Jurassic saw rapid 'burst' of mammal evolution, scientists say

An artist's impression of a docodont, the now extinct mammal that saw an explosion of skeletal and dental changes (including the special molar teeth that give them their name), in the Middle Jurassic.

The movie franchise Jurassic Park may centre on dinosaurs, but the era that gave the films their name was noted for the extremely rapid evolution of mammals, scientists say.

Mammals were evolving up to 10 times faster in the middle of the Jurassic than they were at the end of the period, coinciding with an explosion of new adaptations, new research shows.

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.

Oxford University researchers did the first large-scale analysis of skeletal and dental changes in Mesozoic mammals as they tried to discover how rapidly new body shapes emerged.

They discovered that mammals underwent a rapid burst of evolutionary change that reached its peak around the middle of the Jurassic (200-145 million years ago).

The team comprised researchers from Oxford University in the UK and Macquarie University in Australia. A report of the research is published in Current Biology.

"What our study suggests is that mammal 'experimentation' with different body-plans and tooth types peaked in the mid-Jurassic," said Dr Roger Close of Oxford University's Department of Earth Sciences, lead author of the report.

"This period of radical change produced characteristic body shapes that remained recognisable for tens of millions of years."

The team recorded the number of significant changes to body plans or teeth that occurred in mammal lineages every million years. During the mid-Jurassic the frequency of such changes increased to up to 8 changes per million years per lineage, almost ten times that seen at the end of the period. This is exemplified by therian mammals, the lineage leading to placental mammals and marsupials, which were evolving 13 times faster than average in the mid-Jurassic, but which had slowed to a rate much lower than average by the later Jurassic. This 'slow-down' occurred despite the increase in the number of mammal species seen in this later period.

The researchers say they don't know what set off the evolutionary burst, but that it could be due to environmental change, "or perhaps mammals had acquired a 'critical mass' of 'key innovations' - such as live birth, hot bloodedness, and fur - that enabled them to thrive in different habitats and diversify ecologically".

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