Heterodonty where you least expect it

One of the first things I was ever told about what makes reptiles different from mammals was that reptile teeth were the same all throughout their jaws (called a homodont condition) and that mammal teeth were different throughout their jaws (called a heterodont condition). The fact that mammals had fur, mammary glands, and had a high metabolism/constant body temperature were all obvious, but I had never heard about the difference in teeth. That was elementary-school simplification, though, and the fact of the matter is that the distinction is not so sharp. There have been both homodont mammals and heterodont crocodylians, dinosaurs, and lizards.

In terms of homodont mammals, toothed cetaceans are probably the best example. Sperm whales, orcas, and dolphins all possess rounded, conical teeth best suited to graping food. We know that whales evolved from primarily terrestrial mammalian carnivores like Pakicetus, but over time their heterodont condition was lost. If look at the skull of Basilosaurus for example, the tooth toward the from of the jaw are conical and useful for grabbing fish and squid, while the teeth towards the rear of the jaw are still triangular slicers. Baleen whales eventually lost their teeth altogether (although they have them for a short time in the womb and have "fossil genes" that once were involved in the production of teeth), but toothed whales kept their teeth. The conical teeth of creatures like dolphins are most useful for grabbing prey which is then swallowed whole, the loss of the heterodont dentition perhaps being triggered by a change in feeding habits millions of years ago.

The focus of this entry is not so much on homodont mammals as heterodont representatives from groups traditionally thought to be otherwise. In 1989, a paper was published in Science describing an unnamed Cretaceous crocodylian from modern-day Malwai. The specimen would eventually be dubbed Malawisuchus (as stated in Jacobs 2000), but it surely did not fit the regular crocodile type. The face was short and deep, the teeth being separated into canine-like and molar-like forms. Much like the skull of Basilosaurus mentioned above, the canine-like teeth were probably used to grip and pin prey, the molar-like teeth being used to slice through prey once it had been killed. Just what drove the evolution of the ancestors Malaqisuchus to develop this type of dentition is still unknown, however.

Even more recently, a paper was published in the Journal of Vertebrate Paleontology (Nydam et al. 2000) describing the teeth of the Cretaceous lizard Peneteius aquilonius. In Peneteius the teeth at the back of the lower jaw are more like molars than those in the front of the jaw, with the teeth of the upper jaw differing from both. The material is more fragmentary than for Malawisuchus, but it still shows a lizard with a dental pattern different from almost all other known lizards, living or extinct. The authors describing the tooth pattern presume that it might have been feeding on insects, having a more advanced "toolkit" of teeth being an advantage. The problem with this hypothesis is, however, that many lizards eat insects today and have not converged on this pattern, so why Peneteius is different is still poorly understood.

Finally, the most famous of all dinosaurs appears to have been developing heterodont dentition as well. Tyrannosaurus rex had the beginnings of a differentiated set of teeth suited to different uses, the most remarkable being the change in the teeth at the front of the jaws into incisor-like forms. According to a paper published in the Journal of Vertebrate Paleontology, (Smith 2005) Tyrannosaurus had three distinct tooth types labeled according to their position in the jaw; premaxillary (front of the upper jaw), maxillary (most of the length of the upper jaw),
and dentary (lower jaw). Smith mostly concentrates on the differences between the teeth that can be applied towards understanding taxonomy (especially since theropod teeth are common fossils that traditionally have been thought to be too generalized to allow for robust attribution to one species or another), but the condition in Tyrannosaurus might also give us some clues about how it ate. In the upper jaw, the smallest, most blade-like teeth are packed together in a somewhat blunted area at the front of the snout, the teeth becoming larger moving back and then smaller again.

Speaking for myself, this might suggest that the front teeth were scrapers, useful is getting the most flesh off the bone. Predators that just rip the muscle off bones might leave a lot behind, but being able to scrape flesh off long bones with some precision might maximize the amount of meat that Tyrannosaurus was able to get off each carcass. Of course, Tyrannosaurus probably could crunch bone, an operation that would not require delicacy, but the differentiated dentition might still be a reflection of the feeding habits of this animal.

This sort of feeding reminded me of an earlier paper describing tooth marks on the ilium of a Camarasaurus in which the bone showed several long, non-intersecting grooves in parallel to each other (Chure et al. 1998). The authors interpreted the fossil to mean that an Allosaurus (or similar theropod) had accidentally bitten the ilium while feeding, its incisors scraping the bone. The hypothesis that the marks are the result of an accident might be correct, but perhaps the marks were left as a result of a more "delicate" de-fleshing of the hip area? If large theropods were using this technique to get the most from their meals, then perhaps this could explain the differentiated dentition in the premaxillary of Tyrannosaurus (although it would have to be checked if a similar condition arose in other groups). A more profound understanding of theropod tooth morphology and differences would also benefit our knowledge of the evolution of predators in response to prey animals, as well. In a review conducted by Van Valkenburgh and Molnar (2002), for instance, it was noted that theropods show almost no tooth differentiation as opposed to mammalian carnivores. This no longer appears to be the case, and a reevaluation of large theropod teeth, jaws, and feeding habits is certainly warranted.

The idea that Tyrannosaurus was using its more varied dental toolkit to make the most of carcasses has to make sense in terms of other evidence, though. In a study of gut contents preserved for the Tyrannosaurus-relative Daspletosaurus (Varricchio 2001), it is suggested that tyrannosaurs crushed and ingested much of their prey, their meals then being broken down by strong stomach acids, following the same sort of pattern seen in crocodiles and birds today. This does not rule out the hypothesis I mentioned above, but it's something important to keep in mind.

Likewise, the fact that the jaw of Tyrannosaurus was strengthened (presumably for bone crushing) should be noted. Rather than having a "hinge" in the lower jaw that would allow it to expand to the sides to gobble down even larger pieces of meat, the Tyrannosaurus jaw is optimized for power. This would have been especially important for killing, but it might have constricted the size of the mouthful Tyrannosaurus could have swallowed. Could strengthening of the jaw (and hence lack of flexibility) driven tyrannosaurs to develop somewhat specialized premaxillary teeth? I can't say, but it is a hypothesis that I'd definitely like to look into.

References;

Chure, D.J.; Fiorillo, A.R.; Jacobsen, A. (1998) "Prey Bone Utilization by Predatory Dinosaurs in the Late Jurassic of North America, With Comments on Prey Bone Use by Dinosaurs Throughout the Mesozoic." Gaia, Vol. 25 (4), pp.865-887

Clark, J.M.; Jacobs, L. L.; Downs, W.R. (1989) "Mammal-Like Dentition in a Mesozoic Crocodylian." Science, Vol. 244, pp. 1064-1066

Jacobs, L. (2000) Quest for the African Dinosaurs. The Johns Hopkins University Press. Baltimore, MD.

Nyam, R.L.; Gauthier, J.A.; Chiment, J.J. (2000) "The Mammal-Like Teeth of the Late Cretaceous Lizard Peneteius aquilonius Estes 1969." Journal of Vertebrate Paleontology, Vol. 20 (3), pp. 628-631

Smith, J.B. (2005) "Heterodonty in Tyrannosaurus rex: Implications for the Taxonomic and Systematic Utility of Theropod Dentitions." Journal of Vertebrate Paleontology, Vol. 25 (4), pp. 865-887

Van Valkenburgh, B.; Molnar, R.E. (2002) "Dinosaurian and mammalian predators compared." Paleobiology, Vol. 28 (4), pp. 527-543