Brink et al. 2015 - What does it tell us about phytosaurs?

So new tooth news seems to be coming at a rapid pace this year! Several tooth-related papers have come out already and we're barely past the halfway mark. Most recently we have Brink et al. (2015) discussing funky features in meat-eating dinosaurs and, tangentially, other archosaurs. While I obviously love theropods and dinosaurs in general my interest at the moment is with non-dinosaurian archosaurs like our strange Chinle friends from Comb Ridge.

Duane Nash already did a good breakdown of what the article means in terms of theropod dinosaurs and how to relate the findings of Brink et al. to modern correlates as well as exploring what they could mean in terms of feeding and prey capture methods in various dinosaurs. If you haven't read his blog I'll wait.

Okay. Back? Good. As you can tell from both the article and the blog Brink et al. reject the stress-induced formation hypothesis for these interdental folds, as has been suggested previously. Instead they find that these structures are present even before stresses are placed on the teeth - while the unerupted teeth are still in the alveoli. So what does that have to do with Triassic teeth?

If you read the article you will see they sampled a few non-dinosaurian taxa (a phytosaur and an indeterminate Cretaceous croc) as well as the Triassic theropod Coelophysis. We have an abundance of phytosaur teeth at Comb Ridge and have picked up a few teeth we have tentatively IDed as theropod. So not only is Brink et al. a cool paper, it deals with some of our Triassic friends too!

Two views of phytosaur teeth in SEM and thin section, both from Brink et al. (2015), CC-BY
Image C shows mesial denticles under SEM and thin section. D shows a thin section with enamel, globular dentine, and primary dentine.

Brink et al. are looking mainly at the evolution and development of the structures with limited discussion on how the structures would have directly influenced prey capture and processing (though Nash, linked above, goes into that more). One of the more interesting things to me to come out of this is that phytosaurs have interdental fold structures like theropods and unlike crocs, Spinosaurus, ominivorous animals like Troodon and pure herbivores like ornithischian dinosaurs. Brink et al. further state that these adaptations are best interpreted as ways to capture large prey and crush bone. When we talk about phytosaurs, though, most people tend to interpret them as crocodile analogs. Sometimes this means perhaps ambushing large prey, other times preying on fish. This second option has been especially favored for the narrow-snouted forms, viewed by some as not robust enough to deal with large struggling prey.

A Redondasaurus attacks a decent sized prey item - a silesaurid. From Edyta Felcyn: go support her art!

There is some other evidence to suggest that phytosaurs were not just meekly eating fish and moderate-sized animals like dinosauromorphs (see image above). Coupled with their teeth that were perfectly adapted to ripping up large struggling prey items and mashing their bones, we have trace behavioral evidence to indicate this is exactly what happened. Last year Drumheller et al. documented a phytosaur attack on a living rauisuchian. You can read PastTime Podcast's take on the paper if you don't want to read through the paper itself. In short, though, they find evidence that a phytosaur tried to wreck shop on a rauisuchian, an animal that was basically a cross between a tyrannosaur and a crocodile. Wreck so much shop, in fact, that the phytosaur tooth went almost completely through the femur of the rauisuchian. This unfortunate fellow was then attacked by another rauisuchian and finally scavenged by a smaller phytosaur. Times were rough in the Triassic, even if you were the biggest, baddest fellow on the land.

Damaged psuedosuchian femur. The phytosaur attack is represented by the embedded tooth in Box A. Image from Drumheller et al. (2014).

If encounters like this were rare and the exception to the normal behavior of phytosaurs then the fossils described by Drumheller et al. are truly remarkable. Between the marked heterodonty found in adult phytosaurs described by Hungerbühler (2000) and the new evidence that they possessed dental adaptations that enabled them to capture, kill, and process prey larger than them it seems unlikely that this was a one-off chance encounter.

Ventral view of phytosaur snouts from Hungerbühler (2000). Note the different size and shapes of the teeth in this view.

 Instead our view of phytosaurs as fish-eaters occasionally attacking small-to-medium-sized land prey needs to be challenged. Phytosaurs were equipped with a dental battery that enabled them to routinely tackle large, dangerous, struggling prey as adults. This would include animals that were significantly larger than them. While juvenile phytosaurs seem to lack these dental adaptations (see, for example, my earlier post on this topic) and likely pursued prey smaller than themselves, adults would have been terrifying creatures to behold.

An interesting point to consider too: if phytosaurs were more like Nile Crocodiles than gharials, why don't we see ziphodont dentition in crocs? Certainly wildebeast and zebra don't give up after a fight. Brink et al. note that their Cretaceous croc also lacks ziphodont dentition, suggesting the behavior of crocs and their prey haven't changed much. Modern crocs are obviously capable of tackling large prey (though usually not larger than their own body). If they have gone hundreds of millions of years without the interdental folds and can eat large land prey, what were phytosaurs doing different?

Crocodiles and their prey in Africa - 2:57 from National Geographic

We don't have the fossils to answer that definitively but it would appear that modern crocodiles are not as good of an analogy for phytosaurs as has long been supposed. Hopefully future work at Comb Ridge and across Triassic collections will lead to new insights, clarifying what this unique clade was doing.

As an end note, Brink et al. suggest that ziphodont dentition with interdental folds is basal to all theropods, even thought phytosaurs possess the same tooth structure. It would have been nice to look at things like pseudosuchians from the Triassic to see if similar dental structure existed. If so, perhaps this sort of adaptation dates back to the rise of archosaurs in general. I guess that's another paper for another time.

Next up from me: a return to the lighter side. I'm going to be reviewing Richard Delgado's new Age of Reptiles comic series, Ancient Egyptians!

References:

Brink, K. S., Reisz, R. R., LeBlanc, A. R. H., Chang, R. S., Lee, Y. C., Chiang, C. C., ... & Evans, D. C. (2015). Developmental and evolutionary novelty in the serrated teeth of theropod dinosaurs. Scientific reports, 5.

Drumheller, S. K., Stocker, M. R., & Nesbitt, S. J. (2014). Direct evidence of trophic interactions among apex predators in the Late Triassic of western North America. Naturwissenschaften, 101(11), 975-987.

Hungerbühler, A. (2000). Heterodonty in the European phytosaur Nicrosaurus kapffi and its implications for the taxonomic utility and functional morphology of phytosaur dentitions. Journal of Vertebrate Paleontology, 20(1), 31-48.

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

A Toothy Issue

I am going to talk about teeth today. When I first knew I was going to get into paleontology I didn't think I would every really study teeth. I mean, teeth are neat and everything but I wanted to study dinosaurs! Dinosaurs, especially when I was younger, were mainly known for having relatively simple and easily-identifiable teeth that didn't tell us much besides diet. The only people who studied teeth were mammal paleontologists (which I foolishly looked down upon in my middle and high school years).

Even as I progressed through college I didn't pay much attention to teeth. Sure there were some odd teeth known from the Triassic Period, like Revueltosaurus and Tecovasaurus, but they were rare and the exception to the rule. I figured that they provided only marginal information on the ecosystem and that the major components were well known and understood - things like phytosaurs, metoposaurs, aetosaurs, and rare dinosaurs like Coelophysis. Well it turns out, unsurprisingly, that this view is naive and wrong.

Some of this change has come about from the work of Andy Heckert in the early years of this century. Although his treatise on Chinle microvertebrates is somewhat out of date now (it was published by the New Mexico Museum of Natural History and Science in 2004) it helped establish that the diversity of animals living in western North America was much higher during the Triassic Period than people had previously suspected. In addition to naming new taxa like Krzyzanowskisaurus, Protecovasaurus, and Crosbysaurus, his PhD work showed many new tooth types from the Chinle Formation and Dockum Group that had never been reported in the scientific literature!

Our work at Comb Ridge has focused on teeth. This is not because we set out to find lots of teeth. As with most things in paleontology you focus on what you find. At Comb Ridge we haven't found phytosaur skulls and troves of fossil fish like we do further north. We haven't found aetosaurs like in Arizona or mass graves of dinosaurs like in New Mexico. Instead we are finding teeth. Lots and lots of teeth. So many teeth that one locality, The Hills Have Teeth, may be the most productive microfossil site in Utah - it is certainly the most productive microsite in the Chinle of Utah. We have a dozen species represented, possibly more, from this one hill and they are all known from their teeth. So let's have a brief overview of tooth anatomy and terms so that it doesn't seem like I'm speaking gibberish in future posts.

Handy guide for some of the most common tooth terms I made based on an image from Lopez et al. (2015). Scale bar = 1 mm. CC-BY 4.0

List of Dental Anatomical Terms and Definitions

• Apex - the "top" or tip of a tooth; the portion furthest away from the gumline.
• Apical - a directional term, referring to things towards the apex.
• Asymmetrical - a tooth, viewed from the apex, that does not have the same profile on the lip-side as it does on the tongue-side.
• Base - the "bottom" of the tooth; the portion of the tooth at the gumline.
• Basal - a directional term, referring to things towards the gumline.
• Carina - a distinct ridge or edge, usually found along the leading or trailing edge of the tooth.
• Cingulum - a ridge, "waist", or "belt" of thickened enamel running around the tooth near the gumline.
• Circular - refers to a tooth that is circular in outline when viewed from the apex.
• Conical - a tooth that when viewed from the side has a roughly cone-shaped or pyramidal outline.
• Crown - the portion of the tooth from the gumline to the tip. What most people think of when they use the word "tooth."
• Denticles - triangular or angled protrusions along an edge used for cutting food. Can be angled towards the apex or facing perpendicular to the crown height. In some species these can be subdivided into smaller denticles.
• Dentine - the tough inner material that makes up most of a tooth. Very hard but not shiny.
• Distal - the part of the tooth facing the back of the mouth. In older literature this is sometimes referred to as "posterior."
• Enamel - the tough, shiny, outer surface of a tooth. A very hard material!
• Infolding - used to be commonly referred to as "labyrinthodont", which means "maze tooth." These are places on the tooth where the enamel is folded in towards the center of the tooth. It appears wrinkled.
• Labial - the side or portion of the tooth that faces the outside of the mouth. Labial literally means "lips."
• Laterally compressed - refers to a tooth that is much thinner "side to side" than it is "front to back" when viewed from the apex.
• Lingual - the side or portion of the tooth that faces the inside of the mouth. Lingual literally means "tongue."
• Mesial -  - the part of the tooth facing the front of the mouth. In older literature this is sometimes referred to as "anterior."
• Occlusal - the surface, face, or point of the tooth that would rub against ("occlude") the opposite tooth from the opposite jaw. Sometimes used in place of apical when referring to a viewing angle.
• Recurved - a tooth that, when viewed from the side, has the back (distal) side curved inward, so that the edge looks like a half-moon.
• Resorption pit - a pit on the base of a tooth, showing where bone and dentine were reabsorbed by the animal to allow the tooth to be shed.
• Root - in animals with teeth set into sockets, the root is the dentine that extends below the gumline into the jaw to anchor the tooth.
• Serrations - like on a steak knife, these are small notches on the edge of a tooth for cutting or slicing food.

Okay, so there are a number of terms there but I think I've given the definitions in terms that aren't too hard to follow for the average person. Let me show a few examples of teeth so I can sort of show how these terms are used "in the real world."

Crosbysaurus tooth. Scale distance = 1 mm.

The above picture is of part of a Crosbysaurus tooth from one of our sites at Comb Ridge. It shows denticles, the pointed cutting parts on the distal edge (or carina) of the tooth. Each of the pyramid-shaped structures has smaller bumps on them - these are the accessory denticles. This picture is in labial view.

Crosbysaurus tooth. Scale distance = 1 mm.

Here is another view of the same tooth. Here we are looking at the tooth in mesial view with the apex on the right and the base on the left. You can see a resorption pit at the base - it looks like the tooth is hollow. You can notice that this tooth is laterally compressed - it is much narrower than it is tall.

Archosauriform tooth. Scale distance = 1 mm.

Last example. Here is an archosauriform tooth in basal view. The front of the mouth, or mesial side, would be towards the right while the back of the mouth, or distal side, is to the left. You can see in this view that the tooth is asymmetrical - the labial and lingual sides are not equal. This picture also gives a decent view of the resorption pit located in the middle of the base here. That tells us that this is a shed tooth crown.

Thanks for making it through this! I know there were a lot of terms but I promise they will come in handy for many of my future posts. And now you can impress your dentist with your knowledge of dental terminology! The paleontology of teeth (Odontology) is not just for mammal paleontologists. All of this work with microfossils and Triassic teeth has certainly given me a new appreciation of how important these little things can be and what they can tell us about an ecosystem. Just what specifically can they tell us? That sounds like another blog post in its own right.