Just what is a Nimravid, anyway?

28 06 2007

Holophoneus Skull
Hoplophoneus sp. via Wikipedia

The saber-toothed cat is one of the most famous of prehistoric icons, but perhaps one of the most neglected when it comes to public understanding. While we know dinosaurs by their genus names (names like Tyrannosaurus, Apatosaurus, and Ankylosaurus are easily come to mind), few people are familiar with saber-toothed cat genera like Smilodon, Metailurus, Dinofelis, or Xenosmilus (and there are many more). What’s even more confusing is that what we often call a saber-toothed cat is not really a cat at all, but a related carnivore called a nimravid that was molded by a striking trend in parallel evolution.

Up until a few months ago, I have never even heard of the term “nimravid”, and I was quite surprised to find out that Barbourofelis and Hoplophoneus, two creatures I had always assumed were just another kind of saber-toothed cat, could not be called true cats at all. Skulls of these two genera (or manufactured facsimiles) usually sit in the same displays as those of Smilodon and other more-familiar saber-tooths, and I never thought twice to look for differences. How careless I was not to pay attention, and how careless of museums to keep lumping the remains of these separate lineages together with minimal comment.

Part of the problem with tracing the evolutionary history of mammalian carnivores is that they have generated an amazing amount of different forms; there is much diversity and plenty of branches, so every new fossil certainly can shake the tree. To keep things simple, however, all living carnivores evolved from a line of primitive carnivorous mammals called Miacids, with the Order Carnivora first becoming recognizable sometime during the Eocene (approx. 56-34 million years ago), the groups giving rise to modern dogs (Family Canidae) and cats (Family Felidae) diverging about 43 million years ago. Not all the groups that arose from the first true carnivores left living descendants, however, and such is the case with the nimravids.

Hoplophoneus mentalis via Wikipedia

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

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

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).

You can see how complicated things can get; three different groups of animals arriving on the same body form from the same group of ancestors within a short amount of time. Indeed, saber-teeth seem to be a very common consequence for carnivores in this particular group, and oddly enough some living herbivores like the Musk Deer have impressive fangs as well. I’m not well-versed in evo-devo, but perhaps studying why musk deer develop such impressive teeth might give us some clues as to how it happened in these extinct cats, despite different ancestry. I should also perhaps mention that I’m curious about any sexual dimorphism between male and female saber-bearers; could sexual selection had a role in the extension of these massive canines? I don’t think it’s unreasonable to think so, especially if (as we’ll discuss) they were so long that they seemed to make these carnivores even more specialized in hunting, feeding, and social behavior than living carnivores.

Given the prevalence of massive canines amongst extinct felids and other groups, it’s a wonder why there are none living today (it should be noted, however, the Clouded Leopards have very long and impressive canines, even though they don’t peek out of their mouths when closed). It should also be noted that I have essentially left out a number of other, more distantly related saber-toothed carnivorous mammals called creodonts, which held saber-toothed hyenas like Hyaenodon in its ranks. For a time, it must have seemed like everyone and their mother had impressive fangs, and I can only wonder as to how these impressive structures became so-widespread.

It is not enough to merely say that nimravids are different, however; if they are not true saber-toothed felids, how closely are the groups related? Initially, some scientists thought that nimravids were ancestral to true cats based upon their more-primitive skull structure. As more fossils came out of the ground, the hypothesis that nimravids are closely related to true cats without being ancestral to them became favored, but this was overturned by the idea that nimravids and true cats are not very closely related, the nimravids diverging from the line that led to cats much earlier. This third view seems to make the most sense given the current fossil evidence, but I have to wonder how the reassignment of Barbourofelis will affect things, especially if it’s considered to be closer to felids than nimravids.

Here is a visual representation of the three hypotheses (which could be entitled “I can has MS Paint?”), after Hunt’s diagram in his 1987 paper;

Nimravid/Felid evolution

I included the “ancestral line” label in order to enforce the changing ideas about how evolution works, as well. In the first example the animals just kept evolving in the same line (they were the same genetic line, just with different species names as we came across them in the fossil record), but the third diagram shows that just because a new branch emerges does not mean that the ancestral line stops immediately. I have omitted Thylacosmilus and Barbourofelis as to keep things as simple as possible, and the fact that whatever I came up with would merely be a guess. I would also be remiss if I did not point this fact; while true saber-toothed cats do belong to the Family Felidae, they are all grouped together in the Subfamily Machairodontinae and do not have any living descendants. They diverged fairly early during felid evolution, ultimately becoming extinct, and I have hence tried to avoid the term “saber-toothed tiger” as much as possible. Because I’m trying to focus on nimravids for this entry I will keep the designation of “felids” for true saber-toothed cats, and hopefully I’ll eventually write a piece with more detail about the more well-known carnivores.

The big question involving these animals, however, is “How in the hell did they actually use those teeth?” Given that saber-toothed mammalian predators evolved three times in a geologically short time in three separate groups of predators suggests that they were useful for something, but how do you bite with teeth that extend past your lower jaw? In considering this question, it’s important to remember that when biting only the lower jaw is actually moving, so if a saber-toothed mammal wanted to impale a prey item with its long canines, it would have to throw its neck around with considerable force to achieve that end. In fact, this kind of action has already been proposed by some, the dynamics of felid saber-tooth skulls making it difficult to conceive how such huge canines could be used to effectively bite prey.

Part of the problem with having saber-teeth is that you need to open your jaw exceedingly wide in order to get food in your mouth. The oft-cited measurement for the gape of the felid Smilodon is 120 degrees (no source I’ve seen references where this measurement came from), and even if this is wrong we know that in order to get food into their mouths, many of the hyper-saber-toothed mammals would need to open their jaws to a 90 degree angle or more, otherwise they would not be able to get food in their mouths. What this means, as far as muscle strength is concerned, is that the muscles would not be as strong as in other cats, getting the mouth open being more important to a strong bite, so saber-toothed mammals would not have the crushing power of modern tigers or lions. Likewise, owning saber-teeth can make hunting difficult; if you stick your teeth into a live animal and it struggles, you could very well lose a tooth. Likewise the teeth would be more fragile, so putting extreme stresses on them (like crushing bone) would largely be out of the question too; it would be more effective and safe to attack soft parts of an animal than to try for the take-down neck-bites that modern cats employ.

We should be careful in our assumptions, however; we’re dealing with extinct animals, and their method of capturing/subduing prey may have differed significantly from any living carnivore. While I just mentioned that saber-toothed mammals likely had weak jaws, a 2005 study suggests that they had jaws as strong or stronger than living big cats, with different killing strategies depending on the overall durability/robustness of the saber-teeth. Likewise, an earlier study (1996) based upon tooth wear in Smilodon was unable to match wear indicative of bone crushing/chewing/abrasion with living hyenas, canids, and cats, suggesting that Smilodon may have avoided contact with bone as much as possible. Indeed, even though all these animals had impressive canines, not all their canines were equal, and some would be better suited to dealing with stresses involved with prey capture than others. Still, I would regard many of these teeth as delicate, and I can only imagine the pain these mammals must have endured when one of them broke.

Other hypotheses about how these animals employed their teeth involves the white shark-like tactic of disemboweling the softer underbelly of prey, then waiting for the eviscerated creature to die. This would be a rather risky move, the predator essentially sticking its head right between both sets of sharp hooves (assuming the prey was an ungulate). What seems more reasonable would be a strategy based upon cooperation, much like modern lions taking down huge water buffalo. If the group could bring down the prey with their claws, one animal could deliver the killing bite to the neck, minimizing the amount of potential harm to itself. This hypothesis, however, requires the study of behavior that we are no longer privy to, and it would be unreasonable to infer such a pattern on all saber-toothed mammals as the rule.

In his own paper studying the various methods of attack saber-toothed mammals could have used, G.G. Simpson concluded that they were best adapted for stabbing, not as much for slicing (although he conceded that they likely did this as well), the dentition of these animals showing their predatory habits (it had been hypothesized earlier that these animals may have been scavengers). Simpson’s study is interesting, but prey is generally not taken into account; only the effectiveness of different strategies for ripping up the assumed prey. While it certainly serves as a good reference point from a mechanical point of view, the skulls of the animals are considered out of context, and so the major mysteries of these animals remain unsolved.

Ultimately, all the known saber-toothed predators died out, regardless of their affinities. One of the most popular views (which I am surprised to still hear) is that the teeth of these animals simply became so huge that they could not properly open and close their mouths, driving the species to extinction. If there are urban legends in paleontology, surely this is one of the most annoying and persistent. G.G. Simpson refutes this idea in his popular work The Meaning of Evolution, published more than 30 years before I had heard it from various documentaries claiming scientific accuracy;

The sabertooth is one of the most famous of animals just because it is often innocently supposed to be an indisputable example of an inadaptive trend. In fields far remote from paleontology the poor sabertooth has some to figure as a horrible example, a pathetic case history of evolution gone wrong. Its supposed evidence is thus characteristically summarized in a book on (human) personality: “The long canine tooth of the saber-toothed tiger grew more and more into an impossible occlusion. Finally, it was so long that the tiger could not bite effectively, and the animal became extinct.” Now, like so many things that everyone seems to know, this is not true… Throughout their history the size of sabertooth canines varied considerably from one group to another but varied about a fairly constant average size, which is exactly what would be expected if the size were adaptive at all times and there were no secular trend in adaptive advantage but only local and temporary differences in its details. The biting mechanism in the last sabertooths was still perfectly effective, no less and probably no more so than in the Oligocene. To characterize a finally ineffective a mechanism that persisted without essential change in a group abundant and obviously highly successful for some 40,000,000 years seems quaintly illogical! In short, the “inadaptive trend” of the sabertooth is a mere fairy tale, or more fairly, it was an error based on too facile conclusion from imperfect information and it has since been perpetuated as a scientific legend.

Why saber-teeth seemed to be so trendy among predatory mammals, only to disappear entirely, I have no idea. Obviously they must have been good for something, some common developmental, ecological, or other trend driving canines to be longer, only to (perhaps) cause the animals to be so specialized that they could no longer compete with other carnivores who did not have to be so concerned about their teeth. At the very least, however, I hope this post have served to bring to attention a group generally overlooked, often mistaken for their cousins, when they have a rich evolutionary history of their own.


Hunt, R.M. 1987. “Evolution of the Aeluroid Camivora:Significance of Auditory Structure in the Nimravid Cat Dinictis“, American Museum Novitiates, Number 2886, pp. 1-74

Simpson, G.G. 1941. “The Function of Saber-Like Canines in Carnivorous Mammals“, American Museum Novitiates, Number 1130

Further Reading;

The Big Cats and their Fossil Relatives by Turner and Anton

The Velvet Claw by MacDonald

Evolving Eden by Turner and Anton

Fatalis by Rovin (fiction)

Wild Cats of the World by Sunquist


Excellent book: The Dechronization of Sam Magruder

28 06 2007

I’m not a huge sci-fi fan, but I have to say that G.G.Simpson’s novel The Dechronization of Sam Magruder is one of the most enjoyable works of fiction I’ve ever read. I’ll get to my take on who the narrator and Sam Magruder actually are and why this matters, but first I wanted to share this particular passage dealing with evolution;

It has been said by some theorists that cases like that of the crocodile, virtually unchanged for 100 million years and more, represent a failure of the evolutionary force, a blind alley, or a long senescence. As I gazed at my antagonist, it occurred to me how false this is. Here was no failure but an adaptation so successful, so perfect that once developed it has never needed to change. Is it, perhaps, not the success but the failure of adaptation that has forced evolving life onward to what we, at least, consider higher levels? The crocodile in his sluggish waters had perfectly mastered life in an unchanging environment. No challenge arose. Our ancestors lived in a more evanescent world, where what was adaptation at one time became an inadaptive burden in 100,000, 1,000,000, or 10 million years. For them, their adaptation was always a blind struggle to keep up, to face new conditions, to exploit new opportunities. Only changing races met that challenge. Of the others, those to whom chance close the adaptive avenue of change, the unlucky became extinct and the lucky, like the crocodile, found and settled into some way of life where the challenge was absent, and there they stagnated.

Indeed, crocodiles are fortunate; adaptation and evolution has perfected them to exist in a niche that does not require major changes provided that conditions don’t drastically change overnight. Even before there were actual crocodiles, there were tetrapods essentially filling the same niche of aquatic ambush predator, and for one reason or another crocodiles beat them all out, so well-adapted to their role in the ecosystem that there hasn’t been much need for big-time evolutionary change. If there was anything like a communal memory of crocodiles, there wouldn’t be a memory of a time when there was not water to conceal themselves in, and they have been preying upon the various creatures that have visited the water’s edge the whole time. While there was once a greater diversity of crocodiles in type and habit, they have dominated their niche for so long that there is seemingly nothing they cannot handle.

This passage also speaks to what wrote in popular works like The Meaning of Evolution, in which he refutes the idea that life has some sort of vital force pushing it forward to be constantly evolving (or the idea that species, like individuals, had lifespans of birth, growth, and “senescence”), nor does evolution strive toward one perfect, unchangeable end (although crocodiles probably fooled many in this regard). While crocodiles will continue to change little by little through time, I can’t think of a reason why they should not continue to persist much as they are now for many millions of years to come.

As for the book itself, I feel that our protagonist Sam Magruder and the narrator are dual-voices of G.G. Simpson. Magruder, a scientist who is utterly lonely and stuck in a world where he feels he cannot make any lasting contribution to science, is the most powerful voice in the story, but I feel the narrator tells us quite about Simpson as well. Magruder’s story is told via 8 stone tablets that Magruder inscribed and buried, but not all the content of the slabs is shared with the reader; the academics who are disseminating the information have edited it, inserting lengthy footnotes, snide remarks, and editing what they deem to be distasteful. One such example is Magruder’s discussion of his sexual frustration, no female of his species existing for more than 65 million years into the future, but the editors decide this discussion is too frank and omit it from the public’s copy. Likewise, the narrator makes mention of, but omits, various footnotes of academics apologizing for Magruder’s behavior (such as using a colloquialism, which is apparently unbecoming of a scientist) and his ignorance of a topic (such as when sauropods became extinct in North America). These were Simpson’s little jabs at academia, perhaps venting his own frustration at fellow scientists who lacked imagination or developed a “holier-than-thou” attitude over the years.

While I did not know this about Simpson, the afterword by Stephen Jay Gould makes it clear that Simpson was a very lonely man despite his accomplishment. This was not for lack of company, but Simpson wanted so much to contribute something lasting to paleontology that he could seemingly could not bear much criticism or brown-nosing (which was worse), and the book makes it apparent that he was worried he would be forgotten. While Magruder buried his slabs, never knowing whether someone would find them (he’d be dead for at least 65 million years when they did), Simpson seemingly buried this bit of fiction, which did not come to light after his death. At the end of his life, perhaps Simpson felt like Magruder; contributing much to our understanding, but isolated from anyone who would study his work in the years to come, isolated from satisfaction and afraid that his work would be left to collect dust on a shelf somewhere. What gratification he might have gotten from people like me who are still impressed by his work, I don’t know, but it seems that intellectually Simpson felt he was stuck in the Cretaceous, never knowing what would become of the messages he left to future generations.

If you enjoy science fiction at all, and especially if you like tales about dinosaurs, I highly recommend this book; it can be read cover-to-cover in less than an evening and gives us a very personal look at one of the greatest paleontologists ever to have lived. Immediately after I closed Simpson’s book, I picked up Sapolsky’s A Primate’s Memoir, and it only furthered my enjoyment for the evening. I have absolutely loved the first 60 pages of Sapolsky’s book, and I can’t wait to get to the rest of it; it is another must-read if you’re looking for good books this summer.

In the field with Barnum Brown

28 06 2007

Lilian MacLaughlin Brown’s Bring ’em Back Petrified was an interesting read as it provided a look at one of the world’s greatest fossil hunters looking for his quarry in one of the hardest locales to tackle; the jungle. While Barnum’s wife doesn’t seem nearly as interested in fossils as being a sort of housewife in the field, she does prove some interesting (if short) accounts of work in the field.

He hitched up his pants, spat on his hands, stepped forward and brought his pick down like a sledge hammer, shattering the limestone rock. He picked up a fragment. Out of his pocket came the hand lens, and, training it on the specimen, he squinted down into the glass. Rufio and Lolo stood a few paces behind, exchanging looks and whispers. There are times when Barnum is as finicky about noise as a golf champion on the green. This was one of them. Now an irritate cough hinted that his concentration was being disturbed by the men’s whisphering. They took the hint. We all stood like statuary – waiting.

At last he came to life with a, “hmmmmmm,” which I recognized as a good sign – particularly when he followed this by pulling out his pipe and tobacco pouch. Barnum always pulls out his pipe when he prepares to develop a dig. It’s an English briar, a smelly thing he latched onto sometime before we were married, and so heavy he has to rest his jaws between puffs. Why he keeps it is a mystery, although it does make him look a little like Sherlock Holmes.

I had to nudge him with a questioning, “Yes-s-s?” before bringing him entirely out of his trance. Even then he filled and lit his pipe before favoring me with a smiling,
“It’s our sea-bed alright, Pixie.”
Picking up another rock fragment, he examined it through the lens, then suddenly emitted a jubilant,
“FORAMINIFERA! Stand back, everyone. Nobody touch the rock. We’ve got foraminifera!”
Lolo threw Rufio a look of alarm. “Is this bad? Are we get a sickness?”
“No,” said Rufio, “I think it is something scientific. And many times,” he added, in a whisper, “that not so good either.”

Both turned to me for enlightenment but I could only shake my head and nod towards Barnum. No scientist is in an explanatory mood when he has just made a discovery. Having prospected the rocks for his information, he himself must be prospected if one is to pry that information loose. Only one doesn’t do it with his pick. One wheedles-particularly at night when a scientist’s defenses are down. Since this was daytime, I decided on the direct approach. Barnum was hunkered down working his pick point carefully over the rock.
“what,” I asked, giving the question a casual twist, “is foarminifera?”

The pick halted in mid-air. His head and neck took on a peculiar rigidity. Turning slowly, he eyed me in such a “violent” manner that Rufio stepped gallantly forward to my defense.
“Señor, we only like to know-”
“You stay out of it, Rufio,” Barnum barked. “This is a family affair.”

Although I soon caught the melodramatic undertone, for an instant indignation welled up in me.
“Can’t a person even…?” I began.
It was Barnum’s clue to clap a hand to his forehead, actor-like, and go on with the melodrama:

“That a woman of mine should ask what a foraminifera is!” he orated. “A woman who was with me when I dug up the Colossochelys atlas in India; who watched me unearth the Samotherium and Sivatherium; who helped me with my Wyoming sauropods; who…” He broke off his act, and laughed at the blank look on the men’s faces.

I laughed. The men laughed. The Señor Doctor was loosening up.

These sharks just keep getting pregnant!

27 06 2007

Because of my public school education, I have been trained to immediately think of the New Mexico Whiptail lizard (Cnemidophorus neomexicanus) whenever I hear the word “parthenogenesis.” What exactly parthenogenesis was or how it occurred was never covered, but the association between reproduction without a male and these lizards was certainly hammered in to my brain. Lately, however, there have been various cases of other animals exhibiting parthenogenic reproduction, including Komodo Dragons (Varanus komodoensis) and Bonnethead Sharks (Sphyrna tiburo).

The latest news deals with a Blacktip Reef Shark (Carcharhinus melanopterus) that had died after a physical had gone awry, the shark (named “Tidbit”) biting one of the aquarium staff before dying shortly thereafter. Upon autopsy, veterinarian Bob George discovered a near-term baby shark inside Tidbit, making this perhaps the second known case of shark parthenogenesis known. Tests still need to be done to confirm this, but Tidbit was not kept with any males of her species and sharks are not known to hybridize, so it is fairly certain that this is another case of parthenogenesis (it should also be noted that both bonnetheads and blacktip reef sharks belong to Order Charcharhiniformes, or the “Ground Sharks”).

In the case of the bonnethead shark, the parthenogenic offspring was female, and the CNN article linked above makes no mention of the sex of the offspring found in Tidbit (whatever I guessed, I’d have a 50/50 chance at being right). It is possible that in times of sexual isolation some sharks can produce offspring on their own, but if no males show up (or no males are produced through parthenogenesis, as komodo dragons apparently can achieve), then the population is no better off. I would also be interested to see if this particular ability exists across all shark groups or just in this particular order; are the “warm-bodied” lamniform sharks like Makos, Porbeagles, and Great White Sharks capable of this as well? Is this ability something new that evolved in this one group, or is it an ancestral condition that more derived sharks have lost? For now we’ll have to await Tidbit’s test results, but if nothing else aquarium curators should be keeping their eyes open for more instances of parthenogenesis going on right under their noses.

More books?!

27 06 2007

As if I didn’t have enough to read already, I’ve added a few more books to my planned reading for the summer. I finished Lilian McLaughlin Brown’s Bring ’em Back Petrified last night, so I’ll hopefully be able to jump right into whatever arrives in the mail today when I get home from work. Anyway, here’s what’s been added;

The Tempo and Mode of Evolution by George Gaylord Simpson (1984 paperback)

I’ve been meaning to read this book for quite some some, but until now I had been unable to find an affordable copy. I certainly can’t wait to dig into it.

How Animals Work by Knut Schmidt-Nielsen

I know a bit about skeletal anatomy, but I have to admit that my understanding of physiology and biomechanics is pretty poor. I’m hoping Schmidt-Nielsen’s book will help to fix that.

Walker’s Mammals of the World (2 Volumes) by Ronald M. Nowak

I first happened across Walker’s Mammals of the World while petsitting for the late Dr. Ted Stiles, and I knew that my library would be pretty poor without a copy. While I plan on accumulating other books detailing mammals of the Neotropics and Africa, it will be great to have a set of books that I can use to further the number of taxa I’m familiar with.

Historical Geology: Evolution of Earth and Life Through Time by Reed Wicander and James S. Monroe

In the fall of last year I took a course called Evolution in Geologic Time, and while I’m pretty familiar with big-time evolutionary events, I could do a lot better remembering exactly when they happened. Likewise, my understanding of evolution is focused primarily around tetrapods, the majority of earth’s history prior to the Cambrian still a bit foggy upon recall, so I definitely want to help my understanding of what happened in “Deep Time” a bit more.

God’s Own Scientists: Creationists in a Secular World by Christopher P. Toumey

I haven’t yet read Ronald Numbers’ The Creationists, but I saw this book mentioned on a comment thread over at Respectful Insolence and it sounded very interesting. I’ve become relatively well-familiar with changes and shifts in creationist thought over the past few hundred years, but I definitely want to get a better idea of what goes on behind closed doors when “creation scientists” get together.

The Theory of Island Biogeography by Robert H. MacArthur and Edward O. Wilson

After a tip from Bora that Quammen’s Song of the Dodo has a fair number of mistakes, I thought I would pick up what is considered to be the landmark work in the field of biogeography. I plan to read the two books in succession and write up a post on the subject, but that’s probably a few weeks (if not a month) away.

The way I go through books, a few more will likely be added before all is said and done, but for now I think I’ve got my work cut out for me.

Walking With Triceratops

26 06 2007

It’s funny how certain things come into style, only to be swept away by a new trend. Clothes, music, and television all are mediums in which the bang-and-bust cycle of trends is apparent, but things can come “back into style” in paleontology as well. Not very long ago I read Robert Bakker’s The Dinosaur Heresies, a wonderful book that certainly shook things up. Not everything within its pages is valid, but Bakker certainly did a lot of heavy-lifting in order to show that dinosaurs were not glorified lizards. One of the points he makes involves the posture of one of the most beloved dinosaurs, Triceratops, but as I have learned the problem of Triceratops posture is a bit more complicated than I had originally thought.

Triceratops Sprawling
From “Mounted Skeleton of Triceratops elatus” by Henry Fairfield Osborn, American Museum Novitiates, Sept. 6, 1933

On the fourth floor of the American Museum of Natural History, the mounted skeleton of Triceratops horridus (the same mount referred to as T. elatus by Osborn, above) stands today in the same posture as it was mounted in the 1930’s; back legs directly under the hips like a good dinosaur, but with the front limbs sprawled out to the sides, almost resembling a body-builder who bulked up his arms but neglected his legs. In fact, this analogy is not very far off from what Osborn himself envisioned, writing;

The pose of the fore limb, set out widely apart from the body, is also designed to withstand attack, like the widely spreading feet of the pugilist or wrestler.

This is Osborn’s idea of the adaptive value for the stance, but it does not explain why his Triceratops is sticking out its upper arms nearly horizontal to the body. The problem in determining a proper mount for the composite skeleton (fossil material coming from 4 skeletons) was that the front limbs would not articulate in the way a mammals forelimbs do. After observing various reptiles and mammals, as well as trying different poses, it became apparent that Triceratops could not carry its forelimbs directly under the body without disarticulating its own skeleton; the sprawling pose was the only one that seemed to work. I’m sure the appearance of Triceratops as being front-heavy played into the pose as well, such a massive head requiring plenty of support from the arms, although in the position chosen a huge amount of strain would be put on the front limbs. It’s arms would be spread out away from the body in a push-up position, with the legs directly under the body; if this dinosaur walked in such a position it would be a very amusing sight.

Then I read Bakker, who argued that fossil trackways (in addition to his own research and position that dinosaurs were active, dynamic animals) showed that Triceratops did, in fact, carry its front legs underneath its body and was even capable of galloping. How fast this dinosaur may have been is something for another day (I’m not even aware of any studies that have attempted to answer that question, in fact), but Bakker seemed very confident in his idea of posture, and I had my own misgivings about the super-sprawl of the AMNH mount. The trick is, however, remebering that this is not an either or question where the super-sprawl or “good dinosaur” pose must be right to the exclusion of all others. In truth, the real answer seems to lie somewhere in between.

In the 1933 paper, Osborn mentions the analysis of W.D. Matthew, tells us that other scientists were mounting their ceratopsians in different ways;

The heavy strain of supporting the great body on these widely-spread fore limbs is very apparent but there seems to be no other way to pose the skeleton. A compromise pose, such as that of the National Museum mounted skeleton (or of Marsh’s restorations so far as they can be interpreted) serves to reduce, not to banish the anatomically impossible disjointing.

Triceratops Profile
From “Mounted Skeleton of Triceratops elatus” by Henry Fairfield Osborn, American Museum Novitiates, Sept. 6, 1933

With such problematic limbs, different mounts were bound to take different poses, and the Smithsonian dinosaur is of further interest to us because it recent has helped to answer the very questions we’re asking. In May of 2001, the Smithsonian unveiled its new mount for Triceratops (dubbed “Hatcher” after the man who discovered it), featuring limbs that were closer to being under the body than Osborn’s dinosaur, but still bend slightly outward from the body. The reanalysis that led to this new pose was undertaken by Ralph Chapman, who used computer modeling to help properly restore the dinosaur (i.e. it, too was a composite, and its back feet were actually from a hadrosaur) and figure out how it might have moved. Not everyone is behind the semi-sprawl, however, and just prior to the Smithsonian skeleton being unveiled Gregory S. Paul and Per Christiansen issued the paper “Forelimb posture in neoceratopsian dinosaurs: implications for gait and locomotion” in the journal Paleobiology, arguing that the problem with Triceratops posture often stems from problems articulating other parts of the skeleton; if you get the vertebrae and ribs wrong, you’ll probably get the arms wrong, too.

So what about trackways? If we have the ichnofossils, then we should be able to tell what the limb posture of this animal was, right? This seems to be a lot to ask; both in Bakker’s book and the Louie Psihoyos’ book Hunting Dinosaurs (the only two books with depictions of Triceratops tracks I’ve come across) the trackways are illustrated. In Bakker’s book, the tracks align just the way he says they should; the tracks suggesting the feet were under the body. Psihoyos’ book, however, depicts tracks made by the front limbs being just slightly outside those made by the rear limbs, suggesting a semi-sprawl. If that was not enough, fossil trackways are notoriously hard to match up with known fossils, tracks of the same type receiving their own name (like Grallator here in New Jersey) when we can’t figure out what dinosaur made them. This isn’t to say that we can’t identify any tracks or even any tracks made by ceratopsian dinosaurs, but given how massive the heads of ceratopsian dinosaurs could be, it’s important information to know when determining posture that trackways don’t tell us much about.

And so the arguments will likely continue; while we now know that Triceratops and its relatives like Torosaurus did not hold their front limbs out to the point where their chests would be scraping the ground while their butts were in the air, the way their skeletons are constructed seems to suggest a small amount of sprawl in the front limbs. Carrying such a massive skull would have certainly affected the front limbs, and if the Triceratops carried its limbs directly under its body it may have been a little more unstable/more likely to tip over. I don’t know if it has been done, but I for one would love to see what happens to the animal’s center of gravity when switched between the various postures, and I think that the semi-sprawl would have offered Triceratops more support without putting undue strain on its limbs.

It’s funny how old ideas come back; in such a short amount of time, Triceratops was thought to have its arms out to the side, then under its body, and now we’re at a point of compromise. While the logic of uniformitarianism may be “the present is the key to the past,” the history of the science of paleontology has taught me that the past is the key to the present, and old “fossils” can still tell us a lot about debates that continue today.

Not quite Big Bird

26 06 2007

If you can’t wait for my analysis of the PNAS article (slated for the June 29th issue) about the giant Peruvian penguins Icadyptes salasi and the smaller Perudyptes devriesi, here’s a list of articles covering the story. While the new finds certainly have implications for the evolution of penguins, everyone is acting especially surprised that these birds lived in an area much warmer than the Antarctic, but I guess they’ve never heard of the Galapagos Penguin (Spheniscus mendiculus) or even the African (“Jackass”) Penguin (Spheniscus demersus). This reminds me that I need to pick up G.G. Simpson’s book Penguins: Past and Present, Here and There, but I’ve got lots of books to get through before I start ordering anything new. Anyway, here is the abstract and the entries relevant to the big birds;

Julia A. Clarke, Daniel T. Ksepka, Marcelo Stucchi, Mario Urbina, Norberto Giannini, Sara Bertelli, Yanina Narváez, and Clint A. Boyd (2007). “Paleogene equatorial penguins challenge the proposed relationship between penguin biogeography, body size evolution, and Cenozoic climate change.” Proceedings of the National Academy of Sciences (PNAS) June 29, 2007.

New penguin fossils from the Eocene of Peru force a reevaluation of previous hypotheses regarding the causal role of climate change in penguin evolution. Repeatedly, it was proposed that penguins originated in high southern latitudes and arrived at equatorial regions relatively recently (e.g., 4 to 8 million years ago), only well after the onset of latest Eocene/Oligocene global cooling and increases in polar ice volume. By contrast, new discoveries from the middle and late Eocene of Peru reveal that penguins invaded low latitudes over 30 million years earlier than prior data supported, during one of the warmest intervals of the Cenozoic. A diverse fauna includes two new species, here reported from two of the best exemplars of Paleogene penguins yet recovered. The most comprehensive phylogenetic analysis of Sphenisciformes to date, combining morphological and molecular data, places the new species outside the extant penguin radiation (crown clade: Spheniscidae) and supports two separate dispersals to equatorial (paleolatitude ~14� S) regions during greenhouse Earth conditions. One new species is among the deepest divergences within Sphenisciformes. The second is the most complete giant (>1.5m standing height) penguin yet described. Both species provide critical information on early penguin cranial osteology, trends in penguin body size, and the evolution of the penguin flipper.

LiveScience – Giant Ancient Penguins Liked it Hot
PhysOrg.com – Prehistoric equatorial penguins reached 5 feet in height
BBC News – Tropical giant penguin discovered
Mongabay.com – Past global warming produced monster penguins
The Guardian – Pick up a penguin? Not this one you wouldn’t
EurekAlert! – March of the giant penguins
San Francisco Chronicle – Rethinking penguins — big, warm-weather fossil