Tuesday, July 7, 2015

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 phytosaursmetoposaursaetosaurs, 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 Although his treatise on Chinle microvertebrates is somewhat out of date now (it was published by the New Mexico Museum of Natural History and Science in 2004) it helped establish that the diversity of animals living in western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 KrzyzanowskisaurusProtecovasaurusand 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.

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