The well-preserved fossil of the early mammal Liaoconodon hui sheds light on the evolution of the mammalian middle ear, researchers have suggested.
Credit: Jin Meng / AMNH
EDINBURGH: The fossil of an extinct species of mammal from China with excellently preserved ear bones has shed new light on how the middle ears of mammals evolved.
Since Darwin's time, scientists have believed that the small middle ear bones of mammals evolved from the jaw bones of reptiles, but there has been little direct evidence to confirm this theory. Now a newly discovered fossil of an early mammal that lived alongside the dinosaurs has allowed researchers to catch these shifting bones in the act.
"[this study] provides the first ever clear morphological evidence to illustrate the transition" from reptilian jawbones to mammalian ear bones, said lead researcher Jin Meng of the American Museum of Natural History in New York.
From jaw bones to ear bones
The lower jaws of reptiles are made up of several different bones, while mammals have only a single jaw bone.
By contrast, where reptiles have only a single middle ear bone, mammals have three. These small bones amplify sound waves and transmit them to the inner ear: the more complex bone structure in mammal ears gives them greater sensitivity to high frequency sounds.
This may have been a crucial evolutionary advantage for our nocturnal ancestors, helping them to survive in low light conditions.
Scientists have long theorised that reptile jaw bones gradually migrated through evolution to become the middle ear bones of modern mammals. However, fossil evidence showing how this transition occurred has been difficult to come by.
Tiny bones formed middle ear
Published in the journal Nature, Meng and his colleagues describe a fossil of a previously unknown species of triconodont - a small, furry, four-legged mammal that lived 120 million years ago at the height of the dinosaur age.
In this fossil, two of the lower jaw bones have clearly started to evolve into the middle ear bones found in later mammals, but are still connected to the lower jaw by a gristle and bone structure called Meckel's cartilage.
Researchers uncovered the fossil in the Jiufotang Formation, part of the Jehol group of rock formations in North East China, a rich source of fossils including feathered dinosaurs and primitive birds.
In this new species, dubbed Liaoconodon hui, two bones called the ectotympanic and malleus have moved away from the main tooth-bearing jaw bone, on the way to forming part of the middle ear. Crucially, however, they are still attached to the jaw bone by Meckel's cartilage, which is itself partially transformed into bone.
An evolutionary hearing aid
Meng and his colleagues argue that this ossified Meckel's cartilage performed a vital stabilising function. In modern mammals these tiny bones are attached to the skull, but this is not the case during the transition from jaw bones to ear bones.
But these bones can only play a role in hearing if they are held in place by something, and this new study suggests that the ossified Meckel's cartilage kept the bones stable and secure. This enabled Liaoconodon hui to hear better than its reptilian ancestors, and to survive in a hostile environment still dominated by dinosaurs.
The results were welcomed by Alistair McGowan of the School of Geographical and Earth Sciences at the University of Glasgow in Scotland, who described it as "yet another instance where a superbly preserved specimen from the Jehol deposits has provided critical evidence of a sequence of morphological transformations responsible for a major evolutionary transition."
McGowan also suggested this study may prompt scientists to look again at their fossil collections, adding that "we can expect other researchers to re-investigate other specimens already in museums to search for the structures described in this paper."
