In the wake of dinosaurs’ extinction, mammals got brawny before they got brainy.
— the immediately post-extinction epoch spanning 66 million to 56 million years ago — does reveal a flourishing menagerie of weird and wonderful mammal species, many much bigger than their Mesozoic predecessors. It was the dawn of the Age of Mammals.
Bertrand and her colleagues used CT scanning to create 3-D images of the skulls of different types of ancient mammals from both before and after the extinction event. Those specimens included mammals from 17 groups dating to the Paleocene and 17 to the Eocene, the epoch that spanned 55 million to 34 million years ago.
To assess how the sizes and shapes of those sensory regions also changed over time, Bertrand looked for the edges of different parts of the brains within the 3-D skull models, tracing them like a sculptor working with clay. The size of mammals’ olfactory bulbs, responsible for sense of smell, didn’t change over time, the researchers found — and that makes sense, because even Mesozoic mammals were good sniffers, she says.
To track changes in mammal brain size through time, researchers created tracings of the brain cases inside mammal skulls using CT scanning. At left is the cranium of the Paleocene mammal, with sensory regions including the olfactory bulbs and the neocortex highlighted in purple.
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