Fossil Friday: Kayentatherium

Species: Kayentatherium wellesi (Kah-yen-tah-thee-ree-um wells-eye)

What it means: Kayenta beast

Other Species: none

Where I live: Arizona in the U.S.A.- The Kayenta formation

When to find me: The Early Jurassic period, about 196 million years ago.

My favorite food: Plants! I’m an herbivore.

My neighborhood: The Kayenta formation used to be a tropical floodplain, a bit like African savannah today- but no grass or flowers. Ferns cover the open plains, dotted with islands of spiky cycad groves. Rivers crisscross the land with lush tree ferns, ginkgo trees, and conifers. Every year during the wet season the plains turn into a flooded marsh, but the hottest months bring no rain, and the rivers shrink until the plains are almost as dry as the great desert that lies to the north.

A few of my neighbors: I’m surrounded by meat-eating dinosaurs like Dilophosaurus, Kayentavenator, and Coelophysis, and plant-eating dinosaurs like Sarahsaurus (an early sauropod) and Scutellosaurus (armored dinosaur). I have to be careful not to get stepped on, or end up as lunch for crocodile cousins the prowl the rivers. In the sky, flying pterosaurs with long tails keep an eye out for insects like beetles, dragonflies, an ancient cousin of the moth, and something called a snakefly. While frogs and turtles swim with the fish in the river.

Fun Facts:

  • I look a little like weird beaver (or capybara!) , but I can lay 38 eggs in a single clutch! I’m from a group of animals called cynodonts, and the kits are ready to explore the world as soon as they hatch!
  • You’ll see a lot of prehistoric mammals and mammal-ish things with therium at the end of their names. Therium is the ancient Greek word for beast. My name, like many other prehistoric critters, just means Beast from the Kayenta Formation. Kayenta-therium. Kayenta beast. 🙂
  • My species name, wellesi, is in honor of the paleontologist Samuel Welles. It’s pretty common to do this with species names.
  • Some paleontologists think I liked to be around the water a lot, a little like a modern day muskrat or capybara.

Fossil Finds: Several individuals, including fragmentary teeth and skull, and the head and body of an incomplete (but beautifully preserved) individual that includes 38 tiny babies with jawbones only 1cm long. Paleontologists used a microCT scanner to get a 3D x-ray of the delicate fossils.


KERMACK, D.. (2008). A new tritylodontid from the Kayenta formation of Arizona. Zoological Journal of the Linnean Society. 76. 1 – 17. 10.1111/j.1096-3642.1982.tb01953.x.

Sues, Hans-Dieter & F. A. Jenkins. 2006. The Postcranial Skeleton of Kayentatherium Wellesi from the Lower Jurassic Kayenta Formation of Arizona and the Phylogenetic Significance of Postcranial Features in Tritylodontid Cynodonts in: Carrano, Matthew T., Gaudin, T. J., Blob, R. W. and Wible, J. R., Amniote Paleobiology: Perspectives on the Evolution of Mammals, Birds, and Reptiles: The University of Chicago Press. pp. 114-152.

Pickrell, John (2019-10-23). “How the earliest mammals thrived alongside dinosaurs”. Nature.

10 thoughts on “Fossil Friday: Kayentatherium

  1. Kayentatherium is one of the earliest examples of semi-aquatic specialism in mammaliamorphs in the mammal fossil record. It deserves credit for that. It appears to have laid eggs like a platypus. Makes me wonder if a platypus should be considered a mammaliamorph instead of a mammal. And why would a platypus survive extinction but not the Kayentatherium.

    Liked by 1 person

    1. Hi! It’s good to see you here. Thank you for stopping by 🙂

      Kayentatherium was a Tritylodontid cynodont, a group of highly specialized animals that were the last known non-mammalian synapsids.

      Basically, it has everything to do with what traits or “characters” are present in the skull, mostly, but also in the rest of the skeleton. Synapsids are all the animals that are moderately like mammals based on a single feature in the skull called a temporal fenestra. It’s a hole in the skull behind the eye orbit. A huge range of animals are in the group, including mammals, cynodonts like Kayentatherium, and critters like Dimetrodon. In fact, because Dimetrodon is a synapsid, it is more closely related to us than the dinosaurs it’s usually lumped with!

      Cynodonts were animals that possessed some mammalian traits and some reptilian traits, but many of them would be easily mistaken for mammals if we saw them walking around (except for the lack of external ears). Tritylodontid cynodonts like Kayentatherium are even closer to mammals, and are the last known group before true mammals appeared.

      A platypus is a monotreme, a true mammal that lays eggs. Like all true mammals, their young drink mother’s milk. She sweats it out for them to lick up. The platypus in particular comes from a specific group that appeared after the dinosaur extinction. There are a number of animals that superficially look a lot like a platypus, but it’s more because of convergent evolution and they are not direct relatives to modern platypus. The earliest known relative to the modern platypus is somewhere between 19-48 million years ago (according to Wikipedia), but the modern platypus as we know it today only appeared 100,000 years ago.

      I forgot to add the fun fact about Kayentatherium possibly being semiaquatic! I did mention it may have been a bit like a capybara, but I’ll edit the fun facts. 🙂


  2. I think Kayentatherium is a very underrated mammal. And I like your sir of “foreshadowing” based around it scientific name.

    This is a bit off topic, but I think it’s time to go update your Dinotoyblog Forum thread. It is just a reminder since you last updated it over a year as far as I’m aware.

    Regardless, great cynodont and the perfect animal to come after your Castorocauda COTM post.


    1. Thank you for stopping by Angel! I’m glad you like this little guy. I never knew there were mammal-ish critters this interesting so early in the fossil record. It’s pretty cool to find out about these animals that get no publicity. 😀

      Oh my, I really should update the forum. It’s been forever. I’ll see if I can get a routine down, because that really would be good to do. Thank you for the reminder!


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