"Why does science have to name every little thing?"

One of the most common questions I got when I was teaching as some variation on that theme. "Why do scientists have to make things so complicated?" "Why can't they just call it something simple?" It is a question bigger than just high school level as well; one of the barriers to effective science communication and education seems to be the general "fear" of overly technical language . The general public seems to view scientists as speaking in convoluted and complex terms.

It is true that scientists have devised some very complex ways of describing things that might seem simple to a layman. And sometimes we can get wrapped up in using the terms we are familiar with when talking about our research, to the detriment of any non-technical audience. The media is also partially to blame as well, with perpetuating ideas such as all prehistoric reptiles are dinosaurs. But why do these terms exist in the first place?

This brings me to Hendrickx et al. (2015). Hendrickx and his coauthors published a paper last year breaking down theropod dinosaur teeth and analyzing many different aspects of their morphology. In addition, the authors created a standardized terminology for future paleontologists to use when describing their dinosaur teeth. Hendrickx and his coauthors explain why creating such a terminology is needed succinctly; in effect answering the question of why scientists create names for so many things.

...several pivotal theropod taxa with well-preserved dentitions still lack a thorough dental description...leading numerous authors to identify isolated theropod teeth to broad clades with uncertainty...isolated teeth are key pieces of evidence to assess vertebrate paleoecological diversity and are often used for stable isotopic studies with various applications...A better understanding of theropod anatomy and morphological variation is therefore central to help resolving systematic relationships and to provide paleoecological clues. Tooth morphology is tied to diet, which has extensive evolutionary repercussions, such as morphological convergence, more than other parts of the skeleton. Yet, theropod teeth have been shown to possess many diagnostic features of taxonomic value...Although theropod teeth seem simple at first sight, this is effectively a result of the absence of comprehensive studies on tooth anatomy and morphological variation among theropods, as well as the lack of a uniform anatomical nomenclature.

What does the wall of text mean? Basically, theropod dinosaur teeth can be used to study evolutionary relationships, paleoecology, and several other important things in paleontology, but no one has bothered to come up with a good way to talk about them.

That's the crux of scientific terminology; coming up with a good way to talk about things. Good, in this case, means usable. Terms should describe well-defined parts of an organism's anatomy. If we say, "the tip of the tooth," on a tooth that has multiple "tips", how are we to know which specific tip we're talking about? Are there differences between "wrinkles" and "grooves?"

Figure 1 from Hendrickx et al., 2015

Without understanding the distinctions between subtle anatomical differences in different taxa, how are we to find out if these features are actually taxonomically important? A quick example from the Morrison Formation. Here in western Colorado we have basically two relatively common large theropods from the Morrison: Allosaurus and Ceratosaurus. Skeletal remains of Allosaurus, including teeth, seem to dominate in the Morrison making up 3/4 of all the theropod remains (Foster, 2007). Teeth attributed to Ceratosaurus do turn up in the field, however, and are usually distinguished by the presence of ridges near their bases. Now that we have Hendrickx et al.'s paper, we can go into a bit more depth. We can say, for instance, that the teeth attributed to Ceratosaurus have basal fluting, and these flutes are not seen in the contemporaneous Allosaurus. So this may help us distinguish between these teeth in the field and keeps us from mistaking Ceratosaurus teeth (with their flutes) with wrinkled or ornamented teeth (or tooth fragments).

The description of Ceratosaurus teeth by previous authors, however, has been lacking in detail and confusing, often using different terms for the same anatomical feature. As Hendrickx et al. note, having their framework in place will help facilitate such a description and they specifically mention Ceratosaurus as being in need of such a redescription. Hopefully such a project will be forthcoming. This topic will also be the focus of my next blog post!

Going forward I am hoping to see a theropod-wide tooth catalog. While Hendrickx et al. do point out that teeth are quick to change, evolutionarily speaking, to changes in diet and feeding behavior, they also note the taxonomic utility of teeth. While many theropod teeth can't be narrowed down to a genus or species, being able to address higher-level taxonomic questions with teeth is important. In addition, some taxa appear to have diagnostic dental modifications. Doing systematic studies and descriptions of theropod teeth may yield more information on what characters are taxonomically useful and potentially add autapomorphies to established genera.

My biggest complaint is that the authors did not examine what a theropod tooth is. They identify problems with past work, the utility of teeth, and the need for a framework but there is no way to determine if this framework is applicable to a given tooth. Obviously for teeth attached to theropod jaws this isn't a problem, but the majority of the dental fossil record for archosaurs consists of isolated shed teeth. While workers in the Cretaceous and Jurassic strata have this problem to a lesser degree (though it is possible that some crocodylomorphs developed similar tooth morphologies), those of us working in the Triassic are confronted with a host of dental convergences! One need look no further than the saga of Revueltosaurus to find examples of teeth that look similar between widely divergent clades. In the Triassic there are plenty of carnivorous reptiles, many with laterally compressed teeth. While in truth the terms developed by Hendrickx et al. (2015) are likely to be broadly applicable, a brief discussion of what synapomorphies exist among the dentition of theropods would have been appreciated, so that those of us working under all that overburden could sort our rauisuchian teeth from our dinosaur teeth just a little easier.

Works Cited

Foster, John. 2007. "Allosaurus fragilis". Jurassic West: The Dinosaurs of the Morrison Formation and Their World. Bloomington, Indiana: Indiana University Press. pp. 170–176

Hendrickx, C., Mateus, O. and Araújo, R., 2015. A proposed terminology of theropod teeth (Dinosauria, Saurischia). Journal of Vertebrate Paleontology,35(5), p.e982797. 

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.