In a paper published in the journal Science, researchers say they also noticed enlargement in the areas of the brain that correspond to the ability to sense touch through fur, a sense acutely developed in mammals.
Zhe-Xi Luo of the Carnegie Museum of Natural History in Pittsburgh was part of a team that used high-resolution CT scans to study rare 190-million-year-old fossil skulls of Morganucodon and Hadrocodium, two of the earliest known mammal species.
The scans revealed the tiny mammals from the Jurassic fossil beds of China had much larger brains than expected for specimens of their period, a Carnegie release reported Thursday.
"Our new study shows clearly that the olfactory part of the brain and the part of the brain linked to tactile sensation through fur were enlarged in these early mammals," Luo said. "A sophisticated sense of smell and touch would have been crucial for mammals to survive and even thrive in the earliest part of our evolutionary history.
"I have spent years studying these fossils, but until they were CT scanned it was impossible to see the internal details unless you were willing to destroy the skulls to look inside," Luo said. "I was absolutely thrilled to see the shape of the brain of our 190-million-year-old mammal relatives."
Even that long ago, the researchers said, the brains of the earliest mammals were notably large relative to body mass, with brain-to-body sizes approaching the proportions seen in modern mammals.