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Machines Like Us

Inner ear may hold key to ancient primate behavior

Thursday, 14 June 2012

This is a three-dimensional reconstruction of the cranium and semicircular canals from the fossil anthropoid primate Aegyptopithecus zeuxis. Credit: Tim Ryan, Penn State

CT scans of fossilized primate skulls or skull fragments from both the Old and New Worlds may shed light on how these extinct animals moved, especially for those species without any known remains, according to an international team of researchers.

The researchers looked at the bony labyrinth in fossil remains and compared them to CT scans previously obtained from living primate species. The bony labyrinth of the inner ear is made up of the cochlea—the major organ of hearing—the vestibule and the three semicircular canals which sense head motion and provide input to synchronize movement with visual stimuli.

"Almost in every case where there is a fossilized skull, the semicircular canals are present and well preserved," said Timothy Ryan, assistant professor of anthropology, geosciences and information sciences and technology, Penn State. "They are embedded in a very dense part of the skull and so are protected."

Normally, researchers assess the locomotor behaviors of extinct animals, including primates, by examining limb bones. However, frequently the only fossilized remains found are from the head. By comparing the semicircular canals of extinct species to those of existing species, the researchers could determine if the extinct animals moved with agility—leaping like monkeys or lemurs or swinging from limb to limb like gibbons—or travelled more slowly like baboons or gorillas.

They could make this determination because the size of the three semicircular canals is closely related to their sensitivity.

Previous research showed that there is a direct relationship between the size of the semicircular canals and the degree of agility an animal exhibits. There is also a direct connection between the size of these canals and the size of the animal.

Correcting for animal size, the researchers compared scans from 16 fossil species spanning New World monkeys, Old World monkeys and apes, to living primates whose locomotor behaviors are known. Included in the study are some of the oldest fossil anthropoids—the group that includes monkeys, apes and humans—from the Fayum Depression in Egypt.

"The fossil anthropoids analyzed here clearly fall into the range of variation of modern primates, making agility reconstructions based on extant taxa relatively robust," the researchers reported in today's (June 13) issue of Proceedings of the Royal Society B: Biological Sciences.