WASHINGTON — American researchers confirmed a fossil evidence that mammals, which have the biggest brains and produce some of the smallest litters of offspring, traded brood power for brain power during their evolution.
The study published on Wednesday in the journal Nature described fossil of an extinct mammal relative and her 38 babies, a rare one because it contains the only known fossils of babies from any mammal precursor.
The presence of so many babies or more than twice the average litter size of any living mammal revealed that it reproduced in a manner akin to reptiles, probably developing inside eggs or having just recently hatched when they died, according to the researchers from the University of Texas at Austin.
The findings may help reveal how mammals evolved a different approach to reproduction than their ancestors, which produced large numbers of offspring.
“These babies are from a really important point in the evolutionary tree,” said Eva Hoffman, who led research on the fossil as a graduate student at the University of Texas’s Jackson School of Geosciences.
“They had a lot of features similar to modern mammals, features that are relevant in understanding mammalian evolution,” said Hoffman, who co-authored the study with her adviser, Jackson School Professor Timothy Rowe.
The mammal relative belonged to an extinct species of beagle-size plant-eaters called Kayentatherium wellesi that lived alongside dinosaurs about 185 million years ago. Like mammals, Kayentatherium probably had hair, according to the study.
A 3D visualizations analysis 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. However, the brain is an energy-intensive organ, and pregnancy is an energy-intensive process.
The discovery that the mammal precursor had a tiny brain and many babies, despite otherwise having much in common with mammals, suggested that a critical step in the evolution of mammals was trading big litters for big brains.