Just what is a Nimravid, anyway?

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;

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.

References;

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