Prehistoric jawbone reveals evolution repeating itself

A 115-million-year-old fossil of a tiny monotreme, an egg-laying mammal related to the platypus, provides compelling evidence of multiple origins of acute hearing in humans and other mammals.

The discovery of a prehistoric jawbone, reported in February in the journal Science, suggests that the transformation of bones from the jaw into the small bones of the middle ear occurred at least twice in the evolutionary lines of living mammals after their split from a common ancestor some 200 million years ago.

“The earbones are still attached to the lower jaw, which implies this shift had to occur in later monotremes and independently of the shift occurring in the common ancestor of marsupials and placentals,” said James Hopson, Professor in Organismal Biology & Anatomy and an author of the paper.

At a dig on the south coast of the Australian state of Victoria, paleontologists found a lower jawbone of the world’s oldest known monotreme, Teinolophos trusleri, a small primitive mammal hardly bigger than today’s shrew.

Many paleontologists have doubted that such a seemingly complex adaptation could have originated more than once in mammals, but according to the authors, the evidence of T. trusleri indicates that it did.

“Nothing like that has ever been found before,” said Tom Rich, lead author of the paper and curator of vertebrate paleontology at Museum Victoria in Melbourne, Australia.

“These jaws may be the oldest evidence of monotremes on Earth,” Rich said. “Some of these jawbones show facets for what scientists call accessory bones—bones that humans and most other mammals do not have.”

The lower jaw of living mammals, including humans, is made up of just one bone, the dentary. Some accessory jawbones (called the angular, the articular and the prearticular) that are present in mammal-like reptiles, which gave rise to the mammals, eventually ended up as part of the middle ear in mammals: the angular became the ectotympanic or tympanic ring that supports the eardrum; and the articular and prearticular became the malleus—one of the three bones in the middle ear that transmits sound from the eardrum to the inner ear, where nerves pick up the vibrations from sounds and make it possible for mammals to hear.

Some of the most advanced mammal-like reptiles had some of these bones already functioning in hearing, and they occurred earlier in time than T. trusleri, Rich said.

This suggests that the development of the acute hearing system with the chain of bones from the eardrum to the inner ear developed at least twice in the history of mammals, he said, once in the group that gave rise to the placentals and marsupials, and another in monotremes, which include T. trusleri.

Fossil jaws from other groups of early mammals also appear to have supported accessory bones, though the evidence is not as clear as in Teinolophos. This suggests that the independent transformation to earbones occurred more often than previously thought.

How can this supposedly rare and unexpected evolutionary change have occurred so commonly in early mammals?

“Recent studies of jaw and ear function in primitive mammal-like reptiles indicate that the larger angular bone may have supported an eardrum while still part of the lower jaw,” Hopson said. But once the dentary bone made a new jaw hinge with the skull in the immediate predecessor of mammals, the accessory jawbones may have abandoned their job of supporting the jaw and evolved exclusively to perform the middle ear, sound-transmitting function.

“The evidence of the fossils indicates that though this did eventually occur, it took place gradually and piecemeal in each of the descendant lineages, so that the complete freeing of the earbones from the jaw and their attachment to the skull occurred many times independently,” Hopson said. “Only the evidence of fossils has been able to unravel this tangled history of a complex adaptation.”