Abnormal Interests

The news from Nature is that the three bones of the enter ear (malleus, incus and stapes, commonly known as the hammer, anvil and stirrup) of mammals may have developed in separately in placental (animals like us) and marsupial (opossum and kangaroo and the like), on the one hand and monotremes (duck-billed platypus for example)on the other. Up till now, most zoologists thought they evolved from a common ancestor, over time move from being part of the jaw bone to their current place in the ear. Well the jaw bone part still seem correct but a fossil monotreme (Teinolophos trusleri)from Australia may have changed the common ancestor part of the story.

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."

Interesting, new evidence requires new ways of thinking about what (and how) one hears.