A new species of prehistoric mammal, a chipmunk-sized insect-eater found in China, is revealing more about how mammals' tiny, complex ear bones evolved, scientists say.
Maotherium asiaticus lived in the Cretaceous period, about 123 million years ago, a time when life on land was dominated by dinosaurs.
Maotherium was a small, nocturnal mammal, about 15 centimetres long and weighing about 80 grams, that fed on insects and worms, according to the study published in this week's Science.
"What is most surprising, and thus scientifically interesting, is this animal's ear," study co-author Zhe-Xi Luo of the Carnegie Museum of Natural History said in a statement.
Mammals have a middle-ear structure that is unique among vertebrates. The sensitive hearing seen in mammals (including humans) is made possible by three of the tiniest bones in the body: the hammer, the anvil and the stirrup.
These bones, along with the eardrum, are believed to have evolved from the jaw hinge of a common ancestor of mammals and reptiles, although how the bones separated from the jaw and moved to the ear is not well understood.
Sensitive hearing was critical for small mammals living at the same time as dinosaurs, allowing them to be active at night.
The Chinese and American scientists who examined the fossils of Maotherium say its middle-ear bones are somewhat similar to those of modern mammals, except for an unusual connection to the animal's lower jaw.
The connection is seen in the primitive ancestors of mammals, in the embryonic stage of modern mammals and in modern-day adult mice with a certain genetic mutation.
The researchers say examining the well-preserved fossils will help biologists understand how complex structures, such as the middle ear bones, can arise in evolution.