While dinosaursruled the daytime, the earliest mammals may have prowled the night, guided by an elaborate sense of smell that set them apart from their ancestors and paved the path to modern mammals.
By recreating the size and shape of the brains of two ancient great-uncles of modern mammals, researchers, whose work is published today in the journal Science, find that the initial boost in mammalian brain size came largely from a bigger olfactory system.
"One of the things about mammals that's unique is that they have huge brains. There are no other animals that have brains that are so big," said Timothy Rowe of the University of Texas at Austin, who led the new study. "That's been one of the questions: Why, how and when did the brain get so large and what's driving that?"
His team found the answer by scanning tiny fossilized skulls of two 190-million-year-old mammal relatives found in China using high-resolution CT scanners similar to those used in medicine, but with more intense X-rays.
The CT scans can detect the precise contours of the skull's insides without destroying them, allowing researchers to make virtual molds of the minuscule brains.
When the researchers looked at the more ancestral species, Morganocudon, they found its brain was 50 percent larger relative to its size when compared with its reptilian ancestor. Most of the size increase, they found, came from a much bigger olfactory system.
The other fossil skull, from Hadrocodium, a paperclip-sized animal that is more closely related to modern mammals, showed a further 50 percent increase in relative brain size, again with much attributable to a more refined sense of smell.
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