Analysis of 200 million-year-old teeth suggests mammals from that time functioned more like their cold-blooded counterparts.
The research was led by the University of Bristol and involved scientists from the University of Southampton.
Published in Nature Communications, it is the first time palaeontologists have been able to study the physiologies of early fossil mammals directly.
Fossils of teeth from two of the earliest mammals, Morganucodon and Kuehneotherium, were scanned for the first time using powerful X-rays.
This allowed the team to study growth rings in tooth sockets, deposited every year like tree rings, which could be counted to tell how long these animals lived.
The results indicated a maximum lifespan of up to 14 years – much older than their similarly sized furry successors such as mice and shrews, which tend to only survive a year or two in the wild.
Dr Elis Newham, now Research Associate at the University of Bristol, led this study that was conducted during his time as a PhD student at the University of Southampton.
He said: “We made some amazing and very surprising discoveries.
"It was thought the key characteristics of mammals, including their warm-bloodedness, evolved at around the same time”
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