An ancient critter may shed light on when mammals middle ear evolved – Science News

Exceptionally preservedskulls of a mammal that lived alongside the dinosaurs may be offeringscientists a glimpse into the evolution of the middle ear.

The separation of the threetiny middle ear bones known popularly as the hammer, anvil and stirrup fromthe jaw is a defining characteristic of mammals. The evolutionary shift of those tiny bones, which started out as joints in ancient reptilianjaws and ultimately split from the jaw completely, gave mammals greatersensitivity to sound, particularly at higher frequencies (SN: 3/20/07). But finding well-preserved skulls from ancientmammals that can help reveal the timing of this separation is a challenge.

Now, scientists have sixspecimens four nearly complete skeletons and two fragmented specimens of a newlydescribed, shrew-sized critter dubbed Origolesteslii that lived about 123 million years ago. O. lii was part of the Jehol Biota, an ecosystem of ancientwetlands-dwellers that thrived between 133 million and 120 million years ago inwhats now northeastern China.

The skulls on the nearlycomplete skeletons were so well-preserved that they were able to be examined in3-D, say paleontologist Fangyuan Mao of the Chinese Academy of Sciences inBeijing and colleagues. That analysis suggests that O. liis middle ear bones were fully separated from its jaw, the team reports online December 5 in Science.

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Fossils from an older,extinct line of mammals have shown separated middle ear bones, but this newfoundspecies would be the first of a more recent lineage to exhibit thisevolutionary advance.

O. lii apparentlymoved its jaw both in side-to-side and in rolling motions as it chewed. Suchchewing ability, the team says, may have played a role in the evolutionaryseparation of the jaw and middle ear bones.

This paper describes aspectacular fossil, says vertebrate paleontologist Zhe-Xi Luo of theUniversity of Chicago, who was not involved in the new study. But hes notconvinced that O. lii represents anevolutionary leap forward in mammalian ear evolution.

Luo notes that O. lii is closely related to the mammalgenus Maotherium, which lived aroundthe same time and in roughly the same location. In Science in July, Luo and colleagues reported that a new analysis ofMaotherium revealed that its middle ear bones were still connected to its jawbones by a strip ofcartilage (SN: 7/18/19).

That finding, Luo says, wasexpected. Maotherium is well-known asa transitional organism, in which the middle ear bones had begun to rotate awayfrom the jaw but were still loosely connected by that cartilage. There arenumerous branches and twigs on the mammal family tree, Luo says, and evolutionoccurred at a different pace on them. But, he says, its unlikely that O. lii wouldhave had separated ear bones when Maotheriumdidnt, given the pairs close positioning onthe tree.

Luo says he also doesntfind the studys evidence that the separation was complete in O. lii convincing. Three of the fourskulls in the study were missing all or part of the middle ear, and the gapbetween the middle ear bones and jaw in the fourth skull may have been a breakthat occurred during fossilization, he adds.

However, the new studysresearchers reject this idea. Its common that different interpretations mayexist for a discovery in paleontology, says vertebrate paleontologist Jin Mengof the American Museum of Natural History in New York, a coauthor of the study.

But, Meng says, none of the ear bones or the cartilage in any of the skulls show fractured or broken edges. That, he says, suggests that these features were already separated in the animals before their demise.

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An ancient critter may shed light on when mammals middle ear evolved - Science News