The discovery of the prehistoric jawbone, reported in the Feb. 11, 2005, issue of 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.
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 much like today's shrew.
"The ear bones are still attached to the lower jaw, which implies that 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, Ph.D., professor of organismal biology and anatomy of the University of Chicago and one of the authors of the paper.
Many paleontologists have doubted that such a seemingly complex adaptation could have originated more than once in mammals, but according to the authors of the paper, the evidence of T. trusleri indicates that it did.
"Nothing like that has ever been found before," said Tom Rich, Ph.D., 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 the human is made up of just one bone, the dentary. Some accessory jaw bones (called the angular, the articular and the prearticular) that are present in mammal-like reptiles that gave rise to the mammals eventually ended up as part of the middle ear in humans: 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 transmit sound from the eardrum to the inner ear where nerves pick up the vibrations from sounds and make it possible for us 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 includes T. trusleri.
The presence of a trough in the lower jaw of the T. trusleri supports the view of independent origins of the mammalian middle ear, said co-author Pat Vickers-Rich, Ph.D., of Monash University in Victoria, Australia.
"We suspect in that groove lay some accessory bones, which in more advanced forms were incorporated into the middle ear," she said. "The trough tells us that such a change had not yet occurred in Teinolophos, even though 115 million years ago monotremes had split off on their own evolutionary line from the marsupials and placentals."
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 into 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 ear bones 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."
Anne M. Musser, Ph.D., of the Australian Museum and Timothy F. Flannery, Ph.D., of the South Australian Museum also were co-authors of the paper.
The study was funded by the National Geographic Society.