A relative of today’s mammals that lived in the Jurassic offers clues about brain development

Kayentatherium wellesi adult with its babies (Image courtesy Eva Hoffman / The University of Texas at Austin)
Kayentatherium wellesi adult with its babies (Image courtesy Eva Hoffman / The University of Texas at Austin)

An article published in the journal “Nature” describes a research on the origin and evolution of mammals’ great brains. Eva Hoffman and Timothy Rowe of the University of Texas at Austin’s Jackson School of Geosciences examined fossils of Kayentatherium wellesi, a relative of today’s mammals that lived in the Jurassic period, concluding that a key moment in their evolution was a kind of trading brood power for brain power.

Big brains and a limited amount of offspring are two characteristics that differentiate mammals from other animals and there may be a connection between them. This idea gets some confermation thanks to the discovery of fossils of Kayentatherium wellesi, a tritylodontid cynodont, dating back to about 185 million years ago.

Timothy Rowe, now a professor at the Jackson School, collected the first fossils 18 years ago in Arizona. Initially it seemed a single specimen but in 2009 Sebastian Egberts, then a student of the Jackson School, discovered the traces of a group of babies. A CT scan uncovered a series of bones still in the rock and their extraction was risky. The consequence is that only the progress of the application of this type of examination to paleontological research over the years finally allowed to identify them accurately for a total of 38 babies.

It’s not clear whether the babies were already born or if they were still developing, perhaps within their eggs. In fact, cynodonts are part of the order of therapsids, which also includes groups of animals that had characteristics more similar to reptiles than to modern mammals including in some cases the laying of eggs. There are no traces of eggshells near the babies’ bones but it’s possible that they were of a type that hardly fossilized so their absence proves nothing.

The examination of the fossils revealed that the babies skulls were a smaller version of the adult’s. Again it’s a difference compared to mammals, whose puppies have characteristics a bit different that then change in the course of their growth to become those common to adults.

According to the researchers, pregnancy and the care of the offspring is a process that requires a lot of energy but the brain consumes a lot of energy as well so a species can either have many children or a big brain. In the Kayentatherium wellesi’s case, the fossils show a significant amount of babies and a small brain. In the subsequent millions of years, some species of mammals already had a much larger brain than this cynodont and at the same time few babies.

Timothy Rowe has been working for years on the reconstruction of the origin of mammals and the evolution of their characteristics publishing various research on the subject. The study of the fossils of Kayentatherium wellesi adds new information but the discussions will certainly include the previous ones.

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