Day #11 on our Christmas Countdown is…

Kayentatherium!

Kayentatherium: the “Beast of Kayenta”

Kayentatherium was a mammal-ish critter from the Kayenta Formation, which was during the Early Jurassic of what is now New Mexico.

It was about the size of a large cat, and though it looks like a mammal, it was part of a mammal-ish group called cynodonts. So what is a cynodont? In very basic terms, they are a group of animals that still shared a few traits with reptiles, like laying eggs and some far more complex things like certain bones in the skull, but also had many things in common with mammals like us.

Technically, you might say that all mammals (including humans) are cynodonts, but not all cynodonts are mammals. In the same way that one can say all birds are dinosaurs, but not all dinosaurs are birds.

Kayentatherium is interesting because it shows how cynodonts were just as specialized and varied as mammals are today. It looks like this critter would’ve spent a lot of time in the water or by the riverbank. Probably like modern muskrats or capybaras, munching on reeds, horsetails, and other plants along the water’s edge.

Those strong feet would be good for digging a nice little burrow in the soft mud too, which is good, because they had to watch out for predators like Dilophosaurus and Kayentavenator. Burrows are also very good for keeping babies safe for a short while, and this critter had a lot of babies.

One fossil find has mama with 38 of the tiny critters! Considering the size of the babies and the number, that makes it likely that they were inside eggs. Unlike most mammal babies, these tiny fossils have proportions very similar to the adult, so it’s possible that they were already ready to go out into the world on their own.

So does that mean that they were indeed inside eggs, and just about ready to hatch and head off into the big wide world? Or perhaps they had already been raised for a little while, and were soon to venture out on their own? It’s hard to tell without more fossils.

Either way, it’s safe to say that they were doing something quite different than mammals of today, and these fossils provide clues into the lives of a group of animals that no longer exists.

Do you think we should take inspiration from living mammals for ideas on how critters like this one might have lived? For example, mammals that lay eggs like the platypus, or perhaps plant-munching mammals that enjoy a similar lifestyle, size, and body plan…like capybara?

See you tomorrow for day 12 of the Critter Christmas Countdown!