Groovy Bones: Mammalian ear structure evolved more than once

Fossils of an ancient egg-laying mammal indicate that the configuration of the bones in all living mammals’ ears arose at least twice along independent evolutionary pathways, paleontologists say.

IN THE GROOVE. A trough (red arrow) on the lower jawbone of Teinolophos trusleri, an ancient egg-laying mammal, indicates that it had a reptile-style ear structure and its modern descendants’ ear structure evolved independently of that in other mammals. P. Trusler

The tiniest mammalian bones—the middle ear’s sound-transducing trio of the stapes, incus, and malleus—most distinctively distinguish mammals from other vertebrates.

The configuration of these ear bones is similar in all three extant groups of mammals: the placental mammals, which bear live young, the pouched marsupials, and the egg-laying monotremes, such as the duck-billed platypus. Because of the complexity of the bone arrangement, some scientists have argued that the innovation arose just once—in a common ancestor of the three mammalian groups.

Now, analyses of a jawbone from a specimen of Teinolophos trusleri, a shrew-size creature that lived in Australia about 115 million years ago, have dealt a blow to that notion. The recently discovered fossil, one of six jawbones by which the species is known, is also the best preserved, says Thomas H. Rich, a paleontologist at the Museum Victoria in Melbourne, Australia. Rich and his colleagues describe their find in the Feb. 11 Science.

Its teeth place T. trusleri within the monotreme group, says Rich. However, a distinct groove on the rear of the animal’s jawbone indicates that its ear bones were embedded in a mass of cartilage there.

This placement, more characteristic of mammals’ reptilian ancestors, hints that not all early mammals possessed the modern-day configuration of ear bones. It also implies that today’s monotreme descendants of T. trusleri developed the distinctive stapes-incus-malleus configuration independently of marsupial and placental mammals.

In mammals, the series of delicate bones in the middle ear transmits vibrations from the eardrum to the inner ear. That sensitive arrangement lets mammals, especially those that hunt prey by echolocation, detect sounds at frequencies up to 120 kilohertz (kHz), says Zhe-Xi Luo, a paleontologist at the Carnegie Museum of Natural History in Pittsburgh. In contrast, birds typically can detect frequencies up to only 30 kHz, he notes.

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