clipped from: SYDNEY: Part of the human ear evolved from jaw bones of our reptilian ancestors, according to a new mammal fossil find.
Three tiny bones found in the middle ear of all mammals are widely thought to have evolved from bones that in reptiles form part of the lower jaw. However, no fossil demonstrating the transition were known until now.
Researchers reported today in the British journal Nature that Yanoconodon allini, a newly-unearthed ancient mammal from China, has ear bones that appear to be at an intermediate stage.
"Now we have a definitive piece of evidence, in a beautifully-preserved fossil split on two rock slabs," said study author and evolutionary biologist Zhe-Xi Luo from the Carnegie Museum of Natural history in Pittsburgh.
It is remarkable that such tiny bones as those in the ear have survived 125 million years, said the study authors.
reposted from: cosmos mag via clipmark - wildcat
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| Yancodon was found split in half onto two mirror-image siltstone slabs. The 15-cm-long fossil has middle ear anatomy intermediate between reptiles and mammals. Image: Zhe-Xi Luo/CMNH |
SYDNEY: Part of the human ear evolved from jaw bones of our reptilian ancestors, according to a new mammal fossil find.
Three tiny bones found in the middle ear of all mammals are widely thought to have evolved from bones that in reptiles form part of the lower jaw. However, no fossil demonstrating the transition were known until now.
Researchers reported today in the British journal Nature that Yanoconodon allini, a newly-unearthed ancient mammal from China, has ear bones that appear to be at an intermediate stage.
"Now we have a definitive piece of evidence, in a beautifully-preserved fossil split on two rock slabs," said study author and evolutionary biologist Zhe-Xi Luo from the Carnegie Museum of Natural history in Pittsburgh.
"This fossil, some 125 million years old, [reveals] the last stage of this most interesting evolutionary process," said Luo. "The middle ear bones have already acquired similar proportion and size to the modern mammal middle ear, yet the ear structure is still connected to the jaw."
Reptiles today have a single bone (the stapes) in their middle ear, while mammals have three (stapes, incus and malleus). These tiny bones act to amplify sound waves and transfer them from the air to the liquid contained in the inner ear's spiral-shaped cochlea.
Painstaking anatomical studies in the early 1900s suggested that the middle ear bones of mammals and the jaws of reptiles share the same origin. "We can trace this developmental process in developing mammalian embryos," said Luo. "In the early embryonic stage of modern mammals, the middle ear was still attached to the jaw. Later, the middle ear becomes separated from the jaw, and … starts the adult function [of hearing]."
Because of these differences in ear anatomy, reptiles today are not as sensitive as mammals in the higher frequency range of sounds. The unique hearing adaptation is thought to have been critical for nocturnal mammals to succeed, helping them to compete in a world of limited visibility. Today mammals have the best hearing of any vertebrates.
"The reptilian middle ear connected to the jaw can pick up the ground vibration, whereas the middle ear fully separated from the jaw is certainly much better at receiving the air-borne sound," said Luo, who collaborated on the study with experts at the University of Nanjing in China.
He argues that the fact that Yancodon's middle ear bones were still connected to the jaw structure might have made the species capable of hearing vibrations in the air and in the ground - "although not developed to the same high-pitch sensitivity as the modern mammal ear."
"This early mammalian ear from China is a Rosetta Stone type of discovery which reinforces the idea that development of complex body parts can be explained by evolution, using exquisitely preserved fossils," said Richard Lane, an earth scientist at the U.S. National Science Foundation in Washington D.C.
It is remarkable that such tiny bones as those in the ear have survived 125 million years, said the study authors.
The fossil find is very interesting, commented Graeme Clark, auditory scientist and founder of the Bionic Ear Institute in Melbourne. Though "the paper has perhaps drawn a few more conclusions than they should have" about how the fossil ear would have functioned in the living species, he said.
Yanoconodon - which would have been about 15 cm long – had a long body, short and sprawling limbs and claws that were ideal for either digging or living on the ground. It lived during the Mesozoic-era and likely fed on insects, worms and other small fauna.
More information:
A new eutriconodont mammal and evolutionary development in early mammals, Nature
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