Having studied the structure of the hearing system of one of the most ancient mammals, paleontologists have found that the ear bones were originally attached to the lower jaw and also served for chewing. Today, only the Australian platypus and echidna have such bones. The research results are published in the journal Nature.
Most other mammals have three small auditory bones in the ear - the hammer, incus, and stapes - that transmit sound waves and help increase the range of hearing, especially at high frequencies. In early fossil mammals, these bones were attached to the lower jaw, and played another role - helping animals to chew.
The transition from the dental bones to the ossicles of the middle ear is believed to be a hallmark of mammals. A recent discovery by paleontologists from the United States, China, and Australia provides a better idea of how this transition from dual (chewing and auditory) to a single auditory function took place.
The researchers examined the skull and other skeletal parts of a mammal from the order of the Charamid Vilevolodon diplomylos, found in the sediments of the Middle Jurassic Tiaojishan Formation in China. The discovery is 160 million years old and is one of the earliest known species of herbivorous mammals.
Vilevolodon, a small animal the size of a squirrel, lived in trees and was able to soar in the air, flying from branch to branch, like modern flying squirrels. Based on the morphological characteristics of the teeth, scientists suggest that vilevolodon ate a herbivorous diet, probably consisting of seeds and soft tissue of plants.
From the well-preserved auditory bones - the hammer, incus and ectotympanic - a small bone supporting the eardrum, paleontologists have established that the structure of the vilevolodon's ear is surprisingly similar to the ear of modern representatives of the order of monotremes - the platypus and echidna.
Previously, it was believed that these endemic species of the Australian fauna have a unique structure of the inner ear, but now it is clear that they simply have the most ancient variant for mammals - in fact, a transition from early mammals to modern ones.
The authors believe that the early variant of the structure of the entire incudomalleolar joint, which includes the auditory ossicles, was retained in mammals throughout the Mesozoic. Today, in a modified version, it is found in monotremes, and in early ontogeny - also in marsupials and plantations.