Jurassic Gliding Mammal Relative Sheds New Light on Evolution of Mammalian Middle Ear
Paleontologists from the United States and China have examined the middle ear bones (ossicles) of Vilevolodon diplomylos, a gliding haramiyidan that lived during the Middle Jurassic period, some 160 million years ago, and found them remarkably similar to those of modern monotremes (egg-laying mammals).
Paleontologists know that the three ossicles — the malleus, incus and stapes — in extant mammals’ close non-mammal relatives were attached to the lower jaw and functioned with it, serving in both chewing and hearing.
In many mammal lineages, these bones eventually detached from the lower jaw and came to serve only auditory functions. The details of this transition have been mysterious.
In the new research, Professor Shundong Bi from Yunnan University and the Indiana University of Pennsylvania and colleagues analyzed computed tomographic (CT) images of the well-preserved malleus and incus of Vilevolodon diplomylos.
They found the ancient animal’s malleus and incus remarkably similar to those of modern monotremes.
Since haramiyidans and monotremes are not thought to be closely related, this resemblance demonstrates a surprising instance of convergent evolution.
“Although separated by 160 million years of geologic time, it is remarkable how similar these bones are in Vilevolodon diplomylos and the living monotremes, the platypus and echidna,” said Dr. John Wible, a researcher at Yunnan University and the Carnegie Museum of Natural History.
“The preservation of these ossicles in the fossil is amazing; the incus is just a little over a millimeter across!”
The malleus and incus of monotremes have long been considered to be unique adaptations as they are wholly unlike these bones in the remaining living mammals, marsupials and placentals.
“This type of malleus and incus represent an evolutionary stage between fossils with ossicles attached to and functioning with the lower jaw and those where the ossicles are detached from the lower jaw and exclusively function for hearing,” the researchers said.
“We hope the discovery advances the understanding of how mammals, including humans, developed their unique sense of hearing.”
The results were published in the journal Nature.
J. Wang et al. 2021. A monotreme-like auditory apparatus in a Middle Jurassic haramiyidan. Nature 590, 279-283; doi: 10.1038/s41586-020-03137-z