Shelf Life 13: Nothing But the Tooth

MICHAEL NOVACEK (Provost of Science and Curator, Division of Paleontology): The American Museum of Natural History really first went to the Gobi in the 1920s in a series of expeditions. And the original purpose was to actually find evidence for the origins and early evolution of humans.

It turned out that the expeditions were technically a failure, but in the midst of their effort to do that, they basically stumbled upon incredible deposits of dinosaurs and early mammals in the Gobi Desert.

We first returned to Mongolia in 1990 with basically three paleontologists from this Museum – myself, Mark Norrell, and Malcolm McKenna. We found enough on that first expedition to really excite us and ever since we’ve been going back to the Gobi.

I’m Michael Novacek. I’m the Provost of Science here at the Museum, but also a curator of paleontology. 

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PAÚL VELAZCO (Postdoctoral Researcher, Division of Paleontology): I’m Paúl Velazco. I’m a post-doctoral researcher in the Division of Paleontology.

In 2010 and 2011 a joint expedition by AMNH and the Mongolian Academy of Science recovered two tritylodonts. Tritylodonts are- They’re a group of cynodonts.

NOVACEK: It’s hard to describe what a cynodont is. A cynodont- We used to call them mammal-like reptiles because they had some signs of mammalian features – just a few clues in the skull. Now, we’re more inclined to call them reptile-like mammals. But they lie outside of what we call bona fide mammals. So, cynodonts are a sister relative or a sister lineage of mammals. They’re very ancient – over 200 million years – and there are some very strange adaptations in these animals. 

VELAZCO: So, these tritylodonts, they were a group of cynodonts. The smallest was the size of a small rat. The biggest probably was similar to a big domestic cat. It’s thought that tritylodonts, they lay eggs. They had a worldwide distribution and they were living alongside dinosaurs.

I study bats and when I study living] mammals, I have described several new species. And I describe them with tons of evidence. I have, like DNA evidence. You usually have access to big series of individuals from the same species. But when you have a fossil, sometimes, you only have one specimen. And not even one complete specimen. So, you have to describe every single bone, you have to describe angles, curvatures, and compare that with other fossils.  We have to use CT scanning, SEM photography. You have to do your best and try to be as detailed as you can.

Some fossils are known just by one or two teeth.

NOVACEK: We call them, derisively, spare parts. And I’ve worked on spare parts for a long time.  I, you know, basically had to learn how to really study teeth in order to work on these ancient creatures.

VELAZCO: Just by studying the shapes, sizes, and forms in the teeth you can have an idea of how the relationship between the groups of mammals came about.

One of the most important characteristics that distinguishes our specimens from other tritylodontid species resides in the teeth.

Tritylodonts’ teeth, they are really- really amazing. That’s where the name tritylodont comes about. The upper teeth have three rows of cusps that face forward. And the lower post-canines, they’re comprised of, like, two rows of cusps that face backwards. And they were working like this, you know, to shred the vegetation. 

The combination of numbers between each row is called the cusp formula. It’s unique for each genus. And we found that our two fossils, are different from other tritylodontids. and each other.

NOVACEK: I was so excited looking at these tritylodonts with Paul. It appears that these species that we’ve discovered are new genera. They’re completely new lineages.

VELAZCO: To just base the whole description or the whole set of characteristics that will define this genus on one specimen, there’s more pressure to do a better job. 

NOVACEK: There’s an enormous amount of curiosity that is inspired by looking at something that’s so strange And in a sense, these things are alien. They’re from another world. It’s our world, but we’re talking about 80 million years ago. And so, you immediately transport yourself there. Because you’re saying, “Well how did this thing live? What was it related to? What was a day in the life for the tritylodont?” They’re very distinctive animals.