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Ancient mammal gives clues to ear evolution

21 March 2007

Scientists have discovered a mammal the size of a gerbil, living at the time of the dinosaurs 125 million years ago. It reveals clues about how mammal ears evolved from reptile jaw bones.

The fossilised remains of a primitive mammal reveals clues to ear evolution. © Nature/Z.Luo/CMNH

The fossilised remains of a primitive mammal reveals clues to ear evolution in mammals. © Nature/Z.Luo/CMNH

Fossils from the mammal's skull show a key feature of early middle ear evolution, at an intermediate stage between modern mammals and ancient mammal relatives.

The mammal is called Yanoconodon allini , after the Yan Mountains in China where it was found, and after Edgar Allin for his studies of mammalian ear evolution. The fossils show incredible detail because they are so well preserved.

'We are learning so much more about the early evolution of mammals from these new finds of exquisitely preserved articulated skeletons from the Yixian Formation in China, than we did during most of the previous century, when available fossils consisted mainly of jaws and teeth,' says Jerry Hooker, Natural History Museum palaeontologist.

Jaws become ears

Mammals have one lower jaw bone called the dentary. Reptiles, amphibians and birds have, today and in the past, lower jaws composed of several different bones.

Fossils from ancestors of mammals, such as the mammal-like reptiles, allow us to trace an early evolutionary reduction from a jaw bone with several bones to a single bone.

The bones didn't disappear. They reduced in size and migrated to the ear region, becoming the middle ear in modern mammals.

Fossil evidence
Illustration of primitive mammal Morganucodon.

Illustration of primitive mammal Morganucodon.

Examples have been found, such as the primitive mammal Morganucodon living during the Jurassic period (200 -145 million years ago). Its fossil remains show an intermediate stage where the lower jaw bones have reduced but not yet become the middle ear of modern mammals.

This evolutionary process can also be seen during embryonic development of the duck-billed platypus and other mammals. The embryos of these mammals have several bones in the lower jaw, like reptiles, but only one in adults.

New research

The new research on the Yanoconodon fossils shows a more advanced stage of evolution than Morganucodon . In fact, the same stage you see in the embryo of a duck-billed platypus.

It shows how the ear bones first became detached from the new jaw joint but still remained attached to the lower jaw further forwards by a rod-like structure called Meckel's cartilage.

Backbone evolution

Yanoconodon also reveals another aspect of evolution, a backbone with shortened ribs in the lower trunk region. This is an intermediate stage between mammals and reptiles.

Mammals have distinct thoracic (middle) and lumbar (lower) regions of the vertebral column with ribs in the thoracic area and none in the lumbar. Reptiles have no distinction and their ribs continue all the way down.

The Chinese fossils show that the typical mammalian ribless lumbar region evolved independently in two different mammal groups.

Zhe-Xi Luo from the Carnegie Museum of Natural History and his team carried out the research, which is published in the journal Nature

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