54-Million-Year-Old Skull Reveals Surprises about Early Evolution of Primate Brains

brain illustration

This image shows a translucent rendering of the brain as it would fit inside the skull of the 54-million-year-old primitive primate, Ignacius graybullianus.

C.K. Dalmyn

The CT scan technique has become an accepted method for fossil studies but most often involves medical scanners that are repurposed for fossils. By contrast, the primate brain study used a more powerful industrial-strength high-resolution scanner at Penn State University.

Claire Dalmyn, an undergraduate student at the University of Winnipeg and paper co-author, traced more than 800 X-ray images of the braincase. The effort took nearly a year and produced one of the best endocasts they had ever seen for an extinct mammal.

“I couldn’t believe what we were seeing,” Bloch said.

Primate brains tend to be dominated by cerebrum, the most highly evolved part of the brain, making it difficult to look at functional areas and forcing researchers to focus solely on brain size.

“Brain size is interesting, but it’s quite difficult to interpret because in one brain, 50 percent could be made up of olfactory bulbs, and in another brain, 50 percent could be made up of visual processing areas,” Silcox said.

In the primitive primate, the cerebrum had not yet evolved to the point where it covered all of the other functional regions of the brain. As a result, the relative sizes of different parts of the brain provide a better picture of brain function and the early stages of primate evolution.

The specimen came from north central Wyoming, near the entrance to Yellowstone National Park. The intact skull is a rare find that allowed the primate to be studied this way for the first time, Silcox said.

“This is very exciting in terms of the history of paleo-neurology, the history of the study of brains in the fossil record,” Silcox said.

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