The way people have been talking about Tiarajudens eccentricus, you would think that this 265 million year old cousin of ours was unique among herbivores for having saber-fangs. It was not, but it does raise the question of what makes a saber-tooth in the first place.
Tiarajudens was not quite like any creature alive today. Described by Juan Carlos Cisneros and colleagues in Science, this animal belonged to an entirely extinct group of vertebrates called anomodonts, and it had a short, rounded skull topping a somewhat lizard-like body. But Tiarajudens was more closely related to you and I than any reptile.
Over fifty million years before Tiarajudens, there was a great evolutionary split among small, lizard-like vertebrates. On the one side were the early diapsids – the starting point for the full diversity of reptiles – and on the other were the early synapsids; a group of unfamiliar creatures that underwent a major evolutionary radiation before the time of the dinosaurs and, today, is represented entirely by mammals. Since the anomodonts were one of many extinct synapsid lineages, Tiarajudens was one of our very distant cousins.
The most immediately-impressive aspects of Tiarajudens were its long canines. Thanks to the celebrity of the saber-toothed cats of more recent epochs, we tend to think of these teeth as a hallmark of flesh-tearing predators, but Tiarajudens was clearly an herbivore. In fact, the creature’s flashy saber-teeth have directed attention away from a more significant feature of Tiarajudens related to its diet of greens.
Not all the teeth of Tiarajudens were set along a single toothrow as in our own jaws. The synapsid had several short, incisor-like teeth at the front of the upper jaw, followed by a large fang, but there were also thirteen molar-like teeth set in a row on the pterygoid and ectopterygoid bones of the skull. Imagine that your molars were moved a few centimeters toward the midline of your head so that they were sticking out of your upper palate – that is roughly how the flat grinding teeth of Tiarajudens were arrayed.
The fact that Tiarajudens had palatal teeth made it nearly unique among its close relatives. Other, earlier synapsids – such as the sail-backed Edaphosaurus – had palatal teeth, but, with the exception of one other species, this tooth arrangement is unknown among the subgroup to which Tiarajudens belongs.
The palatal teeth of Tiarajudens were a peculiar specialization to a life eating plants. This is reinforced by the pattern of wear on the animal’s teeth.Wear marks indicate that the upper teeth were coming in contact with teeth in the lower jaw to crush food. No lower jaw has yet been found, but, if confirmed by future finds, the jaws of Tiarajudens would be one of the earliest cases of occlusion between upper and lower molar-type teeth known. Tiarajudens was one of the first vertebrates able to chew its food.
Being an early chewer isn’t the sort of thing that makes the New York Times, though, so it’s no wonder that news reports have focused on the saber teeth of Tiarajudens. Just how unusual this feature is depends on how we define what a saber fang actually is.
Tiarajudens was not the only member of its evolutionary family to have elongated, specialized canine teeth. Within the anomodonts – the group to which Tiarajudens belonged – there was another subgroup called the dicynodonts. Their name – “two dog teeth” – is a clue to their anatomy. Represented by forms such as Diictodon, Placerias, Lystrosaurus, and, of course, Dicynodon, these squat, beaked herbivores had prominent, conical canine teeth often referred to as “tusks.” The teeth of Tiarajudens, by comparison, were much longer and more blade-like.
As paleontologist Jörg Fröbisch mentions in a comment on the new Science paper, the difference between the canines of Tiarajudens and its dicynodont cousins raises the question of “When is a saber-tooth a saber-tooth, and when is it a tusk or a slightly enlarged canine?” There is no standard terminology or specific method to distinguish between the various categories, and this quickly becomes frustrating because tusks, sabers, and other elongated teeth have evolved multiple times over the past 265 million years among many different synapsids.
(I’m sticking to synapsids here, but it is worth noting that Tyrannosaurus rex has been said to be a saber-toothed dinosaur. One need only look at the skull of the juvenile specimen “Jane” to see why.)
Up until now, distinguishing between tusks and saber-teeth relied on a “I’ll know it when I see it” type of common sense. Dicynodonts, elephants, hippos, walrus, manatees, and other animals are said to have tusks even though their modified teeth are all different in shape and, in some cases, modified versions of different teeth in the jaw. The tusks of elephants and dugongs are large incisors that lack enamel, for example, while those of the hippos are modified canine teeth coated in enamel. (Thanks to Robert Boessenecker for the correction about walrus tusks and enamel. See comments below.) Dicynodonts further complicate the breakdown by having modified canine tusks that lacked an enamel covering.
Saber-fangs are a bit more consistent in shape and overall anatomy. Tiarajudens, predatory synapsids called gorgonopsians, sabercats, musk deer, baboons, lemurs, extinct plant-eating mammals such as Uintatherium and Titanoides, and many other animals all possess exceptionally elongated, flattened canine teeth have been put to various uses. The fangs of sabercats were devastating while taking down large prey, but, in baboons and some herbivorous saber-toothed mammals, the difference in saber size between males and females indicates that the teeth have more to do with sexual selection than feeding. (The fangs of Tiarajudens may have been used for display and competition for mates, too, but a bigger sample of fossils is required to know for sure.) There was more than one way to be a saber-toothed synapsid.
Clearly “saber-tooth” and “tusk” are unspecific terms that apply to a wide variety of tooth types, shapes, and uses. The basic question of “What is a saber-tooth?” has yet to be specifically answered. Tiarajudens clearly had impressive fangs that would place it in the saber-tooth club and distinguish it from its tusked relatives among the dicynodonts, but, even in the face of obvious differences, we require more precise terms so that we can better identify the features we rely on to figure out how the natural history of extinct animals. If we can answer the simple question of “What is a saber-tooth?”, we may be better able to understand why this versatile feature has shown up time and again since the days when Tiarajudens chewed the Permian vegetation.
Thanks to Maggie Koerth-Baker for encouraging me to write this post.
Top image: The skull of Tiarajudens as seen from the left side (front of the skull is facing left). Created from diagrams in Cisneros et al., 2011.
Cisneros, J., Abdala, F., Rubidge, B., Dentzien-Dias, P., & Bueno, A. (2011). Dental Occlusion in a 260-Million-Year-Old Therapsid with Saber Canines from the Permian of Brazil Science, 331 (6024), 1603-1605 DOI: 10.1126/science.1200305
Frobisch, J. (2011). On Dental Occlusion and Saber Teeth Science, 331 (6024), 1525-1528 DOI: 10.1126/science.1204206