Jurassic Mammal Precursor Had Tiny Brain

A team of paleontologists from the University of Texas at Austin has uncovered fossils of an adult female Kayentatherium wellesi — an extinct tritylodontid cynodont that lived alongside dinosaurs about 185 million years ago (Jurassic period) — and her babies (at least 38 individuals). The find is among the best evidence that a key development in the evolution of mammals was trading brood power for brain power.

An artist’s impression of a cynodont. Image credit: Jorge Blanco / A.G. Martinelli / M.B. Soares / C. Schwanke / CC BY-SA 4.0.

“The find is among the rarest of the rare because it contains the only known fossils of babies from any mammal precursor,” said Eva Hoffman and Professor Timothy Rowe from the Jackson School of Geosciences at the University of Texas at Austin.

“But the presence of so many babies — more than twice the average litter size of any living mammal — revealed that it reproduced in a manner akin to reptiles.”

Kayentatherium wellesi’s babies were probably developing inside eggs or had just recently hatched when they died.

“These babies are from a really important point in the evolutionary tree,” Hoffman said.

“They had a lot of features similar to modern mammals, features that are relevant in understanding mammalian evolution.”

The in-depth analysis of the specimens revealed that the skulls of the babies were like scaled-down replicas of the adult, with skulls a tenth the size but otherwise proportional.

This finding is in contrast to mammals, which have babies that are born with shortened faces and bulbous heads to account for big brains.

A figure representing 38 babies of Kayentatherium wellesi found with an adult specimen. Image credit: Eva Hoffman / University of Texas at Austin.

The discovery that Kayentatherium wellesi had a tiny brain and many babies, despite otherwise having much in common with mammals, suggests that a critical step in the evolution of mammals was trading big litters for big brains, and that this step happened later in mammalian evolution.

“Just a few million years later, in mammals, they unquestionably had big brains, and they unquestionably had a small litter size,” Professor Rowe said.

“The mammalian approach to reproduction directly relates to human development — including the development of our own brains.”

“By looking back at our early mammalian ancestors, humans can learn more about the evolutionary process that helped shape who we are as a species,” he said.

“There are additional deep stories on the evolution of development, and the evolution of mammalian intelligence and behavior and physiology that can be squeezed out of a remarkable fossil like this now that we have the technology to study it.”

The findings were published in the journal Nature.

_____

Eva A. Hoffman Timothy B. Rowe. Jurassic stem-mammal perinates and the origin of mammalian reproduction and growth. Nature, published online August 29, 2018; doi: 10.1038/s41586-018-0441-3