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
Contact: Catherine Gianaro
University of Chicago Medical Center