Mammals underwent a rapid 'burst' of evolutionary change that reached its peak around the middle of the Jurassic age around 145-200 million years ago, a study says.
The findings suggest that different body shapes and functions in mammals evolved rapidly during this period.
"What our study suggests is that mammal 'experimentation' with different body-plans and tooth types peaked in the mid-Jurassic," said study lead author Roger Close from Oxford University.
Early mammals lived alongside the dinosaurs during the Mesozoic era (252-66 million years ago).
They were once thought to be exclusively small nocturnal insect-eaters, but fossil discoveries of the past decade - particularly from China and South America - have shown that they developed diverse adaptations for feeding and locomotion, including gliding, digging, and swimming.
To find out when and how rapidly these new body shapes emerged, the researchers did a large-scale analysis of skeletal and dental changes in Mesozoic mammals.
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By calculating evolutionary rates across the entire Mesozoic, they showed that mammals were evolving up to ten times faster in the middle of the Jurassic than they were at the end of the period.
"Once high ecological diversity had evolved, the pace of innovation slowed," Close said.
Multituberculates, for instance, saw radical changes to their skeletons and teeth during the mid-Jurassic. However, by the end of the period they had evolved their rodent-like body shape and distinctive teeth, a form that, despite diversifying into hundreds of different species, they would generally retain until they went extinct around 130 million years later.
"In the Jurassic, we see a profusion of weird and wonderful bodies suddenly appear and these are then 'winnowed down' so that only the most successful survive," Close said.