(CNN)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.
Earth's first mammals took it easy and lived far longer than their modern counterparts
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 t