Earth's earliest mammals spent their lives at a more leisurely pace than their modern counterparts, but they lived a lot longer, analysis of some 200-million-year-old teeth has shown.

Using X-ray technology, palaeontologists studied the fossilized teeth of some of Earth's earliest mammals, Morganucodon and Kuehneotherium, shrew-sized insectivores that walked the earth alongside early dinosaurs during the Early Jurassic marine transgression, in what is now Glamorgan in South Wales.

Experts from the UK's University of Bristol and Finland's University of Helsinki studied growth rings in the mammals' tooth sockets, discovering the animals lived for up to 14 years -- surprising, given similarly sized modern-day animals, such as mice and shrews, have a lifespan of between one and two years in the wild.

The tiny mammals had fallen into caves and holes in the rock, where their skeletons and teeth fossilized and remained well preserved.

"We digitally reconstructed the tooth roots in 3-D and these showed that Morganucodon lived for up to 14 years, and Kuehneotherium for up to nine years. I was dumbfounded as these lifespans were much longer than the one to three years we anticipated for tiny mammals of the same size," Dr Elis Newham, Research Associate at the University of Bristol, said in a statement.

"They were otherwise quite mammal-like in their skeletons, skulls and teeth. They had specialized chewing teeth, relatively large brains and probably had hair, but their long lifespan shows they were living life at more of a reptilian pace than a mammalian one," Newham said.

"There is good evidence that the ancestors of mammals began to become increasingly warm-blooded from the Late Permian, more than 270 million years ago, but, even 70 million years later, our ancestors were still functioning more like modern reptiles than mammals," he added.

Newham said it was previously thought that the key characteristics of mammals -- including their warm-bloodedness -- all evolved at the same time.

"By contrast, our findings clearly show that, although they had bigger brains and more advanced behavior, they didn't live fast and die young but led a slower-paced, longer life akin to those of small reptiles, like lizards."

Still, while the mammals had a reptilian pace of life, experts found evidence of sustained exercise in the bone tissue -- which contained blood vessels and fat -- of the early mammals.

"We found that in the thigh bones of Morganucodon, the blood vessels had flow rates a little higher than in lizards of the same size, but much lower than in modern mammals. This suggests these early mammals were active for longer than small reptiles but could not live the energetic lifestyles of living mammals," Newham added.

The research was published Monday in the journal Nature.

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