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





Not your average bovid; the Saiga

13 08 2007

Gazelle
A Mhorr Gazelle, a subspecies of Dama Gazelle, on a cold February morning at the Philadelphia Zoo

Gerenuk
A male Gerenuk, taken in the summer of 2006 at Disney’s Animal Kingdom Park

When I think about antelope, something like a Thompson’s Gazelle, Gerenuk, or Dama Gazelle most immediately comes to mind, countless nature documentaries featuring the bovids in such numbers that they are almost easy to ignore as scenery (or as merely prey for the big cats and other carnivores). There is at least one antelope, however, that would make anyone do a double take; the Saiga (Saiga tatarica).

Saiga
A Saiga, Saiga tatarica, from Wikipedia.

By all accounts the Saiga is quite distinct from its cousins on the African plains. Preferring steppe, semi-desert, and desert habitats, the Saiga is known from Eastern Europe through Mongolia, their conspicuous nose warming air coming into the body in the winter and filtering out dust during the summer. While this aspect of the Saiga’s adaptation is certainly fascinating, it is the horns of the animals that have brought it the most attention, however, as well as the most trouble. As I noted in a post about antlers, horns, and sexual selection, research has shown that males and females of bovid species (which includes antelope) that use their horns as defense against predators both have horns, the horns prevented from getting too gaudy in males because they need to retain their defensive function. In the case of the Saiga, only the males carry horns, and this allows for the horns to become relatively large, and this has greatly contributed to the decline of the species.

Saiga
A male Saiga, from Sokolov, Mammalian Species, No. 38, Saiga tatarica (May 2, 1974), pp. 1-4

One of the primary problems that conservation officials face in Africa and Asia today is the poaching of animals for certain parts of their anatomy for use in Traditional Chinese Medicine (TCM), or other ritual aspects of a particular culture. To understand why the Saiga is targeted for its horns, however, we need to first look at the problems with rhinoceros poaching. Rhinoceros horns have long been treasured by various cultures, young men in Yemen traditionally carry a dagger for defense called a jambia, and there is no better material for the handle of this weapon than rhino horn. Initially, the amount of rhino horn taken was mitigated by the poverty in the region, only the most affluent families being able to afford a jambia with a handle made of genuine rhino horn. As oil prices went up, though, Yemen (among other countries) was flooded with income from the sale of fossil fuels, and the demand for rhino-horn-handled jambias skyrocketed.

Jambias are not the only reason rhinos have been slaughtered. In addition to various ornamental products, rhino horn is valuable in TCM as a fever-reducer. When actually tested it seemed to reduce fever in rats, and Saiga horn had about the same efficacy, although author Richard Ellis rightly notes in his book No Turning Back that “Asprin and ibuprofen, for which no animals have to die, would probably work just as well.” In India rhino horn was used as an aphrodisiac, but expensive prices caused practitioners to stop prescribing it. These pressures, along with others, ultimately drove the rhino population down so low that horns were rare even within illegal trade, and demand kept going up. A substitute had to be found.

The horns of the Saiga, the species only recently recovered from population declines in the early to mid-20th century, were actively endorsed by the World Wildlife Fund as a substitute for rhino horn (also see here and here. Strangely, while the WWF is one of the groups responsible for the crash of Saiga populations, they make no mention of their actions on their website, their efforts of helping the Saiga since 1994 being extolled rather than their responsibility for the near-eradication of the animals. Nevertheless, the political destabilization of Russia and other areas in which the Saiga lived made regulation near impossible, and while some researchers claim that the plains were once “blackened” with Saiga, there are probably less than 50,000 of the animals left throughout their entire range (the Mongolian Saiga, a subspecies, being reported as having a population of less than 1,300 in a 1999 paper by Lushchekina et al. in the journal Oryx).

Indeed, the tale of the Saiga is one of the greatest tales of ecological mismanagement in history, and its implications can still be felt today. Ellis, again in No Turning Back, tells us how it is unlikely that Saiga were as well-established by the time the WWF endorsed hunting Saiga as some conversationalists claim;

By the time of the Soviet Revolution, there were only a few thousand saigas left. To forestall their total eradication, the Soviets protected them in Europe in 1919 and in Soviet Central Asia in 1924. In the 1950s commercial harvests of saigas by local groups began.

Strangely, Saiga was not especially well-established in TCM until recently. Earlier texts like the 1597 text Chinese Materia Medica having no mention of the Saiga, but the 1989 Rare Chinese Materia Medica, and the ground-up horn of the male Saiga can be used to;

…check hyperactivity of the liver and relieve convulsion, treat the up-stirred liver wind, infantile convulsion and epilepsy; calm the liver and suppress hyperactivity of the liver-yang; it is efficacious in the treatment of dazzle and vertigo due to hyperactivity of the liver-yang; it improves acuity of vision, cures headache and conjuctival congestion; clears away heat and toxic material; and can be used to treat unconsciousness, delirium, and mania in the course of epidemic febrile disease.

So it does seem that the Saiga has been a more recent alternative to other traditional remedies, gaining the status of an “ancient treatment” only towards the end of the last century (although I am no expert on TCM and there could be earlier references than the ones mentioned by Ellis). As you probably have guessed by now, though, the major problem in Saiga conservation is that it is the males who are constantly targeted, much of remaining populations being females. This greatly reduces the amount of offspring that are likely to be produced, as well as marking a steep drop-off in genetic diversity and possibly even fitness in the species, the Saiga going through two near-extinctions in less than 100 years.

Saiga
Male Saiga skull. Note the large shelf in front of the large nasal opening. From Sokolov, Mammalian Species, No. 38, Saiga tatarica (May 2, 1974), pp. 1-4

While the earlier near-extinction of the Saiga typically gets less attention than the more recent run on their horns, I feel that it echoes recent policies in the United States to remove wolves and other animals from protection; when stocks seem to approach levels that are barely adequate, many want to open up hunting again when the animals should actually be left alone to reestablish themselves and their genetic diversity. While some are optimistic about the recovery of Saiga, I do not share the same hope that they do. Even if raw numbers of Saiga continue to rise, I worry that the decreased genetic diversity will make them more susceptible genetic problems caused by inbreeding and disease epidemics, one “bad day” in an ecological sense being able to wipe the species out.

I would be remiss, however, if I attributed the problems of Saiga conservation entirely to hunting. As I just suggested, disease and parasites can be an important factor in terms of whether populations die off or not, something that can not be planned for by merely making sure there is more of the animals next season. In 1992, Dukes, et al. published a study that showed that paratuberculosis (or Johne’s Disease) was transmittable from domestic sheep to captive Saiga and back again. Even more recently (Morgan et al, 2005) it has been found that Saiga pick up many of the parasitic worms that also infect grazing livestock along their range, both suffering from the parasites and also allowing them to spread further. Global climate change may also adversely affect the Saiga, the Saiga depending on local climatic cues to dictate their migrations. Heavier snowfall/harsher drought may cause them to have to move to new habitats or be decimated, and at present it is unknown whether they’d be able to cope with ecological changes associated with the current warming trend.

In all, things are not looking too good for the Saiga. While there has been a ban on the trade of products made from them and conservation agencies are trying hard to preserve this species, I believe that it not exists in a weakened state, which (despite population size) will make it more susceptible to extinction. Given enough time the species may recover fully, but in my own view this has “heath hen” written all over it, a species that seemed to recover until disease, predation, and weather changes were too much for the remaining birds in New England to handle. I am not suggesting that we say the Saiga is a lost cause; far from it. What I am recommending is that we actually learn something from the terribly mistakes we’ve made with this species, and stop thinking that merely because populations increase it somehow equals immediate species stability.





Taking in the Carboniferous Atmosphere

3 08 2007

In many a time-travel story, little attention is paid to the local ecology of the destination of the intrepid travelers. If you’re headed back to visit Leonardo da Vinci, this probably isn’t going to be a problem, but if you’re headed back much further, like the Carboniferous or Permian, you might run into some trouble. Indeed, nothing could kill such a “time safari” quicker (like the one in L. Sprauge de Camp’s “Crocamander Quest,” collected in The Ultimate Dinosaur) than stepping out of the time portal/capsule and not being able to breathe. Fanciful notions of time travel aside, we can tell something about the ancient atmosphere without having to actually step foot there, and it has some very interesting implications for evolution and ecology.

One of the defining features of the Carboniferous is that things got big. Even though they may have seemed small compared to titanosaurs or mammoths, a 3-foot-long millipede and giant dragonflies are nothing to belittle. Indeed, terrestrial arthropods in all sorts of groups seemed to achieve sizes unheard of today. Why should the Carboniferous be dominated by giant arthropods?

Well, let’s think about the name of the time period we’re talking about: Carboniferous. Carboniferous deposits, sandwiched between the Devonian and Permian, are well-known for containing coal. The coal was originally laid down by the thick plant growth that marked changes in terrestrial ecology, and although the first plants (likely resembling liverworts) colonized the land as early as the Ordovician, the Carboniferous marked the development of lignified bark (“brown coal” is called lignite), which may have led to the huge amounts of coal deposited as bacteria needed some time to evolve a way to break down this new material. Due to Devonian drops in sea level, plants were also had more space to grow, so there was a greater number of swamps and forests produced, filling up with plant and insect life. The changes in global fauna had a major impact on the atmosphere, not only producing more oxygen due to photosynthesis, but the burial of so much biologically produced carbon also increased the amount of oxygen in the atmosphere, perhaps being as high as 35%! (It’s at about 21% today)

Another important piece of the puzzle is that nitrogren levels did not drop, and so not only was there hyperoxic conditions due to plant evolution, but the air pressure was increased as well, and these two factors probably had a lot to do with the evolution of insects flight and insect gigantism. When there is more oxygen in the air, organisms that diffuse oxygen over their trachea are able to obtain more oxygen with less effort, and it doesn’t take as much energy to become large being oxygen is so “cheap.” Likewise, increased air pressure makes flying less costly in terms of energy, as wing size has to go up with decreasing air pressure in order to keep flying animals aloft (as is that case with hummingbirds that live at high altitudes). Not only was it easier to grow large, but it didn’t take as much energy to fly, resulting in the giant dragonflies like Meganeura. In a 1998 paper, Dudley hypothesized that early tetrapods would have taken advantage of the extra oxygen as well, being that they would have “breathed” through their skin. This hypothesis is largely refuted, however, (see the comment of johannes below) and size of early tetrapods does not seem as dependent on the atmospheric oxygen make up, although it likely could have made oxygen intake for amphibious tetrapods more efficient overall.

There are costs to be adapted to an atmosphere with high oxygen content and high atmospheric pressure, however. Part of the cost is the buildup of metabolic “wastes” as a result of taking in so much oxygen, and unless there’s also an increase in enzymes to take care of these wastes, organisms may have led shorter lives. Experiments in the lab with fruit flies, however, have shown that an increase in enzyme production to combat the harmful metabolic products can be selected, and it’s reasonable to think that Carboniferous organisms underwent such selection naturally. Still, atmospheric oxygen levels began to sharply drop at the beginning of the Permian, and by the beginning of the Triassic atmospheric oxygen levels were as low as 15%. Creatures adapted to higher oxygen content and pressure would need to be adapted quickly again if they were to survive, and size/ability to deal with lower oxygen levels probably made a big difference in the great Permian extinction.

Oxygen levels were not always to remain low, however, and it has been hypothesized that hyperoxic conditions (which means more oxygen in the air plus heightened pressure) might be correlated with the evolution of flight, or at least evolution of larger flying organisms. When it’s less expensive to obtain oxygen and less expensive to fly, it would be much easier to develop flight, and there may be correlations with such atmospheric conditions and the evolution of bats, birds, and pterosaurs. Testing still needs to be done (and the more fossils we can get, the better), but if nothing else, the fluctuations in atmospheric oxygen content show us the importance of considering ecology in terms of evolution and extinction.

Resources:

Dudley. “ATMOSPHERIC OXYGEN, GIANT PALEOZOIC INSECTS AND THE EVOLUTION OF
AERIAL LOCOMOTOR PERFORMANCE.” The Journal of Experimental Biology 201, 1043–1050 (1998)





Photos from the AMNH

21 07 2007

Today my wife and I headed into New York and had an overall more enjoyable day at the AMNH than our last visit in February. We mostly chalked up the lower attendance to people being at home reading the new Harry Potter. Before continuing though, I have to say that I probably will not being visting the museum as much as I have in past years; prices for everything involved a trip are going up, so much so that it has become probhibitively expensive. $40 for train tickets, $55 for museum admission (museum + special exhibit + 2 IMAX films), a few extra bucks for subway fare, and a few dollars at Penn Station for a meal (we didn’t have enough foodstuffs at home to make a sandwhich), and overall I spent over $100 for the two of us to go to the museum. Granted, we could have skipped out on the special collection, the IMAX films, and paid $10 to get in (being that the admission prices are only “suggested”), but as much as I love the museum I don’t love it enough to spend over $100 on a day trip there every other month or so.

Enough with the rant, on to the pictures!

Pigeon

Sitting over the main entrance doors at the front of the museum was a group of pigeons, this one apparently taking pride in standing on an eagle’s head. I just found it funny to see what was probably intended to be a finely sculpted example of one of our national symbols being roosted and crapped upon by an even more “American” bird, the “Super Pigeon.”

Barosaurus

Allosaurus

Upon entering the Grand Rotunda one can’t miss a rearing Barosaurus protecting its baby from an Allosaurus, although I’m still pretty doubtful as to whether such a scene ever actually occurred (it’s as if the sauropod is saying “Here’s my underbelly, why don’t you slash it?” to the carnosaur). It’s still impressive, nonetheless.

Allosaurus

While the Allosaurus threatening the Barosaurus has the proper posture, however, not all the museum’s depictions of the dinosaur have been renovated. This model, on a dinosaur phylogenetic tree, makes me think “Halp! Sum1 haz broken my tail! I can has update?”

Orang

My wife and I did get to actually enjoy the recently renovated Human Origins hall this time however, and while at first I thought the super-bright flash of my camera ruined the picture, I thought the above photo of an orangutan skeleton at the exhibit’s entrance was pretty neat all the same.

Proconsul

The flash had a similar effect on the above skull of the ancient primate Proconsul.

Skulls

Unfortunately the lighting of the exhibit was not condusive to photography, but that’s me with some skulls of ancient “family.”

Lucy

And that’s my wife with a replica of “Lucy.”

Reindeer

I was also very impressed by this ancient piece of artwork, originally drawn on an antler (here shown unrolled) 16-13 thousand years ago, found in Lortet, France.

Potto

While not in the Human Origins hall, this skeleton of a loris also proved to be an interesting subject (although the focus was on the shoulder, not the skull like I wanted).

Me

Moving on to the Hall of Ornithiscian Dinosaurs, my wife snapped this one of me in front of the famous Sternberg hadrosaur “mummy.”

Tylosaurus

And here’s another next to one of Sternberg’s other famous finds, a Tylosaurus.

Stego

My wife wanted one of her in front of a Stegosaurus (armored dinosaurs appeal to her much more than others).

Deinonychus

It’s a little blurry, but this head-on view of Deinonychus came out alright as well.

Croc

I also came across a fossil I had never seen before; a well-preserved skeleton of Protosuchus from the Late Triassic of Arizona.

Thylacosmilus

I also made sure to check in on some of my favorite mammal fossils. The above skull is of the marsupial saber-toothed predator Thylacosmilus.

Leo

I must have not been paying attention during my earlier visits, as this time I also came across the skull of the marsupial “lion” Thylacoleo.

Saber

I also came across the above skull in the Human Origins hall, although I cannot recall to what genus this cat (it was a true saber-tooth cat) belonged. [Update: After a little bit of comparison, I believe that this is the skull of Megantereon, although I could still be wrong.]

Sloth

I was also impressed by the claws that the giant sloth Megalonyx wheatleyi bore on its forearms.

Charlotte

And what would a picture post be without a shot of my cats doing something cute? Here’s Charlotte, cured up to Song of the Dodo. Where her interest in biogeography came from, I don’t know.

As for the Dinosaurs Alive IMAX film and the Mythical Creatures Exhibit, they were both “ok” but not especially thrilling. What was most curious about the IMAX film is that it has footage of the grad students who just published the Science paper working in New Mexico, with a few shots of their new fossil. Kevin Padian can clearly be seen in one shot, although for some reason he’s never introduced in the film.

The Mythical Creatures exhibit had some interesting artifacts, but there was no photography allowed (the security guards were very vehement about this) and there was a museum tour guide with the most nasal, grating voice I’ve heard in a while explaining different points of the exhibits. Barring these unfortunate circumstances, it will probably appeal to those who are unfamiliar with some of the scientific background of popular myths, but for those already familiar with dragons, mermaids, and the Kraken, it’s not especially exciting.

Overall, though, it was a good day at the museum. I wish that I could visit on a really slow day, even have the museum to myself for a day (or night) to peruse and study at my leisure, but I don’t think that’s ever going to happen. Still, every time I go back I notice something new or can apply some new bit of knowledge I’ve acquired to the fossils I see, and it’ll always be one of my most favorite places.





If the dinosaurs could wait, I guess I can too…

19 07 2007

Tomorrow a new paper in Science well help to explain the pattern of the origin and radiation of dinosaurs, revealing a much more gradual process than has previously been assumed. Although I’ll save most of my comments until after I get to read the paper (while everyone else is lining up to get their copies of Harry Potter), it could do a lot of good in helping to dispel myths about evolution. The “classic” dinosaur story is that their ancestors were adapted to have their limbs under their bodies, allowing for a much more efficient gait that also freed their hands and allowed them more speed. This adaptation allowed them to quickly overtake all other amniotes of the time, dominating the world for well over a hundred million years, other organisms only becoming large or becoming more specialized after dinosaurs disappeared. Darren at Tetrapod Zoology has done a lot to help dispel the idea that dinosaurs were the be all and end all of Mesozoic critters, and the recently (re)discovered Effigia okeeffeae has shown that dinosaurs were not the only group to develop an advanced bipedal posture.

The LiveScience article announcing the paper doesn’t provide much detail other than the potential shake-up involved for thoughts on evolutionary turnover in the Triassic (as well as the announcement of a “dinosauromorph” named Dromomeron romeri, after the famous paleontologist A.S. Romer). I’m sure more capable bloggers will be able to give some more in-depth insights than I can, but the fact that dinosauromorphs persisted for so long, even coexisting with dinosaurs, requires a bit of re-thinking when it comes to how dinosaurs evolved and became the dominant large animals on the earth. I certainly look forward to learning more tomorrow.





Extinction is apparently all the rage…

13 07 2007

One of the topics that drew me in to the study of evolution was, oddly enough, extinction. While this might seem to be counter-intuitive, evolution being the origins of species and extinction being their end, if we cannot understand or recognize extinction we’re going to have a pretty poor idea of evolution (in fact, the establishment of extinction of fact was one of the key antecedents to the establishment of evolution as fact by Charles Darwin). Despite the important of understanding how extinction happens, it’s a matter that doesn’t seem to get as much attention as it probably should, and I’ve decided to summarize a few recent papers/articles on the subject here for anyone interested in the “life and death” of species.

[Note: By “‘life and death’ of species” I do not mean the antiquated and incorrect belief that species go through stages of birth, growth, and death just like an individual organism does, owing it’s premise to a vitalistic approach to evolution. If you want to read a good refutation of such ideas, see G.G. Simpson’s The Meaning of Evolution.]

First up, a recent Nature article by Nick Lane dubbed “Reading the Book of Death,” that focuses on the Permian extinction event. While the late-Cretaceous extinction that wiped out the non-avian dinosaurs is more widely known (thanks in part to the controversy about an impact at the K/T boundary during the 1980’s and 1990’s), the end-Permian extinction was much more severe, and Michael Benton aptly called his book on the subject When Life Nearly Died (although Peter Ward is perhaps a better known “spokesman” for the Permian, especially because of books like Rivers in Time and Gorgon). . While I have yet to read Benton’s book, I am familiar with Ward’s work and the general global-warming hypothesis that was brought to life in the BBC documentary Walking With Monsters (and, in fact, Ward has a new book out that I assume attempts to tie the Permian to the present in terms of climate change, Under a Green Sky); due to volcanism or other changes, massive amounts of carbon dioxide were dumped into the atmosphere, the temperature rising by about 43 degrees Fahrenheit. Such changes, if they occurred quickly, would have dire implications for life on earth.

There is certainly more to the story, however; volcanoes don’t simply spew out CO2 and little else, and the sheer amount of volcanic activity would have had potentially larger implications than a rise in temperature. While the volcanic activity in the Siberian Traps at the end of the Permian is well known, just exactly what effects they had is still being debated. We know that there was massive amounts of volcanic activity, we know the temperature shot up, we know that a large amount of volcanism can put chemicals into the air that damage the ozone layer (hence letting more UV radiation in), that oxygen levels likely fell at a gradual rate between the Carboniferous and the end-Permian (from 30-13% oxygen in the atmosphere, the article states), and that there were carbon spikes (probably from released methane) prior to the Permian extinctions. How do all these factors come together to explain which groups survived and which did not?

Lane sums up the situation at the end Permian as a hellish nightmare of a world gone awry, the pressures on organisms at that time being so great that many would not survive;

So with dangerously low atmospheric oxygen levels, ecosystems were compressed and fragmented. The deep oceans were largely uninhabitable. Land plants were dying back in the arid greenhouse climate, making food hard to come by. And then came the hammer blows of fate, the greatest volcanic outpourings in the history of our planet, releasing vast quantities of methane and carbon dioxide, raising global temperatures by 6 °C, and lifting the Strangelove conditions to the very surface.

Head for the hills and there’s no oxygen; stay on the shores and you risk breathing hydrogen sulphide. High carbon dioxide levels sabotage your respiratory pigments and choke you from within. Even if death doesn’t get you right away, you’re unlikely to have much spare energy for sex. Population sizes fall; so do body sizes. Even for those that survive the immediate toxicity, slow extinction was likely over a few generations — the blink of an eye in geological terms.

While the importance of each of these factors may vary globally or locally, it is important to realize that extinction events are ecological catastrophes; if we try to find just one smoking gun we’ll likely be mistaken. In fact, extinction is more life a firing squad than a single bullet that knocks the whole system off-kilter. Likewise, Lane also correctly notes that the creatures that survived largely did not do so because they were able to react more quickly to a changing world. Instead, they were already pre-adapted, the precursors of dinosaurs and mammals (and even later lines that would go extinct in the Triassic) having unique systems of air-sacs, palates, or turbinate nasal bones (see the opening of Schmidt-Neilsen’s How Animals Work) that may have helped them cope with an arid world in which water was the most precious commodity of all. While the mystery of the end-Permian catastrophe is still far-from-solved, the more integrated approach of scientists working on the problem may give us a more accurate look at why some groups died while some continued to great success in later periods.

Extinction is not something that has only occurred in the past, however; it is still occurring today. In a new PLoS Biology paper “Projected Impacts of Climate and Land-Use Change on the Global Diversity of Birds,” Jetz, Wilcove, and Dobson determine that global climate change in high latitudes and destruction (and restriction) of habitat in tropical regions will probably significantly reduce a number of bird species. Part of the modern extinction problem, as outline in David Raup’s book Extinction: Bad Genes or Bad Luck?, narrowing the range of a species can set them up for extinction, even when they seem to be doing well or even recovering. The classic case study is of heath hens in New England, where despite the fact that the population was recovering, the species died out in the blink of an eye because enough adverse factors accumulated to wipe all the birds out almost overnight (see an earlier post here).

Indeed, small geographic range or isolation are strongly correlated with extinction, both past and present. Even amongst animals that are not highly specialized, the inability to spread beyond a small area makes them much more vulnerable to extinction if they cannot withstand local pressures. Conversely, wide geographic range can be a good indicator of survivorship amongst species, and this was recently detailed amongst benthic marine invertebrates by Payne and Finnegan in their paper “The effect of geographic range on extinction risk during background and mass extinction” published in PNAS. The findings of the researchers make sense; widely distributed genera fare much better in the long run than those that have more narrow environmental needs that keep them from spreading. This spread also can help spur evolution (given that ecologies are constantly changing, to a greater or lesser degree), and so a greater diversity can arise from genera that can be found over a wider range for a longer amount of time. This does not exempt such groups from extinction, the researchers warn, as more catastrophic events that effect ecology on a global scale can wipe out groups that are widely distributed just as they can specialists. Hence, wide ranges for a group is important for survivorship during times of relatively slow global ecological change, but this does not necessarily exempt then from devastating extinction events like those seen during the end-Permian (as mentioned above).

In any case, I would highly recommend that anyone interested in extinction or evolution check out the above papers and articles, as the constant overturn of life is something vital to our understanding of the history of life on this planet. In case you’re in the mood for some further reading, check out Jeremy Bruno’s post on how mass extinctions affected chelicerates, or check out any of the books mentioned above.





She sold sea shells down at the sea shore

10 07 2007

The Wellcome Image Bank is now online under a Creative Commons License, and various science bloggers are picking out their favorites from the collection of old and obscure images. Mine is an 1823 letter from Mary Anning concerning the discovery of a plesiosaur;

Plesiosaur
[From the Wellcome Collection]

Such an image might not seem especially evocative, most of us are familiar with what a plesiosaur looked like, but during this time it was an amazing discovery. Mary Anning is the unsung heroine of paleontology, contributing as much (if not more) to our understanding of extinct Mesozoic life than many of her contemporaries like William Buckland, Charles Lyell, Richard Owen, and Gideon Mantell. Indeed, while Anning was the one to many of the first ictyhosaurs and plesiosaurs ever recovered from the blue lias of Lyme Regis, she never got credit for her discoveries when the famed scientists spoke about them in public. Indeed, it’s fitting that I happened across this image as I just finished Christopher McGowan’s excellent book The Dragon Seekers about Anning, as well as other scientists who “paved the way” for Darwin’s On the Origin of Species.

Prior to Anning’s discoveries, extinct marine reptiles were largely unknown (or at least never identified for what they were), but her discoveries helped to establish that life in the seas was once very different than today, supporting Cuvier’s assertion that species can become extinct. The discovery of the celebrated dinosauria would come soon after Anning’s finds, but the discovery of the large seagoing “reptiles” helped to establish the fact that such creatures once did exist, and if they had not been found perhaps the first dinosaur remains would have wallowed in obscurity or been misidentified for a long time.

I do my best not to forget Mary Anning, however; on my back wall rests a facsimile of the first articulated plesiosaur discovered (bits of jaw were found before, but no one was sure what they belonged to until an articulated skeleton was found).

And, while you’re at it, check out the picks of Larry Moran, Carl Zimmer, and Skepchick.

[Note: My choice for this image was a close call, however; the current relevance to what I was reading pushed it over the top despite my preference for this distinctive image]