Archosauriform Tooth - New Preprint

As promised, I am back with more tooth news!

My students and I just published an updated version of our preprint describing an unusual archosauriform tooth from the Chinle Formation of Comb Ridge. In this preprint we describe a small, serrated tooth that one of my student co-authors discovered as float in May of 2014. While this article is not peer-reviewed it was submitted for review last week with a few minor changes from the preprint. I caught a few things from my students I had missed before, like calling semionotiform fish tetrapods.

What is the significance of this tooth? In addition to it being the result of my high-school students' fieldwork, this rather plain-looking tooth is somewhat unusual.

MNA V10668. Image from Lopez et al., 2015. A lingual B labial C distal D mesial E apical F basal. Scale bar = 1 mm. CC BY-4.0

At first glance the tooth appears to be relatively nondescript. It is triangular in profile with a slight labial curvature (meaning the tip is deflected towards the center of the mouth). It isn't too wide at the base and is not recurved. All in all, a pretty standard tooth.

A further look at it tells a different story. When my students looked at it and compared it to other Triassic teeth they noticed several differences. It has more serrations on the distal carina than most of the other reported taxa from the Chinle. It is labiolingually compressed, much more than a phytosaur but much less than a dinosauromorph.

My prompt to the students was relatively simple; identify this tooth to the most exclusive group you can. My students spent lots of time describing and comparing MNA V10668. A couple of my students were very stressed out but came through with useful comparisons, as I mentioned above (and detail in the paper).

One thing that was not adequately done in the first draft of the manuscript was a comparison with phytosaurs. The students, including ones who didn't become authors, were either A) not very good at elucidating the similarities and differences between MNA V10668 and phytosaurs or B) didn't attempt to do so at all. This was a problem since no doubt any reviewer would immediately ask to see why we thought this tooth was different from phytosaur teeth. Now adult phytosaurs were easy to distinguish from: they are quite a bit larger than MNA V10668.

Machaeroprosopus skulls at the New Mexico Museum of Natural History. CC-BY 2.0, created by Lee Ruk. No scale is provided but the skull is certainly longer than 1 meter.

Distinguishing from juveniles created a different problem. Juvenile phytosaurs are not as well known; those that have been identified in collections are usually not mentioned or poorly described in the literature. Fortunately the MNA has two juvenile phytosaurs in their collections that helped me address that problem: PEFO 13890/MNA V1789, a paired set of juvenile premaxillae and MNA V3601, a terminal right dentary. Both have teeth and alveoli that are the right size to address the question of whether MNA V10668 came from a phytosaur.

Juvenile phytosaur jaws. Top: PEFO 13890/MNA V1789, Macheroprosopus zunii premaxillae in A) ventral view. Bottom: MNA V3601 right dentary in B) lateral C) dorsal views. Scale bar = 1 cm. From Lopez et al. (2015), CC-BY 4.0

While these are not complete sets of dentition you can get a good idea as to what the teeth of juvenile phytosaurs would have looked like. Generally the bases were circular, not laterally compressed like MNA V10668. The teeth that are present in these specimens are all conical. Some, in MNA V3601, lack serrations. This allows us to feel reasonably certain that MNA V10668 doesn't come from a juvenile phytosaur. Our conclusions would be more solid if we had more preserved dentition from the posterior portion of the jaw, especially since this is the part of adult jaws that have teeth that look more like our specimen. None-the-less it is pretty clear that the juvenile jaws are less specialized than adults in their respective tooth positions - it seems reasonable to suggest that posterior teeth are also conical. This would also be in line with some modern archosaurs and their different juvenile/adult diets. Having conical teeth would help juvenile phytosaurs capture insects and other small prey while adults exhibit heterodonty, allowing them to efficiently process large prey items.

In any case, it appears pretty clear to my students (and myself) that MNA V10668 represents something other than a phytosaur. For that matter, it doesn't correspond to any other identified taxon from the Chinle Formation. Is it unique enough to name a new taxon off of? I don't think so. I admit this is a subjective call, but since the concept of a species in a paleontological sense is subjective anyway I don't see a problem there. In any case it is not like any other identified animal tooth from the Triassic of the southwest.

References
Lopez A, St. Aude I, Alderete D, Alvarez D, Aultman H, Busch D, Bustamante R, Cirks L, Lopez M, Moncada A, Ortega E, Verdugo C, Gay RJ. (2015) An unusual archosauriform tooth increases known tetrapod diversity in the lower Chinle Formation (Late Triassic) of southeastern Utah. PeerJ PrePrints3:e1539 https://dx.doi.org/10.7287/peerj.preprints.1110v2