Neutrons show that 260 million years ago the Pristerodon could already hear airborne sound

A recent study by means of neutron tomography revealed that some forerunners of mammals were already able to hear airborne sound, because these animals already possessed an eardrum at the lower jaw, an impedance matching middle ear and a small cochlea.

Restoration of _Pristerodon mackayi_

Restoration of Pristerodon mackayi. © Dmitry Bogdanov via Wikimedia Commons

We can only hear sound from the air, because we have a specialised area for sound reception – our eardrum. Furthermore, our middle ear consisting of three ear ossicles, the malleus, incus and stapes, amplifies sound impulses from the eardrum, and our cochlea is responsible to transform a wide range of sound frequencies into nerve impulses for the brain.
In contrast, early land-living tetrapods were originally not able to hear airborne sound, because these animals evolved from aquatic ancestors. Up to now it was unresolved whether the forerunners of mammals, the therapsids, already possessed an eardrum and an impedance matching middle ear to hear sound from the air or not.

To shed light on this problem Michael Laaß from the University of Duisburg-Essen investigated a ca. 260 million years old skull of the therapsid Pristerodon from the Karoo Basin of South Africa by means of neutron tomography.

Lass - quote

This investigation revealed the earliest evidence of a cochlea in a far relative of mammals. Moreover, it was possible to reconstruct the ear virtually in 3D and to reconstruct the function of the middle ear. Interestingly, the latter was able to amplify sound and to conduct weak sound impulses from the mandible to the inner ear if the jaw musculature was relaxed.

Skull of the therapsid Pristerodon

Skull of the Pristerodon and image obtained from neutron tomographic data. Courtesy of the author.

Furthermore, the postcranial anatomy of Pristerodon suggests that this animal already had a more upright posture than other therapsids. As a consequence, the lower jaw was usually not in contact with the ground and hearing of seismic sound was impossible. This might be the reason why Pristerodon evolved an airborne-sound sensitive ear, because this was necessary to detect predators or to communicate with conspecifics.

Original publication:  doi:http://dx.doi.org/10.4202/app.00140.2014

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