Roxanne Khamsi, "Ear-splitting discovery rocks mammal identity" (2005)

Ear-splitting discovery rocks mammal identity

Roxanne Khamsi

Triple bone structure arose independently in platypus and humans.

Listen up: mammals seem to have evolved the delicate bone structure of the middle ear at least twice. The surprising discovery comes from a fossil, found off the southern coast of Australia, that belongs to an ancestor of the platypus.

Modern mammals are unique among vertebrates for possessing three tiny bones in the middle ear. The malleus, incus and stapes (commonly known as the hammer, anvil and stirrup) work as part of a chain that transmits sound towards the skull. Birds and reptiles have only one bone to perform this function.

Because the mammalian arrangement is so complex, scientists believed that the set-up had evolved on just a single occasion, in an ancestor that gave rise to placental animals (including humans), marsupials and monotremes (such as the duck-billed platypus).

All this changed when James Hopson, a vertebrate palaeontologist at University of Chicago, Illinois, took a trip to Australia. There he met a team of researchers including Thomas Rich of Museum Victoria in Melbourne.

Rich and his colleagues had recently unearthed a fossil of Teinolophos trusleri, an ancestor of modern monotremes that lived 115 million years ago. "He said he had some new Teinolophos specimens and when he showed them to me I almost fell off my chair," says Hopson, an author of the study, published this week in Science (1).

Hammer time Palaeontologists believe that the middle-ear bones of modern mammals once belonged to the jawbone and later separated to adopt their present location. This is supported by the fact that the middle ear's bones associate with the jaw in the early development of modern mammalian embryos.

What makes the Teinolophos specimen surprising is a large groove in its adult jawbone, which indicates that the smaller bones had not yet detached.

Teinolophos lived after monotremes split from the placental and marsupial mammalian groups. Its jawbone structure, along with its place in the evolutionary tree, hints that a common ancestor to all these mammals lacked the special three-bone ear structure.

This means that natural selection must have driven the same rearrangement in independent groups, after the monotreme split. "Some embryologists had the idea that it might be convergent but nobody really believed this," says palaeontologist Thomas Martin of the Senckenberg Research Institute in Frankfurt, Germany. "I was quite shocked when I heard that such a complex morphological transformation happened twice." The discovery will compel many experts to rethink their appreciation of mammals' common evolutionary heritage. "Until now it was considered to be one of the most important shared derived characteristics of modern mammals," says Martin.

 References

    (1) Rich T. H., et al. Science vol. 307, pp. 910-914 (2005).