A rodent-like mammal that lived 120 million years ago had a weird ear that may have evolved as a result of its unique chewing style.
Yuanqing Wang of the Institute of Vertebrate Paleontology and Paleoanthropology, China and his colleagues discovered an almost complete skeleton of a previously unknown creature – named Jeholbaatar kielana – in the Jiufotang Formation in the Liaoning province of China.
The rodent-like animal’s lower cheek configuration suggests it had an unusual back and forward chewing style that allowed it to grind up and eat plants. This may have contributed to its success as a species, because other mammals alive at the time could only eat insects and other vertebrates.
J. kielana, which weighed about 50 grams, was also different to some other early mammals because its ear bones were separate to its jaw. Its chewing style may have driven this anatomical development, because it forced parts of the jaw to move up to the skull where they ended up forming a unique hearing apparatus, says Wang.
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Like modern mammals, J. kielana had three middle ear bones – the malleus, incus and stapes (or hammer, anvil and stirrup) – that transform soundwaves into electrical signals. But instead of the incus being interlocked with the malleus, as it is in people and other living mammals, it sat partially on top of the malleus.
This configuration was probably necessary to accommodate the animal’s back and forward chewing style, says Wang. In contrast, cats slice their food by biting up and down and cows grind plants between their teeth with a sideways movement. We don’t know if the unusual configuration caused J. kielana to hear sounds in a different way to other mammals.
The separate three-bone middle ear is a defining feature of today’s mammals, distinguishing them from birds, frogs and reptiles, which have connected jaw and ear bones. Research suggests the ear has separated from the jaw at least three times in mammalian evolutionary history, perhaps because it allowed better hearing, says Wang.
“Due to this unique feature, mammals obtained the capability to hear sound in higher frequencies and in a wider frequency range, which allowed mammals to better sense prey and approaching predators,” he says.
Journal reference: Nature, DOI: 10.1038/s41586-019-1792-0
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