Compared with the rest of the animal kingdom, mammals have the biggest brains and produce some of the smallest litters of offspring. A newly described fossil of an extinct mammal relative - and her 38 babies - is among the best evidence that a key development in the evolution of mammals was trading brood power for brain power.
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| A figure representing the 38 Kayentatherium babies found with an adult specimen, which researchers think is their mother [Credit: Eva Hoffman/ The University of Texas at Austin] |
The study, published in the journal Nature, describes specimens that researchers say 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 UT Jackson School of Geosciences. "They had a lot of features similar to modern mammals, features that are relevant in understanding mammalian evolution."
Hoffman co-authored the study with her graduate 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.
A 3D visualization of the skull of a Kayentatherium baby [Credit: Eva Hoffman, The University of Texas at Austin]
Sebastian Egberts, a former graduate student and fossil preparator at the Jackson School, spotted the first sign of the babies years later when a grain-sized speck of tooth enamel caught his eye in 2009 as he was unpacking the fossil.
"It didn't look like a pointy fish tooth or a small tooth from a primitive reptile," said Egberts, who is now an instructor of anatomy at the Philadelphia College of Osteopathic Medicine. "It looked more like a molariform tooth (molar-like tooth) -- and that got me very excited."
A CT scan of the fossil revealed a handful of bones inside the rock. However, it took advances in CT-imaging technology during the next 18 years, the expertise of technicians at UT Austin's High-Resolution X-ray Computed Tomography Facility, and extensive digital processing by Hoffman to reveal the rest of the babies -- not only jaws and teeth, but complete skulls and partial skeletons.
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| The fossils of the mother and babies belonged to Kayentatherium, a close relative to mammals [Credit: Eva Hoffman, The University of Texas at Austin] |
The brain is an energy-intensive organ, and pregnancy -- not to mention childrearing -- is an energy-intensive process. The discovery that Kayentatherium 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," 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, Rowe 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," he said.
Source: University of Texas at Austin [August 29, 2018]
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