After Dinosaurs, Mammals Put Brawn Over Brains

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Hand holding the palm-sized fossil skull of a mammal. Cranium of the Paleocene mammal Arctocyon primaevus, a carnivorous predator most closely related to the group including living pigs, sheep and other even-toed ungulates.
© Thierry Smith, Royal Belgian institute of Natural Sciences

When non-avian dinosaurs died out 66 million years ago, mammals persisted. But a new study shows that this group didn’t go unchanged: in the first 10 million years following the mass extinction event, mammals bulked up, rather than evolving bigger brains, to adapt to the dramatic changes in the world around them.

“Body sizes increasing so much faster than their brains completely changes our understanding of mammal evolution after the mass extinction,” said John Flynn, the Museum’s Frick Curator of Fossil Mammals and a co-author on the new study, which is published in the journal Science today and is led by researchers at the University of Edinburgh. “This novel and surprising insight is only possible through a combination of new fossil discoveries, studying specimens in museum collections, and a large collaborative research team applying advanced CT imaging and statistical analysis methods.”

Mammals first appeared at least 170 million years ago and lived among dinosaurs until a mass extinction event following a catastrophic asteroid impact killed off all dinosaurs except birds.

Previously, it had been widely thought that mammals’ relative brain sizes increased over time in the wake of the wipeout. But the new findings show that compared with their body weight, the size of mammals’ brains initially decreased after the extinction.

The researchers came to this conclusion by performing detailed CT scans in the Museum’s Microscopy and Imaging Facility and elsewhere on a combination of newly discovered fossils and previously known specimens in museum collections from the 10-million-year period immediately after the extinction, called the Paleocene.

Their findings reveal that the relative brain sizes of mammals at first decreased as their body size unexpectedly increased at a much faster rate.

Images of two fossil skulls, with images below each indicating braincases in purple.
Crania and virtual endocasts inside the translucent cranium of the Paleocene Arctocyon primaevus (left) and of the Eocene mammal Hyrachyus modestus (right) show the sizes of each animal’s brain, marked in purple
© Ornella Bertrand and Sarah Shelley

Results of scans also suggest the animals relied heavily on their sense of smell, and that their vision and other senses were less well developed. This suggests it was more important to be big than highly intelligent in order to survive in the post-dinosaur era at first, the team says.

Around 10 million years later, early members of modern mammal groups such as primates and carnivores began to develop larger brains and a more complex range of senses and motor skills. These adaptations would have improved their survival chances at a time when competition for resources was far greater, according to the team.

“Large brains are expensive to maintain and, if not necessary to acquire resources, would have probably been detrimental for the survival of early placental mammals in the chaos and upheaval after the asteroid impact,” said lead author Ornella Bertrand of the University of Edinburgh’s School of GeoSciences. Bertrand was an exchange student in 2009 through the Museum’s Richard Gilder Graduate School Annette Kate Fellowship Program.

Because today’s mammals are so intelligent, it is easy to assume that big brains helped our ancestors outlast the dinosaurs and survive the mass extinction–but that was not so, according to the new study.

“The mammals that usurped the dinosaurs were fairly dim-witted, and only millions of years later did many types of mammals develop bigger brains as they were competing with each other to form new ecosystems,” said senior author Steve Brusatte, a professor at the University of Edinburgh and a 2012 graduate of the Museum’s collaborative Ph.D. program with Columbia University.

Jin Meng, curator-in-charge for fossil mammals in the Museum’s Division of Paleontology, also is an author on the study.