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Our mammal ancestors were cold-blooded for tens of millions of years after their first appearance. In this respect, the early mammals remained similar to the cold-blooded reptiles from which they evolved.

The finding comes from an analysis of fossils of two early mammal species, which suggests the animals lived relatively long lives and had slow metabolisms 鈥� both reptile-like traits.

Mammals are animals that have hair and make milk. The first mammals evolved during the dinosaur era. The oldest known mammal-like animals, like , lived about 230 million years ago in the Triassic period. By the middle of the next period, the Jurassic, true mammals were common.

However, one of the most crucial features of modern mammals leaves no obvious traces in the fossil record, so we don鈥檛 know when it evolved. All mammals are warm-blooded or 鈥渆ndothermic鈥�, meaning they can maintain a constant internal temperature. In contrast, cold-blooded animals like lizards cannot, and must sit in the sun to warm up or hide in the shade to cool down.

Hot-blooded?

To find out when warm-bloodedness evolved, Elis Newham at the University of Bristol in the UK and his colleagues studied two animals from 200 million years ago in the Early Jurassic, Morganucodon and Kuehneotherium. Both were mammaliaforms, meaning they may not have been true mammals: some view them as close relatives with mammal-like traits. Morganucodon聽(pictured above) looked like a shrew or mouse, while Kuehneotherium is only known from teeth and bone fragments.

Newham鈥檚 team studied the roots of the animals鈥� teeth. The roots had a hard coating called cementum, as ours do. New layers were added as the animal aged, so counting the layers gave an estimate of its age 鈥� a bit like counting tree rings.

Based on 61 specimens, the researchers estimated that Morganucodon could live 14 years and Kuehneotherium could live for nine. That is a long time for a small mammal: mice rarely live more than three years. However, it is not unusual for a small reptile like a lizard. It indicates that the animals had slow metabolisms. This suggests they were cold-blooded, as mammals need fast metabolisms to maintain their body heat.

The researchers also examined fossil thigh bones belonging to Morganucodon. By comparing the length of the bones to the area of openings inside them through which blood vessels once flowed, they could conclude that the animal had a low rate of blood flow through its thigh bones. This is more evidence of a slow metabolism and therefore cold-bloodedness.

Slow evolution

The findings fit with other studies of , says Rachel Wallace at the University of Texas at Austin. 鈥淭here really isn鈥檛 good indirect evidence for endothermy until more relatively recent mammalian taxa, where actual fur is preserved.鈥� The oldest mammal known to have fur is the beaver-like Castorocauda from the Middle Jurassic, about 164 million years ago.

The new finding is striking because, in other respects, Morganucodon and Kuehneotherium were mammal-like, says Eva Hoffman at Harvard University. For instance, Morganucodon had both a big brain and a modern-looking skeleton. 鈥淭hat these advanced mammaliaforms still lived more like reptiles than like mammals reflects a mosaic pattern of evolution, in which some ancestral features are retained even as novel features evolve,鈥� she says.

Hoffman says the protomammals may have been similar to the most primitive living mammals: monotremes like the duck-billed platypus, which, unlike other mammals, still lay eggs. 鈥淢onotremes,鈥� she says, 鈥渉ave pretty long lifespans and relatively low metabolic rates.鈥� There is also evidence that they are not fully warm-blooded.

鈥淚t seems to me that Morganucodon and Kuehneotherium may have had a lifestyle similar to that of living monotremes,鈥� says Hoffman. 鈥淚n this scenario, a 鈥榤ammalian鈥� physiology could have evolved in the Middle Jurassic at the earliest, and wouldn鈥檛 have been present in the last common ancestor of living mammals.鈥�