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A Jurassic animal gives clues about the evolution of mammals

A team of scientists from the University of Austin Texas has managed to reconstruct a group of fossils that belong to a species of extinct animal along with its 38 offspring. This is a very important finding, since we are talking about an ancestor of the first mammals that, however, maintains characteristics more typical of reptiles. A very peculiar creature that lived with dinosaurs 185 million years ago, at the beginning of the Jurassic.

The species has been baptized as Kayentatherium wellesi and is included within the group of Mamaliaformes, a clade whose taxonomy is not yet clear and that encompasses the extinct precursors of current mammals.

To better understand why this new species is so important in the study of the evolution of our ancestors, it is necessary to remember that mammals are the animal group with the largest brain. The growth of this complex organ requires a significant energy investment and, for this, we have adopted a reproductive strategy that consists of having few offspring in order to concentrate all our efforts on their growth and survival. Other animals follow the opposite tactic: they have many children in such a way that, to put it in some way, it will be bad if some do not survive.

The passage from one strategy to another implies very important morphological and physiological changes and we hardly know when and how they occurred. That is why the discovery of K. wellesi is so important, because although the animal was surely covered in hair and has other common features with mammals , its abundant offspring as well as the proportions of its brain are even more typical of reptiles.

“These babies are a key point in the evolutionary tree,” explains Eva Hoffman, one of the authors of the work that has just been published in the journal Nature . “They had many characteristics similar to current mammals, and these traits are very relevant to understanding the evolution of the group.”

Brains to scale

Although the fossils were found 18 years ago in a rock formation in Arizona, their discoverers thought the remains belonged to the same individual. It was not until later, with the development of more precise technologies, when scientists found that the smallest bones corresponded to offspring whose skulls were also replicas on a reduced scale of the adult. This finding contrasts with the characteristics of mammals, which are born with more rounded heads and smaller faces to accommodate large brains at maturity.

The fact that the new species had a small brain and many young suggests that this critical step in mammalian evolution, which consisted of having smaller litters and larger brains, came later. “Only a few million years later we already find mammals with these characteristics,” says Timothy B. Rowe, another of the authors.

This change in mammalian reproduction is directly related to human development. “Now that we have the right technology, I think we can discover many fascinating aspects about the evolution of intelligence, behavior and physiology of mammals from the study of this fossil “, concludes the expert.

Reference: Hoffman and Rowe 2018. Jurassic stem-mammal perinates and the origin of mammalian reproduction and growth. Nature. DOI: 10.1038 / s41586-018-0441-3

Photos: Eva Hoffman / The University of Texas at Austin.

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