An "explosion" of mammal evolution took place during the middle part of the Jurassic period with countless new adaptations, then it slowed down by the end of that time period, researchers say.

The Jurassic is the middle period of the so-called Age of Reptiles, the Mesozoic Era, when early mammals were living alongside dinosaurs.

Long believed to be only small nocturnal insect eaters, recent fossil discoveries suggest that in the central millennia of the Jurassic mammals rapidly developed a number of new adaptations in their methods of locomotion - including swimming, digging and gliding - and new feeding behaviors.

The evolutionary sprint took place between 200 and 145 million years ago, researchers say, at a rate 10 times that seen before or after that time.

 "What our study suggests is that mammal 'experimentation' with different body plans and tooth types peaked in the mid-Jurassic," says study leader Roger Close from England's Oxford University.

The most successful evolutionary experiments tended to last the longest, the researchers note.

"This period of radical change produced characteristic body shapes that remained recognizable for tens of millions of years," says Close, a researcher in Oxford's Department of Earth Sciences.

The researchers acknowledged they don't yet know what initiated this "burst" of evolution, suggesting possibilities including environmental changes or reaching a "tipping point" of innovations such as hot-bloodedness, live birth and insulating fur that would have allowed mammals to move into diverse habitats and ecosystems.

"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 says.

"Once high ecological diversity had evolved, the pace of innovation slowed," he suggests.

Similar events have occurred at different points of the timeline of life on Earth, the researchers point out.

One of the most significant is dubbed the "Cambrian explosion," starting around 542 million years ago, that saw the emergence of most major animal phyla, including complex organisms that led to most branches of modern species. 

"What we may have identified in this study is mammals' own 'Cambrian explosion' moment, when evolutionary experimentation ran wild, and the future shape of mammals was up for grabs," Close says.

The study, involving researchers from Oxford and Macquarie University in Australia, appears in the journal Current Biology.

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