Photo of the Day: Playing Grizzly Bears

5 10 2007

Grizzly

Two Grizzly Bears playing at the Bronx Zoo. One of the bears, Jughead (all the Bronx grizzlies are named after “Archie” characters) died earlier this year.





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





The Chimpanzees of Mt. Assirik

25 09 2007

When chimpanzees (Pan troglodytes) appear in documentaries they are often shown inhabiting relatively dense tropical forest, their lives taking place within the green refuge of the forests. As with any other species that is spread over a considerable distance, however, different populations of chimpanzees have different habits, and one of the most remarkable populations are those around Mt. Assirik. Located in the southeastern part of the Parc National du Niokolo-Koba in Senegal, the chimpanzees in this area have to deal with a local ecology that is drier and more open than some of their relatives elsewhere, and their behavioral adaptations to the environment is of great interest to those study human origins.

The Mt. Assirik study area is remarkable in that 55% of the habitat is open grassland, only about 37% being woodland of varying density and only 3% being more dense forest (the remaining area being made up of bamboo forest and isolated trees). Such open spaces allow some of the major Carnivora of Africa to live in close proximity to the chimpanzees; Lions (Panthera leo), Leopards (Panthera pardus), Wild Dogs (Lycaon pictus), and Spotted Hyenas (Crocuta crocuta) are all frequently seen in the area. As if having so many predators at their doorstep were not enough, the Mt. Assirik area seems to have fluctuations of food that aren’t correlated with seasonal changes, and in the dry season water is the most prized of any resource. The apes are not entirely helpless in the face of such pressures, however, and they’ve been behaviorally adapted in some very interesting ways.

Given a choice, the Mt. Assirik chimpanzees prefer to spend their time in the denser areas of forest, but shifting food resources sometimes require them to move across large expanses of open grassland in order to find nourishment. Wandering out onto the open plains alone is so dangerous as to nearly be suicidal, and the apes form large mixed groups when they have to move across the plains. During this time they are at their most vulnerable, especially since they would be unlikely to outrun any of the major predators (especially those that hunt in packs), and they are extremely alert when undertaking such a journey. What is perhaps most striking of all, hearkening back to Raymond Dart’s “Savanna Hypothesis,” is the fact that the chimpanzees sometimes stand up to get a better look at their surroundings, potentially spotting predators before they get too close, although such an observation should not be taken as a sweeping vindication of Dart’s ideas of human evolution.

The presence of just one tree or a few trees spaced far apart doesn’t help the chimpanzees much either; mothers with children and individuals spent much less time in the sparser woodland areas than in the forest, mixed groups seemingly having to issues with the woodlands. Why should this be so? Well, leopards can climb trees (and often do so to stash their kills), as well as lions, and so simply climbing a tree does not equal escape. Lone chimpanzees are far more comfortable in a habitat where they can climb a tree and move through the canopy out of reach of their assailants, something that is not possible in woodlands. The predators may also have another effect on the diet of the chimpanzees; the Mt. Assirik chimps do not seem to eat young ungulates or monkeys, although such behaviors have been made famous where it has been observed (i.e. Gombe). This may be due to some competition, but it may also be due to the restricted forested habitat and the fact that chimpanzees would have to enter the habitat of the carnivores in order to capture young ungulates, predators being likely to quickly learn about any kills that had been made.

Indeed, the Mt. Assirik population is remarkable in that it often moves long distances in order to obtain food as it becomes available, relying on numbers and vigilance to protect itself from predators when it’s habitat only offers a few isolated islands of relief. Although humans did not evolve from modern chimpanzees, this population may give researchers some idea of the behavior patterns of our ancestors when faced with similar constraints when forests became sparser and the plains were filled with predators. Such social behavior is not the only thing that makes the Mt. Assirik chimpanzees stand out, however; they also make use of Baobab trees in a very interesting way.

By now many people are familiar with the ability of chimpanzees to use a piece of wood as a hammer to break a nut placed upon an “anvil” of rock or tree root; such footage has been shown in television programs again and again. Such behavior did not come out of nowhere, however, and the way Mt. Assirik chimpanzees open nuts may represent a stage of tool use that precedes the hammer-and-anvil technology. While it had been disputed for some time whether the Mt. Assirik population used hammers and anvils or just anvils, recent studies have shown that they are cracking open the hard nuts of the tree on branches and not using a hammer. While we might think of an “anvil” as something that can only be used in conjunction with a hammer, mechanically this isn’t necessarily so, and the Mt. Assirik chimpanzees bang the hard nuts they collect on the branches of the tree (therefore staying aloft, not coming down to use stones or the roots of the tree), the tree itself being the anvil.

Given the basal usage of anvils by the Mt. Assirik chimps and the use of hammers and anvils elsewhere, it becomes possible to hypothesize about the evolution of stone tool use in our own ancestors. The starting point was likely similar to what is exhibited by the Mt. Assirik chimpanzees, banging hard nuts on trees or rocks in order to open them (thus preventing damage to the teeth, if it even would be possible to open the nuts using their jaws). The next step would be adding a hammer, possibly wooden (as seen in some groups today) or possibly stone. At this stage any combination of wood or stone hammers and anvils could be used, but tool use would probably not progress until a population was using stone hammers and stone anvils to open foods. In such a scenario, the apes would sometimes miss their targets and flake off bits of stone, an accident that would shape the tools. When a certain cognitive leap was made, the apes could then move from accidentally flaking their tools to doing it intentionally to truly be making tools rather than making use of naturally occurring bits of wood and stone. The reality of the situation may be forever lost to us, ancient tool use before the knapping of stone became prevalent being notoriously hard to discern, but such a line of behavioral descent is not unreasonable and seems to allow further development merely by chance combinations of naturally occurring resources.

Such a discussion is only a brief sketch based upon what I have only recently learned myself, but I hope that it has been at least somewhat informative. Different populations of chimpanzees show different behaviors and live in differing ecologies, and it would be a mistake to assume what the famous Gombe chimpanzees are doing holds true for all the other populations. Another population that I soon intend to write about spends time in caves, probes trees for bush babies, and may even have the beginnings of a fire culture; others do not show the same exact behaviors, but they have their own cultures and reactions to the local ecology. While we should be careful in analyzing the living populations of chimpanzees and their perceived similarities to humans, it would be foolish to think that they can tell us nothing of our own past, and if very well may be that some of traits (behavioral or otherwise) they now exhibit were present in our own lineage, vignettes of evolutionary history being replayed with different actors in our own time.





What a waste of chiropterans

19 09 2007

What is it about some people in Texas wanting to slaughter animals for no good reason? According to news reports out today, a number of bats (an actual estimate of the “infestation” not being given) found their way into Lanier Hall East at Texas Southern University, and a number of the young men in residence thought to make a sport out of the lost (and probably confused) bats. [Note: The following video contains offensive language];

While many of the news reports expressed worries about rabies (although no one seems to have been bitten), the people who killed any number of the flying mammals apparently received no rebuke for their cruelty to the animals. They were not being attacked or molested by the bats, and if there was a problem why not leave? Swinging away at the creatures, probably disoriented and frightened, reveling in the smack of the furry bodies on the linoleum, is far from mature, responsible, or even intelligent. Bats are absolutely amazing animals, and despite the rather pungent smell of guano, I was lucky enough to observe Little Brown Bats leaving their roost over a number of nights last summer at Stokes State Forest. There’s really no excuse for what I can only term cruelty on the part of the students in the videos.

Some Texans take a more healthy interest in bats, however, (although their reactions to a bat in nature and a bat in the house might be different depending on who we’re talking about), and plenty of people show up to watch bats emerge from beneath the Congress Ave. bridge in Austin, TX;

Bat really are amazing mammals, and I am remiss in not writing about them more often (and so they’ve been duly added to my ever-growing list of things to learn/write about). In the meantime, enjoy this clip from The Life of Mammals featuring a particular bat that has a relatively unusual way (as far as bats go) in detecting prey.





The Branching Bush of Horse Evolution

13 09 2007

Note: Welcome, visitors from The Sandwalk and Pharyngula! I am certainly humbled by the amount of praise and attention this post has received, and although it’s not as scientifically rigorous as I would have liked to be (I still have much to learn), I hope that you find it to be an enjoyable read all the same.

Update: I’ve created something of an appendix to this article about how creationists have presented horse evolution in some of their books. It can be found here.

Eohippus
One of Charles R. Knight’s interpretations of Eohippus

When the name of O.C. Marsh is invoked, it is often to tell of his participation in the great “Bone Wars” of the late 19th century, sparring with fellow osteophile E.D. Cope in the pages of the New York Herald. Twisted tales of deceit and sabotage were promulgated in the sensationalist paper, and while both men helped to bring about an American revolution in vertebrate paleontology, the scars of their bitter squabbling have yet to fully heal. Such scientific in-fighting might seem worthy only of a historical footnote or an introduction to the stereotyped image of “smash-and-grab” paleontology of the time which is almost romantically referred to, but the truth of the matter goes far deeper than the public beard-pulling that is so often remembered.

The tiff between Cope and Marsh is strange in that is seems to exist in the popular literature out of time, removed from the context in which it had originally existed. Charles Darwin had published his earth-shaking work On the Origin of Species by Natural Selection a scant 31 years before the ink almost ran red with rage on the pages of the Herald, the question of evolution being of far more importance in the public consciousness than dinosaurs. The full establishment of the dinosaur as a cultural (and dare I say, mythical) creature in the mind of the American public only seemed to take place after the Bone Wars, the appointment of Henry Fairfield Osborn to the American Museum of Natural History (specifically hired to establish a vertebrate paleontology program) and the popular reports of the dinosaur that carried the namesake of Andrew Carnegie, Diplodocus carnegei, being the more immediate beginnings of the public’s love affair with the extinct creatures. Before Brontosaurus and Tyrannosaurus became household names, the public eye was focused upon horses and birds.

The latter half of the 19th century was a stirring time for biological science, especially involving the new areas of vertebrate paleontology and evolution, the august authorities in England keeping on eye on the up-and-comers starting their own careers in the states. Early on, paleontologist E.D. Cope impressed T.H. Huxley with his 1866 discovery of Laelaps aquilunguis, but in a paleontological clean-sweep Marsh would eventually have his name attached to Cope’s dinosaur and the admiration of not only T.H. Huxley, but Charles Darwin himself. As for the renaming of Laelaps, Marsh found that the name was already taken by a genus of mite, renaming the New Jersey greensand dinosaur Dryptosaurus in 1877 (although Cope, throughout the rest of his career, called the dinosaur Laelaps). It would take more than some taxonomic shuffling to impress the eminent British anatomists and paleontologists, however, and Marsh’s ticket into Huxley’s good graces came in the form of toothed Cretaceous birds like Hesperornis (Marsh, 1872).

While the discovery of ancient bones was exciting to some, evolution was an even more popular topic, and the question that surrounded every fossil was “How does this fit into the grand scheme of evolution?” The 1861 discovery of Archaeopteryx from the lagerstatten of Solnhofen, Germany seemed to arrive right on cue to confirm that evolution had taken place in times previously referred to as “antediluvial”, and Marsh’s subsequent discovery of birds with teeth in the American West further confirmed the notion that aves had evolved from reptilian ancestors (Huxley even being so progressive as to name the dinosaurs as the probable ancestral stock). Charles Darwin himself recognized the importance of Marsh’s discoveries as well, and two years after Marsh visited Darwin at Down House in 1878, Darwin wrote the following letter to Marsh on or about August 31, 1880;

I received some time ago your very kind note of July 28th, & yesterday the magnificent volume. I have looked with renewed admiration at the plates, & will soon read the text. Your work on these old birds & on the many fossil animals of N. America has afforded the best support to the theory of evolution, which has appeared within the last 20 years. The general appearance of the copy which you have sent me is worthy of its contents, and I can say nothing stronger than this.

With cordial thanks, believe me yours very sincerely,

Charles Darwin

Toothed birds were not Marsh’s only claim to evolutionary fame, however; by 1876 his assistants had collected enough fossil horse material to show that the horse was not “a gift from the Old World to the New” (as an European authority had once said during a lecture), but quite the reverse. In fact, the timing of the discovery and study of the horses could not have been better for Marsh, as in 1876 T.H. Huxley visited Yale and was duly impressed with the American Professor and his fossil horses. Huxley was absolutely enthralled by Marsh’s fossil equids, and Huxley’s son Leonard wrote of the visitation upon the New World horses as follows;

At each inquiry, whether he had a specimen to illustrate such and such a point or exemplify a transition from earlier and less specialized forms to later and more specialized ones, Professor Marsh would simply turn to his assistant and bid him fetch box number so and so, until Huxley turned upon him and said ‘I believe you are a magician; whatever I want, you just conjure up.'”

Eohippus
Restoration of Eohippus. From “The Dawn Horse or Eohippus” by Chester Stock (1947).

Huxley even featured Marsh’s discoveries of toothed birds and fossil horses in a set of three lectures he delivered at Chickering Hall in New York, the visit of such a famous evolutionist being front page news (with Marsh sharing in good press since his fossils were discussed by so prominent a figure as Huxley). The only thing that could have made the event sweeter would have been the knowledge of an ancestral horse with five toes (what was regarded as the “primitive” condition for mammals as far as digits go), and Huxley prophesied that such a creature would likely be found in North America. In truth, while it did not precisely fit the bill, a horse bearing a vestigial fifth toe had already been found and was collecting dust in Marsh’s Peabody museum. Writing to Huxley on July 12, 1877, Marsh revealed that little Eohippus (a name that was given up when it was discovered that Richard Owen’s Hyracotherium had priority, only to be later changed back to Eohippus in recent years) had been right under his nose all along;

I had him “corralled” in the basement of our Museum when you were there, but he was so covered with Eocene mud, I did not know him from Orohippus. I promise you his grandfather in time for your next horse lecture if you will give me proper notice.

Although the popular press did not take much note of the re-discovery of Eohippus, Huxley was well pleased, and promised to show Marsh all the “lions” of British science during his aforementioned 1878 visit. Such close ties would be important to Marsh later on, serving to keep Cope out of some respectable circles as well as giving Marsh a good amount of prestige. Oddly enough, however, Cope had his own horse genealogy (although ignoring Marsh’s labels) that went from four toes to one, and it was Cope’s “dawn horse” that provided the basis for some of the first Eohippus reconstructions, not Marsh’s. While Cope missed out on a golden opportunity in 1872 when he was provided a jaw fragment of an early horse, Marsh’s skeleton (as far as I can tell) remained locked away while Cope’s employee J.L. Wortmann uncovered the rest of Cope’s specimen of Eohippus in 1880. Cope named his animal Protorohippus, and it was his reconstruction that ultimately influenced Charles R. Knight and, subsequently, Rudolph Zallinger when he painted his famous Age of Mammals mural. For those who did not get at least a chuckle out of the last line, Zallinger created his mammalian masterpiece for the Peabody museum, the very establishment that O.C. Marsh had created to start his professional career.

As can be said of any scientist, however, Cope and Marsh were both products of their time and (especially in their respective cases) their egos, and while the fact that horses evolved was proved beyond doubt, both men made mistakes when it came to evolution. While Cope, late in his career, bemoaned the fact that Marsh had poisoned the well when it came to making connections with Huxley and other British scientists, it is doubtful that Cope would have lasted long amongst those of the Darwinian school of evolution. In the 1896 book The Primary Factors of Organic Evolution, Cope saw the evolution of the horse being orthogenic, or proceeding in such a way as to imply direction towards a more adapted or perfected form. As this concerns horses, Cope wrote;

Examination of all these genealogical lines reveals a certain definiteness of end and directness of approach. We discover no accessions of characters which are afterwards lost, as would naturally occur as a result of undirected variation. Nor do we discover anything like the appearance of sports along the line, the word sport being used in the sense of a variation widely divergent from its immediate ancestor. On the contrary, the more thorough becomes our knowledge of the series, the more evident does it become that progressive evolution has advanced by minute increments along a definite line, and that variations off this line have not exerted an appreciable influence on the result.

Such notions would have gotten Cope banned from Finch’s Beak gentleman’s association (if one had actually existed), the concept of directed evolution undermining one of the most important points that Darwin had attempted to make about the “transmutation” of life on earth. As we shall later see, however, such notions of orthogenesis may have had some influence on one of Cope’s latter-day pupils, Henry Fairfield Osborn, as well.


An illustration of the horse “Clique,” which had an extra toe on each fore-foot. Marsh examined this horse while still alive, and the horse was donated to Yale after its death in 1891. From Marsh, O.C. 1892. “Recent Polydactyl Horses.”

Marsh, as has already been determined, definitely had the attention of the progenitors of evolution by natural selection, and through the efforts of Matt Wedel, Randy Irmis, and Mike Taylor a number of Marsh’s writings have become available for viewing on the internet (The Marsh Repository). In a 1879 paper published some time after Huxley’s visit, “Polydactyle Horses, Recent and Extinct,” Marsh prefaces the rather short fossil section with several pages about known horses within recent history that had extra digits. The most typical condition for the differing equines was having an extra digit on the inside of the front hooves, one that did not touch the ground. Coupled with a brief appeal to similar observations of extra toes from development, this approach was indeed a wise one; not only do most living horses have vestiges of digits that have been lost, sometimes a multi-toed condition still occurs in living animals, seemingly fitting with the same trends seen in extinct genera.

Marsh's Geneology of the Horse
O.C. Marsh’s concept of “The Geneology of the Horse,” a decidedly straight-line progression. From Marsh, O.C. 1879. “Polydactyly Horses, Recent and Extinct.”

What is notable about Marsh’s interpretation of the history of horse evolution is how straightforward it is. Although missing Cope’s differing evolutionary hypotheses, Marsh makes no qualifications about the fossils he found representing only the “types” of different horses; horses evolved along a straight line, and while a few steps may be missing, it was not indicative of the widely branching pattern recognized by later scientists. The “extraneous” toes seem to become reduced in a gradual fashion, while size and tooth height increased (although the patterns on the teeth, as can be seen in the illustration, vary quite a bit in the “higher” forms). Perhaps Marsh’s adherence to a strict linear progression was at least partly inspired by the diagnosis of Huxley. In an obituary written by Marsh to commemorate Huxley’s life, Marsh made special mention of his horses;

One of Huxley’s lectures in New York was to he on the genealogy of the horse, a subject which he had already written about, based entirely upon European specimens. My own explorations had led me to conclusions quite different from his, and my specimens seemed to me to prove conclusively that the horse originated in the New World and not in the Old, and that its genealogy must be worked out here. With some hesitation, I laid the whole matter frankly before Huxley, and he spent nearly two days going over my specimens with me, and testing each point I made. He then informed me that all this was new to him, and that my facts demonstrated the evolution of the horse beyond question, and for the first time indicated the direct line of descent of an existing animal.

Such interpretations of evolution and the fossil record could only exist within a certain paleontological framework; the more bones that were found from different times and locales the more the old notions would splinter and crack. Vertebrate paleontologists who would succeed Cope and Marsh could not study what they did not have, however, but they still recognized the importance of the horse in showing evolution to be a reality. In 1891 Henry Fairfield Osborn, an independently wealthy Princeton professor and one of E.D. Cope’s friends and supporters during the embroiled Herald fiasco, was appointed the first curator of vertebrate paleontology at the American Museum of Natural History in New York City. The museum was somewhat embarrassed at not possessing any sizable collection of vertebrate fossil material, and even though Cope eventually sold some of his collection to the AMNH for a sum that disappointed the beleaguered Philadelphian, the halls of the great institution were still found wanting of ancient creatures that would bring it notoriety.

Osborn, despite his off-kilter ideas about human evolution that plagued his later years, largely made the AMNH what it is today, having some of the best and brightest collectors and preparators of the 20th century under his employ. While such gems as Barnum Brown’s two Tyrannosaurus rex skeletons, the specimen that remains on display today being Brown’s self-confessed “favorite child,” definitely helped to make the museum famous, some of Osborn’s favorite subjects were the fossil horses. Early on in his career, Osborn attempted to raise $10,000 from museum trustees for a project involving horse evolution, but the appeal was denied. Osborn kept at it and eventually succeeded, however, securing $15,000 from William C. Whitney in 1897, funds used to send collectors and curators like James W. Gidley, Bill Thomson, W.D. Matthew, and Walter Granger out into the field to collect ever more horses from Texas, South Dakota, Colorado, and other locales. Indeed, Osborn soon had many new horse fossils to study and display, creating one of the most notable (and among Biblical fundamentalists, controversial) displays of evolution ever presented to the public.

*(WWII caused the museum to send the first, more incomplete Tyrannosaurus rex skeleton, to the Carnegie Museum out of fear that the museum would be bombed and both would be lost. This may seem like an ill-founded fear, but many fossils like Spinosaurus were lost because German museums were struck with Allied payloads.)

Osborn did much to enhance the AMNH collections during the close of the 19th century, although his rather strange views about mammalian evolution (fueled in part by racism and part by Osborn’s membership in the Presbyterian church) never found wide acceptance. Despite his pet hypotheses, Osborn sent paleontologists far and wide in search of specimens to confirm his ideas, and at least in the case of the Roy Chapman Andrews expeditions during the early 1920’s, unexpected boons abounded. While Marsh held that he had moved horse ancestry out of the Old World and safely into America’s domain, Osborn saw the origin of major placental mammalian groups stemming from Asia (including the origin of humans), the hypothetical five-toed ancestor of the horse remaining elusive in North America because it was “really” buried somewhere in Asia. Osborn described his hypothesis as follows;

In the dispersal center, during the Age of Reptiles and the beginning of the Age of Mammals, there evolved the most remote ancestors of all the higher kinds of mammalian life which exist today, including, for example, the five-toed horses, which have not yet been discovered in either Europe or America. That the very earliest horses known in either Europe or America were four-toed indicates that their ancestors may have lost their fifth toe while still resident in the Asiatic homeland.

Roy Chapman Andrews did not bring Osborn any Asiatic five-toed horses from the expeditions into Mongolia in the early 1920’s, although the mammals Paraceratherium and Andrewsarchus were exciting enough in and of themselves. .

The lack of the most ancestral mammalian fossils did not stop Obsorn from attempting to further his own hypotheses, however, and in order to understand how straight-line evolution may have been presented at the AMNH we need to know how Osborn obfuscated the role of “chance” in evolution (using it almost in the same context as modern creationists do), calling the idea that natural selection works on random variations a “dogma.” Osborn instead preferred an Aristotelian “law,” quoting the philosopher in his 1917 book The Origin and Evolution of Life;

So far as law is concerned, we observe that the evolution of life forms is like that of the stars: their origin and evolution as revealed through palaeontology go to prove that Aristotle was essentially right when he said that “Nature produces those things which, being continually moved by a certain principle contained in themselves, arrive at a certain end.”

Such a notion could be regarded as the “Restless Gene” hypothesis, with what Osborn then referred to as the “hereditary-chromatin” in the animal filling needs as they arose in order to achieve a particular end. Despite his confusion about the role of “law” and “chance” in nature, Osborn did recognize that there were certain ratios in limb structures that were present in animals filling different ecological niches, even closely related ones. In the same book, Osborn writes the following about early horses;

No form of sudden change of character (saltation, mutation of de Vries) or of the chance theory of evolution accounts for such precise steps in mechanical adjustment [as in the limb structure of horses]; because for all the proportional changes, which make up ninety-five percent of mammalian evolution, we must seek a similar cause, namely, the cause of acceleration, balance or persistence, and retardation. This cause may prove to be in the nature of physiochemical interactions regulated by selection. The great importance of selection in the evolution of proportion is demonstrated by the universal law that the limb proportions of mammals are closely adjusted to provide for escape from enemies at each stage in development.

This chain of reasoning, such as it is, nearly works backwards from evolution’s “products” (which it is never done fiddling with), much like the lampoon (which I believe stems from Voltaire, although I have been unable to find the quote) that the nose was placed on the human face to hold up ones glasses.

Equus scotti
Assemblage of bones, illustrated as discovery in situ, of the Pleistocene horse Equus scotti. From Gidley, James Williams. 1900 “A new species of Pleistocene horse from the Staked Plains of Texas“. Bulletin of the AMNH ; v. 13, article 13.

Equus scotti
A mounted skeleton of Equus scotti at the AMNH, constructed out of two skeletons. From Gidley, James Williams. 1901. “Tooth characters and revision of the North American species of the genus Equus.” Bulletin of the AMNH ; v. 14, article 9.

Even though Osborn’s ideas of evolution did not catch on, the idea of horse evolution as a more-or-less straight line was still a popular one, at least in works and representations meant for public consumption. The diversity of fossil horses, thanks to many of the expeditions undertaken by Osborn’s department at the beginning of the 20th century, had considerably expanded, and the idea of an evolutionary “bush” for horses began to take root. Such a representation can be seen in one such generalized and “primitive” bush provided by J.W. Gidley in a 1907 paper on horses from the Miocene and Pliocene of North America. Indeed, the diversity of three-toed forms suggested that ancestry was perhaps more complicated than previously thought, more than one form of horse existing at any one time depending on the available habitats. Osborn noted this in his 1917 book as well, but it seemed to be only a supplementary bit of information behind his ideas of a biogenetic law. One of Osborn’s hires, J.W. Gidley, had a more accurate view of horse evolution, however, and produced the first known branching phylogeny of horses through the Miocene and Pliocene.

Old Horse Evolution Tree
A hypothesis as to the relationships of horse subfamilies by J.W. Gidley in 1907. This is the first known branching diagram for horse evolution. From Gidley, James Williams. 1907. “Revision of the Miocene and Pliocene Equidae of North America.” Bulletin of the AMNH ; v. 23, article 35.

As can be seen from Marsh’s earlier phyletic progression, much of horse evolution seemed to be dictated by features of teeth, the number of toes, and certain aspects of the skull, but as Gidley notes in his paper more material is needed if we are to truly understand the relationships of horses. Indeed, things were not so clean and neat as implied by Marsh’s illustration, even with the inclusion of new taxa. In the summary of the research, Gidley concludes;

As at present understood, the fact seems to be fairly well established that there is a considerable phyletic hiatus between the groups of the Equidae as above subdivided, which are as yet not bridged over by intermediate forms. Such a hiatus seems especially marked between the Anchitheriinae and the Protohippinae, while these groups greatly overlap each other in time. So far as indicated by any known species the Anchitheriinae could not well have stood in direct ancestral line to the latter group or to the Equiinae. There seems also to be almost as decided a gap between the Anchitheriinae and the known species of the older group, the Hyracotheriinie. The Equiinae may well have been derived from some species of the Protohippus division of the Protohippinae.

Outside of engaging in a more detailed study, Gidley also made note that various genera of horses overlapped in time with each other. While this does not rule out anagenesis entirely, it is a problem if there is such a large diversity of horses with similar features living alongside each other rather than a few isolated populations moving in a straight-line progression. The overlap was recognized and illustrated by W.D. Matthew almost 20 years after Gidley’s paper, showing where and when fossil horses existed;

Geological and Geographic Range of Equidae
Visual representation of the geological span and geographical ranges of equids through the Cenozoic. Such a representation could easily be misunderstood as endorsing straight-line evolution of horses. From Matthew, W.D. 1926. “The Evolution of the Horse: A Record and Its InterpretationThe Quarterly Review of Biology, Vol. 1, No. 2., pp. 139-185.

While the illustration, if followed closely, does show a branching pattern of evolution, to an untrained eye the evolution of horses through time seems to go in a relatively straight line, the overlap seemingly giving way to an almost orthogenic trend. I doubt that Matthew’s article was regular Sunday night reading for families of the late 1920’s and so I doubt that it contributed directly to mistaken notions of horse evolution, but another illustration from the same paper could more easily cause confusion;

Horse Evolution Simplified
A simplified, “straight-line” version of horse evolution (Click the image for a larger version). This figure was also reproduced in George McCready Price’s The Predicament of Evolution. From Matthew, W.D. 1926. “The Evolution of the Horse: A Record and Its InterpretationThe Quarterly Review of Biology, Vol. 1, No. 2., pp. 139-185.

This model is similar to Marsh’s (see above) in that horses seem to have followed a very simple ancestor/descendant progression through time. While it is true that living horses did have ancestors with multiple toes and we could trace their line backwards through time to the exclusion of other closely related genera, diagrams like this one seem to have “won out” in the public mind over those that more fully encompassed horse diversity. A 1940 paper by R.A. Stirton would be much clearer when it came to the branching horse lineage;

Stirton Horse Phylogeny
From Stirton, R. A. 1940. Phylogeny of North American Equidae. Bull. Dept. Geol. Sci., Univ. California 25(4): 165-198.

Stirton’s illustration is interesting as it shows a fairly straightforward line of descent through Miohippus is the Upper Oligocene, Miohippus giving rise to some side branches that would eventually go extinct before modern times. The radiation of the ancestors and close relatives of modern horses did not start, according to the phylogeny, until the Upper Miocene and Merychippus, Pliohippus eventually giving rise to Equus in the Upper Pliocene. Further, it is interesting to see how close Stirton’s phylogeny is to the work of later researchers, especially that fossil horse authority Bruce McFadden;

McFadden Horse Phylogeny
From MacFadden, Bruce. 1985. “Patterns of Phylogeny and Rates of Evolution in Fossil Horses: Hipparions from the Miocene and Pliocene of North AmericaPaleobiology, Vol. 11, No. 3. (Summer, 1985), pp. 245-257.

The phylogeny is extremely similar to Stirton’s through Parahippus, but the upper branches are a bit more detailed. Instead of having the genus Equus be a descendant of Pliohippus, Pliohippus is relegated to an offshoot that goes extinct before the Pliocene, the genus Dinohippus giving rise to Equus and the recent horses of the New and Old World in that genera. We will come back to the work of MacFadden later, but it is important to note how close the ideas of researchers in decades past were with modern understanding in this area.

Quinn Horse Phylogeny
From Quinn, J. H. 1955. Miocene Equidae of the Texas Gulf Coastal Plain. Bur. Econ. Geol., Univ. Texas Pub. 5516: 102 pp. (Click for larger image)

J.H. Quinn’s 1955 phylogeny of the horses of the Texas Gulf Coastal Plain was even more wildly branching than Stirton’s, and while Quinn’s focus was a bit more narrow, the tree is much more divergent. Other researchers had the genus Equus arising in the late Pliocene (and even as late as the Pleistocene), Quinn’s version has Equus appearing as early as the middle Miocene, Merychippus, again being nominated as the progenitor of all the subsequent forms in the area. While this version of horse evolution has been extensively reshuffled and revised, it is important to note that the idea that horses evolved in a straight line was well out of fashion by the middle of the 20th century at the very latest. Why, then, did it hang on for so long in the public mind?

Mercyhippus
A mount of Mercyhippus isonesus quintus. From Simpson, George Gaylord. 1932. “Mounted skeletons of Eohippus, Merychippus, and Hesperosiren.” American Museum novitates ; no. 587

Part of the problem with museums is that it takes a lot of time, money, and effort to revise exhibitions, and for some time the American Museum of Natural History horse display (THE display that illustrated horse evolution for many years) followed a progression like that of W.D. Matthew’s simplified diagram (see above). While this was eventually changed when the fossil halls were refurbished, it still seemed to show a straight line of descent, and even the display that stands on the fourth floor of the museum today reflects such a transition. If you read the plaques and take the time to compare the skeletons the branching nature of horse evolution is apparent, but the fossils themselves are arranged from Eohippus to Equus in a two parallel straight lines, showing an overall smaller-to-larger and many-to-one toe progression. Likewise, popular books on evolution and paleontology seemed hard-pressed to let go of straight-line evidence. While it could be said that such books were correct in that we could follow the line of descent from modern horses backwards to the exclusion of other groups, this approach seems to do more harm than good in the long run. Take A.S. Romer’s Man and the Vertebrates: Vol. I, for example. Originally published in 1933, my 1954 Pelican Books paperback edition shows the fossil limbs of Eohippus, Miohippus, Merychippus, and Equus from left to right, once again giving the illusion of a pure line of anagenesis. No hint of a larger diversity is given outside a brief mention of the modern forms of Zebra, Ass, and Prezwalski’s Horse.

Eohippus to Equus
Comparison of Eohippus to Equus. There’s a lot of evolution in that dashed line. From “The Dawn Horse or Eohippus” by Chester Stock (1947).

The 1966 edition of Romer’s Vertebrate Paleontology fairs better overall, but is still found wanting. The same straight-line illustration I just mentioned is found in the section treating perissodactyls as a group, and the skeletons of Eohippus, Mesohippus, and Hippidion are shown left to right across pages 266 and 267. While the text does mention an overall diversity of forms, as well as using certain genera for the “type” from which modern horses evolved, the overall visual impression of simple anagenesis remains. Again, I doubt the casual reader picked up Romer’s book for light nightly reading, but it is strange that the progressive ideas about evolution during that time are so poorly represented.

A similar time-capsule is Edwin Colbert’s Evolution of the Vertebrates, originally printed in 1955. The 1966 edition is the one that I acquired, and it is an interesting contrast to Romer’s book. At first Colbert seems to fall into the same trappings of straight-line evolution, showing a simple progression (in text with arrows) from Hyracotherium (Eohippus) -> Orohippus ->Epihippus -> Mesohippus -> Miohippus, spanning the Lower Eocene to the Upper Oligocene. After this progression, however, Colbert does note that there was a proliferation in forms;

By the end of the Oligocene epoch the horses had through these changes attained the status of advanced browsers, capable of eating leaves and soft plants and able to run fairly rapidly and for sustained periods over hard ground. With the advent of Miocene times there was a branching out of horses along several lines of development, probably as a response to an increase in the variety of environments available to them, and especially because of the spread of early grasses and other flowering plants.

An illustration on page 364 makes something of an attempt to reflect this visually, following the phylogeny of R.A. Stirton (see above) but in a more subdued and compressed manner. Being that only the genus names are mentioned, Colbert’s tree looks especially bare, although it must be conceded that it is a more accurate depiction of horse evolution than Romer’s. The illustration on page 148 of the 1961 paperback edition of G.G. Simpson’s Horses more closely follows Stirton’s phylogeny as well, and the plates likewise show the branching of tooth shapes and other characters rather than grouping forms separated by large expanses of time. The relatively rich fossil record of horses would be important to Simpson in another way as well; in his Neo-Darwinian Synthesis-era work, Tempo and Mode in Evolution (1944), Simpson was able to conclude that horses in general seemed to evolve faster than unrelated groups of animals like ammonites but more slowly than mammals like elephants. Although his hypothesis of a near-constant, albeit accelerated, rate for horse evolution has not held up today, the idea that evolution can occur more quickly or more slowly was a very important idea, an idea that took new form in Eldredge & Gould’s hypothesis of punctuated equilibria decades later.

McFadden Horse Phylogeny
From McFadden, Bruce. 2005. “Fossil Horses – Evidence of Evolution.” Science Vol. 307. no. 5716, pp. 1728 – 1730

So what of our current understanding of horse evolution? As I had mentioned earlier, one of the foremost authorities on the topic is Bruce MacFadden, and in 2005 he authored a straightforward summation of the current state of things in an article entitled “Fossil Horses – Evidence for Evolution.” As MacFadden notes, the overall “look” for the tree, featuring lines that did not leave modern descendants, hasn’t changed much since the time of Stirton and other earlier scientists. There has been much shuffling around and plenty of new discoveries, however, and although the diversity of late horses often gets the most attention it is now being revealed that early members of the horse lineage had a wider diversity as well. It almost seems like there’s an evolutionary bottleneck during the Oligocene, with the beginnings of more diversity in the Miocene, Mercyhippus once again leading the charge on to later forms.

MacFadden also takes a moment to correct a common misconception about horse evolution; there was no unalterable progression from small to large consonant with Cope’s Rule;

Although the 55-My old fossil horse sequence has been used as a classic example of Cope’s rule, this notion is now known to be incorrect. Rather than a linear progression toward larger body size, fossil horse macroevolution is characterized by two distinctly different phases. From 55 to 20 Ma, primitive horses had estimated body sizes between ~10 and 50 kg. In contrast, from 20 Ma until the present, fossil horses were more diverse in their body sizes. Some clades became larger (like those that gave rise to Equus), others remained relatively static in body size, and others became smaller over time.

Still, our current understanding is incomplete, and further fossil finds will continue to rustle the branches of the evolutionary bush. In fact, I would not be surprised if more early forms came to light, and I would be especially interested to see if the “Oligocene Bottleneck” is real or merely a factor of fossil collecting bias. There should be no mistake about the amazing entanglement of branches horses represent, however, and it is somewhat surprising that the public does not often hear about the true form of horse phylogeny. While I did not do an in-depth study of how horse evolution was portrayed in the popular media, from what I have seen it seems that past scientists and authors have often opted for simplicity, getting the public to accept evolution has occurred being more important than giving them an accurate depiction of how evolution works. This is a harsh lesson that we are still learning, as inaccuracies in books, museum displays, and other outlets can leave the door open for creationists to spread distrust of science. It is not enough to merely present someone who is unfamiliar with evolution with our “best” example of anagenesis; if we do, it should be in context with the larger theme of unity and diversity of forms, not a throw-away that is supposed to dazzle in and of itself. The evolution of the horse, in fact, is a perfect example of evolution and can be an extremely powerful tool in education if used properly, but for whatever reason the common theme so far has been for many popular science writers and educators to fall out of the saddle.

Evolution is truly an amazing phenomenon; who would have ever conceived of a small, four-toed animal giving rise to Black Beauty? Our overall conception of “more” being better may even make such a move from four toes to one seem counterintuitive, yet the evidence (from fossils to that of development) is clear in its implications. Horses did not spring up out of the ground from the dust, nor were they “spoken into being” by an omnipotent power that decided that Adam should have a faithful steed. Every bone in their body cries out as to their past, and we are all the more enriched to understand that just like the horse, Homo sapiens is a still-changing product of a long and rich evolutionary history, too.





Mega-Post Preview

11 09 2007

I’ve been working on a brand new historical/evolutionary review post all day, and I’m sad to say it’s not yet completed. I was expecting a finished product by now, so as a substitute, here’s the first few paragraphs of the post that should give you some clues as to what I’ll be covering. I haven’t gone back to fully edit yet so please excuse any errors. I hope to have the whole thing finished tomorrow (it’s currently a little more than halfway done);

Eohippus
One of Charles R. Knight’s interpretations of Eohippus

When the name of O.C. Marsh is invoked, it is often to tell of his participation in the great “Bone Wars” of the late 19th century, sparring with fellow osteophile E.D. Cope in the pages of the New York Herald. Twisted tales of deceit and sabotage were promulgated in the sensationalist paper, and while both men helped to bring about an American revolution in vertebrate paleontology, the scars of their bitter squabbling have yet to fully heal. Such scientific in-fighting might seem worthy only of a historical footnote or an introduction to the stereotyped image of “smash-and-grab” paleontology of the time which is almost romantically referred to, but the truth of the matter goes far deeper than the public beard-pulling that is so often remembered.

The tiff between Cope and Marsh is strange in that is seems to exist in the popular literature out of time, removed from the context in which it had originally existed. Charles Darwin had published his earth-shaking work On the Origin of Species by Natural Selection a scant 31 years before the ink almost ran red with rage on the pages of the Herald, the question of evolution being of far more importance in the public consciousness than dinosaurs. The full establishment of the dinosaur as a cultural (and dare I say, mythical) creature in the mind of the American public only seemed to take place after the Bone Wars, the appointment of Henry Fairfield Osborn to the American Museum of Natural History (specifically hired to establish a vertebrate paleontology program) and the popular reports of the dinosaur that carried the namesake of Andrew Carnegie, Diplodocus carnegei, being the more immediate beginnings of the public’s love affair with the extinct creatures. Before Brontosaurus and Tyrannosaurus became household names, the public eye was focused upon horses and birds.

The latter half of the 19th century was a stirring time for biological science, especially involving the new areas of vertebrate paleontology and evolution, the august authorities in England keeping on eye on the up-and-comers starting their own careers in the states. Early on, paleontologist E.D. Cope impressed T.H. Huxley with his 1866 discovery of Laelaps aquilunguis, but in a paleontological clean-sweep Marsh would eventually have his name attached to Cope’s dinosaur and the admiration of not only T.H. Huxley, but Charles Darwin himself. As for the renaming of Laelaps, Marsh found that the name was already taken by a genus of mite, renaming the New Jersey greensand dinosaur Dryptosaurus in 1877 (although Cope, throughout the rest of his career, called the dinosaur Laelaps). It would take more than some taxonomic shuffling to impress the eminent British anatomists and paleontologists, however, and Marsh’s ticket into Huxley’s good graces came in the form of toothed Cretaceous birds like Hesperornis (Marsh, 1872).





Photos from the Philadelphia Zoo, Pt. III

10 09 2007

Here’s the last set of the photos from this past weekend, and as soon as I get the entire set uploaded to Flickr I’ll let you all know.

Gorilla
One of the gorillas at the Primate House. I only saw two while I was at the zoo, so I assume that most of the rest of them were inside or in an area out of view.

Grevy's Zebra
A Grevy’s Zebra (Equus grevyi).

Prairie Dog
A Prairie Dog (Cynomys sp.) having a snack.

African Elephant
The only visible “enrichment” the African Elephants had was a naked metal chain, and the elephant that I was told was named “Petunia” couldn’t stop fiddling with it. I have to wonder if some of this was a way to comfort herself or divert unease, as just before this picture she was driven out of the shade by the two other elephants she shared her habitat with, and overall she seemed to be restless/ill at ease the entire day.

Puzzles
This is “Puzzles,” the Reticulated Giraffe (Giraffa camelopardalis reticulata). I don’t know why the zoo staff has not operated on this animal, and the plaque outside the enclosure is a bit shore on details. Perhaps the condition is inoperable, and it doesn’t seem to visibly inhibit the giraffe, but I do feel sympathy for Puzzles.

Pygmy Marmoset
The most deadly of all creatures, the Pygmy Marmost [Callithrix (Cebuella) pygmaea]. Don’t let their cute appearance fool you; they can deskeletonize a cow in seconds… or is that piranhas…

Sable Antelope
A Sable Antelope (Hippotragus niger) rests in the shade. It’s hard to tell from this angle, but this one has asymmetrical horns.

Squirrel Monkey
A Squirrel Monkey (Saimiri sp.) enjoying some fruit.

Tree Shrew
A Tree Shrew (Tupaia sp.), sitting still long enough for me to squeeze off a shot (albeit a blurry one).

Galapagos Tortoise
One of the Galapagos Tortoise (Geochelone nigra) taking a dip.

Aardvark
A pair of Aardvark (Orycteropus afer) having a snooze in an artificial cave.

Gibbon
“Solstice,” a female White-Handed or Lar Gibbon (Hylobates lar). She shares her habitat with her partner Mercury (who is black rather than blonde) and an Orangutan (Pongo sp.) pair.

Gibbons
Solstice makes her intentions clear; she wants to be groomed by Mercury.

Gibbon
Mercury seemed more interested in grooming his male orang friend than his partner Solstice.

Orang
The male Orang was very shy, and carried a cardboard box on his back like a shell/shade everywhere he went.

Orang

And that’s it. Hopefully I’ll soon upload all my photos (there are thousands of them) onto Flickr soon, but I hope that you’ve enjoyed the ones I’ve put up here.








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