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Mammals went through an 'explosion' of evolution while dinosaurs roamed the Earth

Mammals were evolving up to ten times faster in the middle of the Jurassic than they were at the end of the period, an Oxford University study says
Between 200 million and 145 million years ago, mammals developed a wide range of adaptations for feeding and locomotion including swimming
These adaptations remained in some species until they became extinct

Published: 11:55 EST, 16 July 2015 | Updated: 11:56 EST, 16 July 2015

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Just as dinosaurs were getting in to their stride 200 million years ago, mammals were going through what has been dubbed an 'evolutionary explosion'.

Rapid changes to their size and shape reached their peak in the middle of the Jurassic period, between 200 million and 145 million years ago.

And during this time mammals diversified rapidly, developing a wide range of adaptations for feeding and moving.

Mammals went through an explosion of evolution just as dinosaurs were getting into their stride, new research has shown. This illustration shows docodonts, now extinct mammals that saw an explosion of skeletal and dental changes, including the special molar teeth that give them their name, in the Middle Jurassic

Early mammals, such as steropodon, an ancestor of the platypus, and rat-like multituberculates lived alongside the dinosaurs during the Mesozoic era between 252 and 266 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 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, a team led by Oxford University researchers did the first large-scale analysis of skeletal and dental changes in Mesozoic mammals.

By calculating evolutionary rates across the entire Mesozoic, they found that mammals underwent a rapid 'burst' of evolutionary change that reached its peak around the middle of the Jurassic, 200 to 145 million years ago.

The rapid changes reached their peak in the middle of the Jurassic period, between 200 million and 145 million years ago. For example, therian mammals, the lineage leading to placental mammals and marsupials (a kangaroo is shown) w ere evolving 13 times faster than average in the mid-Jurassic

WHAT IS THE MESOZOIC ERA?

The Mesozoic Era is a large interval of time 252 to 66 million years ago.

It's known as the 'age of reptiles' because it was dominated by the dinosaurs.

The era is subdivided into three periods: the Triassic, Jurassic and Cretaceous, which are sub-divided further.

The era began after the Permian-Triassic extinction event, which is the largest mass extinction in the Earth's history and ended with the Cretaceous–Palaeogene extinction event, which killed off non-avian dinosaurs.

Non-avian dinosaurs appeared in the Late Triassic and became the dominant terrestrial vertebrates early in the Jurassic, occupying this position for about 135 million years until their demise at the end of the Cretaceous.

Now this study explains that mammals evolved particularly rapidly in the middle of the Jurassic period, between 200 million and 145 million years ago.

'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 such as kangaroos, 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.

'We don't know what instigated this evolutionary burst,' continued Dr Close.

'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.

'Once high ecological diversity had evolved, the pace of innovation slowed.'

For instance, multituberculates - a group of rodent-like mammals - saw radical changes to their skeletons and teeth during the mid-Jurassic.

But the evolution of therian mammals (a skeleton is shown in the Gobi Desert) slowed to a rate much lower than average by the later Jurassic. 'We don't know what instigated this evolutionary burst,' Dr Close said

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.

'This is characteristic of other 'adaptive radiation' events of this kind, such as the famous 'Cambrian explosion',' said Dr Close.

'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.

'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.'

The team involved researchers from Oxford University and Macquarie University in Australia and the findings are published in the journal Current Biology.