Fossil hints at mammal evolution

The skull is just 12 millimetres long

A team of scientists from China and the United States has described a tiny fossil creature that could be one of the ancestors of modern mammals.

The shrew-like animal would have run under the feet of dinosaurs at the start of the Jurassic period, nearly 195 million years ago.

Hadrocodium could be our distant cousin

Dr Zhe-Xi Luo

The fossil was found in Yunnan province in China and has been given the name Hadrocodium wui, meaning Fullhead.

Full is a relative term; the animal weighed only about two grams - the same as the smallest land mammal living today. Its skull was just 12mm long

It probably ate the same sorts of things as today's shrews, said lead researcher Dr Zhe-Xi Luo of the Carnegie Museum of Natural History.

Fossil record

He told the BBC: "This was an insectivore It has very fragile teeth, and limited by its very tiny size, it was only capable of eating small insects, small worms, and not much of anything else."

But the animal had, for its size, a significantly bigger brain than the more primitive mammals already known from the early Jurassic.

Writing in the journal Science, Dr Luo and his colleagues say they are particularly excited by the creature's middle ear bones, which are separated from the lower jaw, a key evolutionary difference between mammals and reptiles.

The discovery of this important feature in Hadrocodium wui pushes back by another 40 million years its first appearance in the fossil record.

The researchers argue that an expanding brain in an evolving creature could force the ear bones to move apart.

The anatomy of the cranium in Hadrocodium wui suggests the animal used its newfound brain power for hearing and smelling; both areas of the brain are well-developed.

When it scurried about, the Earth's domination by dinosaurs still had many millions of years to run.

The ancestral mammals had yet to diverge into the present-day groups - placentals, like humans, horses and whales; marsupials, like kangaroos; and monotremes, such as the duck-billed platypus.

Dr Luo said: "Hadrocodium could be our distant cousin, an early mammal that existed alongside the ancestor of living mammals.

"Or it could be our great-great grand uncle, closely related to living mammals but not in our direct lineage.

"Or Hadrocodium could be the direct ancestor of living mammals. The fossil evidence can't distinguish between these three possibilities. But we are satisfied to know that Hadrocodium is the sister taxon (closest known relative) to all living mammals," said Dr Luo.