (but still close) to the Australian “possums” (Suborder Phalangeriformes), the marsupial forms of “the island continent” being perhaps the most familiar and oddly charismatic of any members of the Infraclass Marsupialia.

Kangaroo
A Red Kangaroo (Macropus rufus) at the Philadelphia Zoo (taken in February, 2007). The Red Kangaroo is probably the world’s most recognizable living marsupial mammal.

Much like any group of living mammals, however, the fossil record of marsupial mammals is full of bizarre forms that have left no living descendants. We should not regard such lines of extinct fauna as somehow inferior or flawed, however. As famed paleontologist Stephen Jay Gould once wrote in his book Wonderful Life;

First, in an error that I call “life’s little joke”, we are virtually compelled to the stunning mistake of citing unsuccessful lineages as classic “textbook cases” of “evolution.” We do this because we try to extract a single line of advance from the true topology of copious branching. In this misguided effort, we are inevitably drawn to bushes so near the brink of total annihilation that they retain only one surviving twig. We then view this twig as the acme of upward achievement, rather than the probable last gasp of richer ancestry.

I can scarcely think of a better example of this notion of the spectacular diversity of past life than the extinct marsupial Australian Megafauna, and the carnivorous Thylacoleo carnifex would remind any fossilist that just because an animal is extinct, such status does not imply that it was not a terror in its heyday. Deemed the “Marsupial Lion” Thylacoleo carnifex developed many of the predatory adaptations we seen in living big cats (hence the “leo”, meaning “lion”, in the genus name), and despite the superficially rodent-like appearance of it’s front teeth, it was certainly a powerful predator.

In order to understand why Thylacoleo was such a formidably hunter we first need to understand something about living Carnivora (civets, otters, cats, dogs, bears, etc.) and the way their teeth were arranged. While their have been many large carnivorous mammals since in the past 65 million years, carnivores are set apart by their carnissal, or “scissor”, teeth. If we look at the massive skull of the predatory mesonychid Andrewsarchus of the Eocene, for example, the front teeth appear useful for piercing but the teeth further back in the jaw a large and a bit blunted. While useful in tearing flesh from bone and crushing, they were not especially well-adapted to cutting slicing flesh and such creatures probably ate a fair amount of bone (and possibly had problems with bone splinters in their gastrointestinal tracts) as well.

Andrewsarchus
The only known skull of Andrewsarchus, on display at the American Museum of Natural History in New York City. Note the large, blunted teeth towards the back of the jaw.

The likely ancestors of today’s extant carnivores had their start long before Andrewsarchus was roaming what is present-day Asia. Miacids were weasel-like mammals and are known from the Paleocene and Eocene epochs, and are the first mammal group known to have teeth called “carnissals.” These are the teeth that group all living carnivores together, robust and pointed teeth that seem to be essential to consuming flesh. Another group of mammals, the creodonts (the first of which were discovered by E.D. Cope), also possessed carnissal teeth, but their line died out about 8 million years before the present. Still, the success of the carnivorous mammals seemed to depend on the specialization of the some of the premolar and molar teeth into a sharp, cutting edge, commiting many of the group to a strictly carnivorous lifestyle. Cats are the most specialized today, as they have lost some teeth in the front of their jaw in order to allow their dagger-like canines to have the maximum effectiveness and they no longer have flattened molars at the back of their jaw like canids (dogs) have, allowing those animals a little bit more of a diverse diet in tough times. Indeed, overspecialization in a predatory niche, called “hypercarnivory,” can often put a species at risk if they cannot effectively process other food sources if prey stocks dwindle (such a hypothesis has been put forward about the recent “bone-crunching wolf” discovered in Alaska).

Thylacoleo
A replica of the skull of Thylacoleo, on display at the American Museum of Natural History.

Thylacoleo, however, was an entirely different branch of the mammalian tree, but it seems that its skull and jaws were adapted to similar ends (although arguably were more extreme in their modifications). As easily seen from the skull of Thylacoleo, this marsupial predator was adapted to have it’s own fearsome shearing teeth. The premolars essentially became laterally-compressed blades, more high-ridged and pointed at the front, yet still sharp all the way down their length. These teeth in the upper and lower jaw even helped to sharpen each other as they slid past, allowed the predator to retain a sharp edge. Flattened teeth that might be useful for grinding or processing other foods are entirely absent behind the premolars, showing the Thylacoleo was a specialist of the highest order, having much more scissor-like teeth than the placental carnivores on other continents. Such a gape would have been absolutely fearsome, as exemplified by this recent reconstruction by Jeanette Muirhead;

Thylacoleo
Thylacoleo carnifex, used with permission of artist Jeanette Muirhead.

What is even more surprising than the blade-like teeth of Thylacoleo, however, is how strong its jaws were for a creature of its size. A recent study by Wroe, McHenry, and Thomason found that Thylacoleo, a predator that was less than four-feet long and probably weighed only 220 pounds, had the a bite force equivalent to a modern lion twice its size. The unusual dental arrangement of its jaw might have mitigated this somewhat and technical trials still have to be carried out, but if what the researchers found holds then Thylacoleo could probably have preyed upon most animals living in its range up to sub-adult size on its own, perhaps being the fiercest mammalian predator ever known.

How did Thylacoleo attain such high bite forces? The answer might have to do with the brain and skull differences between marsupials and placental mammals. Many carnivores have relatively large brains in comparison with marsupials, lessening the amount of bone they can devote to massive muscle attachments to enhance bite force. Thylacoleo, by contrast, seems to have had stronger muscle attachments and a smaller brain, and it’s skull superficially resembles that of a big cat. While canids often have elongated skulls, cats have foreshortened ones, and oddly enough Wrote and his colleagues seem to have found that carnivorous mammals that are known to be bone crunchers (primarily dogs or dog-like carnivores) appeared to have overall weaker bite forces than those that did not have the same osteophagous tendencies. This may have to do with the actual killing of prey, big cats and similarly-designed predators depending on strong bite forces in order to choke their prey to death or tear out a large chunk of the prey’s neck with a jugular bite. There are exceptions to this, the bite of saber-toothed cats often being calculated as relatively weak, but overall it seems that a shorter skull with a deep mouth is better of achieving high bite forces than a longer and narrower one. Hence, Thylacoleo actually is not a bad name for the “pouched lion”; it seems to share a large amount of convergences with its modern-day namesake, although it may have been less bright (and less sociable) with a more powerful bite.

Thylacoleo
Ventral view of the skull of Thylacoleo. From E.D. Cope’s “The Tertiary Marsupialia” in The American Naturalist, Vol. 18, No. 7. (Jul., 1884), pp. 686-697.

The predatory affinities of this animal did not always seem so obvious, however. Paleontologist E.D. Cope, in a paper entitled “The Tertiary Marsupiala,” recaps some of the controversy about the feeding habits of Thylacoleo that formed in the late 1800’s;

The discussion between Professor Owen on the one side, and Messrs. Falconer, Krefft and Flower on the other, as to the nature of the food of Thylacoleo, is known to paleontologists. From the form of the teeth alone, Professor Owen inferred the carnivorous nature of the food of this genus, while his opponents inferred a herbivorous diet from the resemblance between the dentition and that of the herbivorous Hypsiprymnus. I have pointed out that the comparison of Thylacoleo with Hypsiprirnnus is weakened by two considerations :

First, the cutting teeth in the two genera are not homologous ; second, the grinding series of molars, complete in Hypsiprymnus, is almost wanting in Thylacoleo. It evidently does not follow that because Hypsiprymnus is herbivorous Thylacoleo is so also. Professor Flower refers to the reduction of the molars in Thylacoleo as slightly complicating the problem, and concludes that the food of that animal may have been fruit or juicy roots, or even meat. It is difficult to imagine what kind of vegetable food could have been appropriated by such a dentition as that of Ptilodus and Thylacoleo. The sharp, thin, serrate or smooth edges are adapted for making cuts and dividing food into pieces. That these pieces were swallowed whole is indicated by the small size and weak structure of the molar teeth, which are not adapted for crushing or grinding anything but very small and soft bodies. It is not necessary to suppose that the dentition was used on the same kind of food in the large and the small species… In Thylacoleo carnifex it might have been larger eggs, as those of the crocodiles, or even the weaker living animals. The objection to the supposition that the food consisted of vegetables, is found in the necessity of swallowing the pieces without mastication. In case it should have been of a vegetable character the peculiar premolar teeth would cut off pieces of fruits and other soft parts as suggested by Professor Flower, but that these genera could have been herbivorous in the manner of the existing kangaroos, with their full series of molars in both jaws, is clearly an inadmissible supposition.

I have to agree with Cope; it is hard to imagine what sort of vegetable matter Thylacoleo would be eating with its specialized dentition. It would have been able to chop plants, surely, but whatever food was not inside the mouth would fall into the ground, that inside the mouth would have to be swallowed whole. This would results in Thylacoleo chewing much more low-quality plant food than other animals with teeth adapted to herbivory, and I doubt that Thylacoleo had a caecum or a habit of swalling smooth stones to aid in the digestion of the hypothetical greenery. Even in 1969, however, there seemed to be some doubt as to whether Thylacoleo was a herbivore, an omniovore, a carnivore, or a hypercarnivore. Leigh Van Valen, in the paper “Evolution of Dental Growth and Adaptation in Mammalian Carnivores”, wrote;

The jaw musculature of Thylacoleo was generally similar to but more powerful than that of Trichosurus, but whether all this increase was an adaptation to greater size is unknown. The question of the diet of Thylacoleo is unresolved. If Thylacoleo was carnivorous, it was in several respects a relatively inefficient carnivore. However, large carnivores were rare in Australia, and the condition of Thylacoleo is what would be expected if a Trichosurus-like phalanger became carnivorous. But the herbivorous diet advocated especially by Flower, Krefft, Lydekker, Charles Anderson, and Gregory remains a real possibility. A decision on this matter will probably not be possible until there is adequate knowledge of the appendicular skeleton.

The initial descriptions of Thylacoleo by Richard Owen were more certain of the carnivorous habits of the marsupial, however. One December 16, 1858, Owen’s paper “On the Fossil Mammals of Australia. Part I. Description of a Mutilated Skull of a Large Marsupial Carnivore (Thylacoleo carnifex, Owen), from a Calcareous Conglomerate Stratum, Eighty Miles S. W. of Melbourne, Victoria” was read before the Royal Society. It states;

The evidence of a large carnivorous marsupial, from pliocene formations in Australia, reached me not many years after my determination of the still larger herbivorous marsupial, Diprotodon australis, which first suggested the idea of the coexistence. The evidence was received in the year 1846…

Thylacoleo
The fragmentary skull of Thylacoleo from Owen’s paper “On the Fossil Mammals of Australia. Part I. …”

The initial fragmented skull of Thylacoleo carnifex (Owen), pictured above, was obtained and sent to the great naturalist by Dr. Hobson of Melbourne. Upon receiving the fossils, it seems that Owen almost immediately recognized the convergences in the skull with modern carnivores, the extant lion (Panthera leo) being his foil for the characters in the new skull. Owen describes the deterministic state and characters of the skull as follows;

The ‘skull’ consisted of the cranial part, similar in size and in the development of the temporal ridges and fossae to that of a Lion. The ‘incisor’ was a large tooth with a trenchant or incisive crown, implanted, with a small tubercular tooth, in a portion of the right superior maxillary bone, including part of the orbit and lacrymal bone. The latter specimen gave decisive confirmation of the carnivorous character of the fossil, the ‘incisor’ tooth answering in shape and function to the great sectorial or ‘carnassial’ and the tubercular tooth to the small tubercular molar of the Lion; being situated, as in that animal, on the inner side of the back part of the sectorial tooth.

Indeed, the bladelike teeth alone were enough to convince Owen of the ferocious nature such an animal must have possessed, writing;

In existing carnivorous mammals the ferocity of the species is in the ratio of the ‘carnassiality’ of the sectorial molar, i. e, of the predominance of the ‘blade’ over the ‘tubercle;’ and this ratio is shown more particularly in the upper sectorial, in which, as the tubercular part enlarges, the species becomes more of a mixed feeder, and is less devoted to the destruction of living prey. From the size and form of the carnassials of Thylacoleo, especially of the upper one, we may infer that it was one of the fellest and most destructive of predatory beasts.

Thylacoleo
A second, more complete skull of Thylacoleo carnifex, from Owen’s paper “On the Fossil Mammals of Australia. Part II. Description of an Almost Entire Skull of the Thylacoleo carnifex, Owen, from a Freshwater Deposit, Darling Downs, Queensland”

Owen’s assertions did not go unchallenged, however. In a later 1886 paper “Additional Evidence of the Affinities of the Extinct Marsupial Quadruped Thylacoleo carnifex (OWEN),” the anatomist includes a quite humorous remark in response to one of his critics. As noted before, some scientists believed that Thylacoleo was essentially a living Cuisinart specializing in cutting up fruit, no more terrifying than some of the arboreal relatives of the Virginia Opossum noted above. Owen, in classic style, writes;

These eminent authors received the support, in reference to objections to my conclusions, of the (then) Curator of the Australian Museum, Sydney, Mr. GERARD KREFFT, who, in his contribution to the ‘Annals and Magazine of Natural History,’ series 3, vol. 18, 1866, p. 148, records his opinion that “the famous marsupial Lion was not much more carnivorous than the Phalangers of the present time.”

The species of carnivorous Phalanger is not named. No evidence of such by fossil specimens has reached me, nor have I found such exceptional habit of an existing species of Phalangista elsewhere noted.

Thylacoleo
Lower jaw (outside view) of Thylacoleo carnifex, as seen in Plate I of Owen’s “Additional Evidence of the Affinities of the Extinct Marsupial Quadruped Thylacoleo carnifex (OWEN)”

Indeed, armed with a more complete lower jaw of the animal, Owen even further extrapolated its carnivorous habits, postulating that it had been the “check” on the large herbivores known from the same period in Australia. All the large forms, in Owen’s view, ceased to exist when “bimanous” forms came to the continent, either eliminating Thylacoleo or putting it out of a job through competition, although the wholesale slaughter of Australia’s megafauna by the people who would become the Aborigines is not an open and shut case. Even beyond the skull, Owen was provided with a claw complete with retractable teeth, now known to occupy the “thumb” position of this carnivore. Some have speculated that its size, ferocity, and retractable claw allowed it to climb trees like a leopard, although others have doubted this an account of how robust Thylacoleo probably was (being twice the weight of modern leopards), which 1) would have made it difficult to climb trees, and 2) would have allowed it to chase off most of the competing predators/scavengers of it’s day. I doubt that there were many creatures that would have crossed the path of Thylacoleo and survived if the “pouched lion” was hungry or territorial.

Despite it’s fearsome reputation, Thylacoleo seems to have disappeared from the land “down under” around 40,000 years ago, probably the very last of its lineage. Indeed, while I have primarily focused on Owen’s T. carnifex here, there were many other earlier species and related genera, each showing different aspects of the skull and form. Why these bizarre creatures, once so prominently disputed, have disappeared from the public understanding of paleontology I cannot say, but it is probably to the relief of living kangaroos and other Australian fauna that they are long gone.

Thylacoleo
Partial skull elements, most notably the incisors, from Owen’s paper “On the Fossil Mammals of Australia. Part IV. Dentition and Mandible of Thylacoleo carnifex, with Remarks on the Arguments for Its Herbivority”





Everyone back in the pool!: From artiodactyl to cetacean

24 08 2007

[Note: This post isn’t quite as comprehensive as I would have liked, and I want to add some more illustrations from my other computer, so don’t be surprised if it changes a bit over the weekend. Still, I hope you enjoy it!]

When I was in the 4th grade, I took one of my first trips to the National Aquarium in Baltimore, MD, and I was absolutely in awe of the building and the creatures it contained. Although I can’t remember much from that early visit, I do recall one particular exhibit on the 2nd floor just above the food court. Sunk into a wall was a glass case, and in that glass case was a replica of half of the body of a Beluga whale, form the tip of the tail to about halfway up the back. Sticking out of that case was a lever, and when pushed up or down the whale’s tail moved up or down in the water in the display; I was definitely impressed by the amount of power it took to get the tail to move at even a constant, slow rate of speed. Unfortunately the display isn’t there anymore, but it did spark my imagination about how evolution could have made an animal that swam moving its spine up and down rather than side-to-side like a shark. How could dolphins and sharks be so similar, but so different in the way they moved?

One of the most celebrated evolutionary narratives is that of the first fish/tetrapod (“fishapod”) crawling out of the “primordial ooze” onto dry land. Even though we are only distantly related to such creatures through common ancestry, the move from the water to a more terrestrial habitat is regarded as one of the greatest evolutionary innovations ever to occur, paving the way for all the great tetrapods of subsequent ages. In popular culture, this is where many satirical (and sometimes serious) “March of Progress” like diagrams start, usually featuring some goofy-looking fish with legs on, monkeys seemingly having evolved from such creatures in the blink of an eye (thus allowing humans, in turn, to evolve). This view, of course, doesn’t do any justice to the larger evolutionary truth of the situations, and if we are to understand why dolphins and whales swim the way they do we need to at least start with the evolution of early amphibious vertebrates.

Although the living coelacanth Latimeria chalumnae did not give rise to the first tetrapods (it’s closest fossil relative seems to be the coelacanth Macropoma from the late Cretaceous of England and Czechoslovakia), it is a sarcopterygiian fish and so it can give us some clues as to what adaptations these “lobe-finned” fish had that allowed them to evolve and colonize more terrestrial habitats. Outside of having the proper bone structure that would provide antecedents for the limbs of later tetrapods, sarcopterygiian fish can move their pectoral and pelvic fins independently of each other, almost in a walking type of motion in the water. This video, shot recently in Indonesia, shows this type of motion (although the fish in the video is trying to stay in one place more than swim away);

This type of locomotion, based upon the movement of the fins/limbs rather than the spine/tail, proved to the be precursor of early tetrapod movement. With the limbs carried out to the sides rather than under the body, the head would have to be swung back and forth in the same manner that many fish swam, the position of the arms and legs making it impossible to do otherwise. This sort of side-to-side S-shape movement can still be seen today in living amphibians like salamanders;

The next big advancement that we are concerned with is the transition from carrying the arms on the side of the body to carrying them underneath the body, allowing organisms that were adapted in this manner to be much more active. This change was originally said to have been the main reason why dinosaurs succeeded when so many other creatures of the early Triassic did not, although recent finds like Effigia have shown that dinosaurs were not alone in developing a bipedal stance. The group that we’re primarily concerned with here, however, is not archosaurs but mammals and their close relatives. While many synapsids like Dimetrodon still had a sprawling stance inherited from its amniote ancestors, by the Cretaceous mammals were carrying their legs directly underneath their bodies, or at least very nearly so (thanks for the correction johannes). This change allowed the animals to move away from a side-to-side wrenching of the vertebral column on the horizontal axis and allow the spine to undulate on the vertical axis, allowing for faster and more efficient movement. The success of this kind of movement can perhaps best be exemplified today by the fastest terrestrial mammal on the planet, the cheetah;

Going back to the Triassic, however, mammals were still evolving and skittering about while dinosaurs, plesiosaurs, icthyosaurs, pterosaurs, and the other superstars of the Mesozoic were undergoing their own evolution. The archosaurs that returned to the water seemed to undertake at least two strategies, plesiosaurs primarily using their flippers for propulsion like modern-day sea turtles while icthyosaurs started off with more snaky, catshark-like motions, the most derived forms becoming evolutionarily convergent with lamnid sharks (like the Shortfin Mako) and tuna. Mosasaurs, which arrived late on the scene, seemed to employ something of a mix of the two strategies, using long bodies with powerful tails and flippers for propulsion. The earliest-known relatives of living whales, however, would not be progressing on their own aquatic evolution until about 13 million years after the last dinosaurs died out at the K/T boundary, the great marine reptiles being long gone by the time of Pakicetus during the Eocene.

Old Pakicetus
Outdated reconstruction of Pakicetus, and how I was first introduced to the creature in a children’s book.

Pakicetus
Modern reconstruction of Pakicetus
[Illustration by Carl Buell, and taken from http://www.neoucom.edu/Depts/Anat/Pakicetid.html]

The problem with the evolution of whales was that their fossil history was largely a mystery until relatively recently. Charles Darwin, in the first edition of his landmark On the Origin of Species by Natural Selection, hypothesized that carnivorous mammals could be adapted to an aquatic lifestyle given enough time. In Chapter 6, Darwin wrote;

In North America the black bear was seen by Hearne swimming for hours with widely open mouth, thus catching, like a whale, insects in the water. Even in so extreme a case as this, if the supply of insects were constant, and if better adapted competitors did not already exist in the country, I can see no difficulty in a race of bears being rendered, by natural selection, more and more aquatic in their structure and habits, with larger and larger mouths, till a creature was produced as monstrous as a whale.

This passage, although there is no real fault in it, gave Darwin plenty of grief as many thought he was suggesting that black bears had evolved into whales. It is easy to tell from the passage, however, that this is not the case, and a letter to Charles Lyell dated December 10, 1859 further shows that Darwin was not trying to prove such a narrow point. Referring to an interview with the “bitter & sneering” Richard Owen about his book, Darwin writes;

Lastly I thanked him for Bear & Whale criticism, & said I had struck it out. — “Oh have you, well I was more struck with this than any other passage; you little know of the remarkable & essential relationship between bears & whales”. —

I am to send him the reference, & by Jove I believe he thinks a sort of Bear was the grandpapa of Whales!

Indeed, the reaction to this passage was certainly overblown (Darwin may yet be vindicated to some extent as some variety of bear seem to be a fair candidate for the ancestor of pinnipeds) and even after the criticism Darwin still maintained that the process of changing a bear into a more aquatic animal is possible. Still, a suitable ancestor for whales was elusive, even the great Basilosaurus offering no definite answers. Discovered in the early 1800’s, fossils of the whale Basilosaurus (=”Zeuglodon“) were found to be exceedingly common in southern states like Alabama, Richard Owen determining the fossil’s mammalian affinities only after Dr. Richard Harlan had deemed the remains to be reptilian and named the creature Basilosaurus. By 1845 enough material had been found and collected by Albert Koch to tour the country with a 114-foot-long skeleton of a “sea monster” named “Hydrarchos”, which was later revealed to be a composite monstrosity made from 5 different specimens of Basilosaurus and other species.

basilo
Koch’s “Hydrarchos”. Via Wikipedia.

Dignity
“Hydrarchos”, the undignified beast. Via Interrogating Nature.

Basilo Knight
One of Charles R. Knight’s renditions of “Zeuglodon” (1913). From the book Monster Hunters

Given that Basilosaurus was discovered and brought to attention long before Darwin published On the Origin of Species, one would expect him to make some mention of it in the book, and indeed he does. On page 349 Darwin writes;

The cetaceans or whales are widely different from all other mammals, but the tertiary Zeuglodon and Squalodon, which have been placed by some naturalists in an order by themselves, are considered by Professor Huxley to be undoubtedly cetaceans, “and to constitute connecting links with the aquatic carnivora.”

From what I have been able to find, however, Darwin was unsure about whether “Zeuglodon” was an intermediate forms within the cetaceans, and he wrote to Huxley in October of 1871 to ask his friend’s opinion on the matter so that he might include a mention in the 6th edition of the book (which is the edition, I assume, that contains the above-quoted passage). Although the letters themselves are not yet online in their entiretly, it does appear that Huxley replied that there was little doubt of the connection between the ancient whales and living ones, whales probably being evolved from animals like living “carnivora” (dogs, cats, bears, civets, etc.). The problem is, however, that even though Basilosaurus showed differentiation in its teeth and greatly reduced hind limbs, it was still much closer in appearance to living whales than the ancestors of the group, only giving hints as to where to look. For almost 100 years the mystery of cetacean origins would remain.

In 1981 Pakicetus was named and described from fragmentary elements of the jaw and back of the skull by Philip Gingerich and Donald Russell , the parts of the skull that were recovered undoubtedly showing its cetacean affinities. This helped to fill in the story of cetacean evolution, and it seemed that the most likely candidates for the ancestors of whales were hoofed carnivorous mammals (“wolves with hooves”) named mesonychids. Still, the problem with Pakicetus was that so little of it had been found, and that for some time it was seen as something of a stubby proto-whale (see illustration above). Not until 2001, when more complete skeletons were found, were researchers able to have a look at the true form of the animal.

Regardless of whether whales evolved from artiodactyls or mesonychids (a controversy I’ll return to later), the discovery of the rest of the skeleton of Pakicetus is important as it gives us some clues as to how different animals might employ different strategies in returning to the water. Looking at the skeleton, Pakicetus was not a big, robust predator like a bear. It was far more like a dog or wolf (the reconstruction sometimes makes me want to say “rat on stilts”), the limbs and paws being relatively thin. Even though I’m sure Pakicetus could’ve doggie-paddled if it wanted to, this might not have been a very efficient or effective way at moving through the water, especially if you’re going to try and catch anything or use your mouth very much. Undulating the spine and using the limbs to give some extra push in moving the body forward would have been a more effective way to move for an animal that wanted to hunt in the water, and this is exactly the kind of motion we see in river otters (like these giant river otters at the Philadelphia Zoo) today;

This sort of stage in the aquatic evolution of cetaceans makes sense given the body plans of their ancestors, although it probably didn’t fully come into play until creatures like Ambulocetus or its descendants evolved. The precursor to the otter-like movement may have been something employed by other living mammals like muskrats, using the hind feet as the main propulsive appendages. Then again, muskrats and other mammals in rivers and lakes use various strategies to move through the water, so the ancestors of cetaceans probably went through a highly experimental stage before a certain type of locomotion started to be more firmly established.

As discussed earlier, mammals and their relatives were carrying their legs underneath their bodies and not out to the sides since the late Permian, and so the motion of their spine adapted to move in an up-and-down motion rather than side-to-side like many living reptiles and amphibians. Thus Pakicetus would not have evolved a tail for side-to-side motion like icthyosaurs or sharks because they would have had to entirely change the way their spinal column was set up first. At this point some of you might raise the point that living pinnipeds like seals and sea lions move in a side-to-side motion underwater. That may be true on a superficial level, but pinnipeds primarily use their modified limbs (hindlimbs in seals and forelimbs in sea lions) to move through the water; they aren’t relying on propulsion from a large fluke or caudal fin providing most of the propulsion with the front fins/limbs providing lift and allowing for change in direction. This diversity of strategies in living marine mammals suggests differing situations encountered by differing ancestors with their own suites of characteristics, but in the case of whales it seems that their ancestors were best fitted to move by undulating their spinal column and using their limbs to provide some extra propulsion/direction.

Sea Lion
Sea Lion, taken at Sea World, Orlando (July 2006)

Looking at the vertebrae of icthyosaurs, sharks, and dolphins, it’s easy to see how mammalian vertebrae were modified to be useful for the mode of swimming exhibited by dolphins. Dolphins, unlike sharks and icthyosaurs, dolphins have two very large processes coming out of the sides of the vertebral centrum (the round part from which the processes branch), with another process sticking up fairly high. This increased surface area allows for much more muscle attachment than in sharks or icthyosaurs, being adapted to the up-and-down motion of the tail flukes. Early whales with increased surface area for muscle attachment along the spine for this kind of movement would be able to have more powerful tail strokes and probably move faster than others, natural selection modifying the spinal column of cetaceans to make the most of their arrangement. Also, the cervical vertebrae of many cetaceans are fused together, stabilizing the neck. If you’re going to be moving through the water with any amount of speed, it’d be a disadvantage to have a long neck with lots of joints that could be stressed or even broken by certain motions, so there would be an advantage in any move towards stability.

Even so, the skeletal specializations in modern cetaceans were just being formed back in the Eocene. Pakicetus, for example, appears as though it would have been a poor swimmer (as I had mentioned before), although its location makes it clear that it had an affinity for freshwater habitats. Also, the bones of Pakicetus seem to be compacted, thus making them relatively heavy, and some have thought that this condition could have acted almost like a diver’s weight belt or ballast (Thewissen & Williams, 2002). The next stage that we are aware of, exemplified by Ambulocetus, appeared to be much more at home in the water, although still far from its later relatives. Shifting towards shallow marine habitats (possibly bays or estuaries), Ambulocetus may have used its feet to swim through the water, and although some have suggested that it would have been too awkward to catch prey, I don’t think an Ambulocetus that actively hunted would be a foregone conclusion, especially if it swam with undulations of its spine as well as with its feet. Ambulocetus is also of interest in that it has characters in its lower jaw that relate to the lower jaw and ear morphology of living cetaceans, the lower jaw of dolphins being extremely important in receiving sounds during echolocation. Being that we know that the mammalian ear developed from the multiple jaw bones of their synapsid ancestors, it is easy to understand how the lower jaw and hearing mechanisms became so related in cetaceans and their ancestors.

There were various other varieties of archaeocetes, and later forms like Basilosaurus would take the successful early marine forms to extremes by adding and elongating their vertebrae, but I think vertebral undulation as a mode of swimming long preceded the known expression of this mode in the late Eocene whales like Dorudon and Basilosaurus. The evolution of modern whales from these forms is another story altogether (more of modifications of forms that were fully marine by that time), but once again we need to go back to the origins of this group to find some more controversy. A number of years ago, while I was still in elementary school, I remember seeing an episode of the TLC series “PaleoWorld” which featured whale evolution. The show definitely made the connection between mesonychid carnivores as being the ancestors to whales, showing one such creature (I assume it was a DinoMotion replica of Andrewsarchus peering contemplatively into a shallow pool). This seemed reasonable enough, Huxley’s idea of whales evolving from mammalian carnivores being somewhat vindicated, but then came a serious of important papers that shook up the phylogenetic tree.

What needs to be understood before we proceed, however, is that the change of ancestry from mesonychids to artiodactyls did not cause evolutionary theory to come crashing down. Mesonychids are closely related to both whales and artiodactyls, but in this case just being “close” doesn’t mean that they’re the right ancestors. To put this in perspective, the change of ancestry from mesonychids to artiodactyls isn’t nearly as big as the change from the hypothesis that birds evolved from pseudosuchian archosaurs like Ornithosuchus to the modern understanding that birds evolved from theropod dinosaurs. Even so, the changes have caused a good amount of controversy. The support for placing cetaceans within the clade Cetartiodactyla, with the hippopotamus and its relatives belonging to a sister group (mesonychids being just outside the new grouping). Morphological studies of the ankle bones and certain skull characters support this relationship as well, suggesting that living cetaceans and hippos shared a common, perhaps semi-aquatic, ancestor in their distant past.

But what happened to the hind limbs of cetaceans? If cetaceans evolved from land-dwelling ancestors, we would expect to see some change or vestige in the fossil record if not in living groups. In fact, that’s just what we have. While archaeocetes like Ambulocetus clearly still used their fore and hind limbs, by the time the group evolved into whales like Basilosaurus the hind limbs were greatly reduced, natural selection working towards eliminating the non-functioning appendages that would only increase drag. Although the reduction of the hind limbs in adults have been reduced or eliminated (save for a few cetaceans with atavisms like small pelvic fins or leg bones in their bodies in their pelvic region), the development of living cetaceans has also shown us that they once had another set of limbs. During development, dolphin embryos actually develop limb buds, but those that would normally become hind limbs or pelvic flippers stop developing and are reabsorbed into the body, showing that it’s not a matter of removing a trait but rather controlling it through development (which also explains the aforementioned atavisms now and then; sometimes the limb production goes forward, just at a stunted rate).

The relationship between cetaceans and artiodactyls (which encompass many groups of mammals like cows, pigs, giraffe, camel, deer, hippos, etc.) however, has cause some creationists to come up with some rather absurd illustrations in order to show evolution to be incorrect. No scientist that I know of is suggesting that a hippo turned into a whale or that a cow turned into a whale, unless you think cows looked something like Pakicetus (and I wouldn’t want to try milking one if I came across it on the shore of some Eocene lake).

My writing here is far from exhaustive or as in-depth as I would like (I need to learn some more physiology/anatomy, for certain), but I hope that I’ve given a fair superficial summation of how evolution can get from fish to tetrapod to whale, only a snippet of the evolution story of modern cetaceans that spans hundreds of millions of years. Even so, the journey out of the water and back into it, over and over again, is one of the most compelling evolutionary narratives known, especially given the intelligence and grace acheived by the descendants of some Eocene artiodactyls walking by the water’s edge.

Sea Lions
Sea Lions that were playing “King of the Rock” at the National Zoo in Washington, D.C. (March 2007)

References;

Bejder, L. and Hall, B.K. “Limbs in whales and limblessness in other vertebrates: mechanisms of
evolutionary and developmental transformation and loss
” EVOLUTION & DEVELOPMENT 4:6, 445–458 (2002)

Buchholtz, E.A. “Vertebral osteology and swimming style in living and fossil
whales (Order: Cetacea)
” J. Zool., Lond. (2001) 253, 175±190

Fish, F.E. “A mechanism for evolutionary transition in swimming mode by mammals” Secondary Adaptation of Tetrapods to Life in Water, J.-M. hlazin & V. de Buffrenil (eds.): pp. 261-287

Geisler, J.H. and Uhen, Md. “MORPHOLOGICAL SUPPORT FOR A CLOSE RELATIONSHIP BETWEEN
HIPPOS AND WHALES
” Journal of Vertebrate Paleontology 23(4):991–996, December 2003

Gingerich, P.D. et al.Origin of Whales from Early Artiodactyls: Hands and Feet of Eocene Protocetidae from PakistanScience, 2239 (2001); 293

Gingerich, P.D. and Russel, D.E. “PAKICETUS INACHUS, A NEW ARCHAEOCETE (MAMMALIA, CETACEA) FROM THE EARLY-MIDDLE EOCENE KULDANA FORMATION OF KOHAT (PAKISTAN)” Museum of Paleontology, The University of Michigan, VOL. 25, NO. 11, p. 235-246

Milinkovich, M.C. “DNA-DNA hybridizations support ungulate ancestry of
Cetacea
” J. evol. Biol. 5: 149-160 (1992)

Motani, R. “EVOLUTION OF FISH-SHAPED REPTILES (REPTILIA: ICHTHYOPTERYGIA) IN THEIR PHYSICAL ENVIRONMENTS AND CONSTRAINTS” Annu. Rev. Earth Planet. Sci. 2005. 33:395–420

O’Leary, M.A. “The Phylogenetic Position of Cetaceans: Further Combined Data Analyses, Comparisons with the Stratigraphic Record and a Discussion of Character Optimization” AMER. ZOOL., 41:487–506 (2001)

Thewissen, J.G.M. and Fish, F.E. “Locomotor Evolution in the Earliest Cetaceans: Functional Model, Modern Analogues, and Paleontological Evidence” Paleobiology, Vol. 23, No. 4. (Autumn, 1997), pp. 482-490.

Thewissen, J.G.M. and Williams, E.M. “THE EARLY RADIATIONS OF CETACEA (MAMMALIA): Evolutionary Pattern and Developmental Correlations” Annu. Rev. Ecol. Syst. 2002. 33:73–90

Watson, D.M.S. “The Evolution of the Mammalian Ear” Evolution, Vol. 7, No. 2. (Jun., 1953), pp. 159-177.