Inner ear may shed light on ancient primate behaviour

CT scans of fossilized primate skulls or skull fragments from both the Old and New Worlds may hold the key to how these extinct animals moved, especially for those species without any known remains, a new study has claimed.

An international team of 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,” Timothy Ryan, assistant professor of anthropology, geosciences and information sciences and technology, Penn State, said.

“They are embedded in a very dense part of the skull and so are protected,” Ryan said.

Normally, researchers assess the locomotor behaviours 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.

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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 behaviours 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 said.

The researchers believe that the relatively high degree of correspondence with known behaviours suggests that this method produces accurate reconstructions of locomotor agility.

They found that the earliest anthropoids moved in the medium to medium slow range, slower than predicted.

The researchers also found that other early anthropoids that predated the split between monkeys and apes also fell in the medium slow category, including the well-known species Aegyptophithecus from about 29 million years ago and other animals from Egypt and Saudi Arabia.

But once the split between Old World monkeys and apes occurs, both monkeys and apes fall in the medium to medium fast range like macaques. This includes Proconsul heseloni found in Kenya and considered one of the first apes.

The scans from New World monkeys, dating from 12 to 20 million years ago, showed the animals were relatively agile similar to cebus monkeys or tamarins.

“Most of the fossil New World monkeys we examined are known only from cranial material with no associated post-cranial fossils,” Ryan said.

“We had no idea about their locomotion,” he said.

The study has bee published in Proceedings of the Royal Society B: Biological Sciences.

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