After studying the structure of the auditory apparatus of one of the most ancient mammals, paleontologists found out that originally the ear bones were attached to the lower jaw and also served for chewing. Today, such bones have survived only in the Australian platypus and echidna. The study was published in Nature.
Most other mammals have three small auditory bones in their ears: the malleus, anvil, and stirrup, which transmit sound waves and contribute to the range of hearing, especially at high frequencies. In early mammalian fossils, these bones were attached to the lower jaw and played another role – helping animals to chew.
It is believed that the transition from the dental bones to the auditory bones of the middle ear is a distinctive feature of mammals. A recent discovery by paleontologists from the United States, China, and Australia gives a better idea of how this transition from a dual (chewing and hearing) to a single auditory function occurred.
Researchers studied the skull and other skeletal parts of a mammal from the order Vilevolodon diplomylos charamide found in deposits from the Middle Jurassic Tiaojishan Formation in China. The find is 160 million years old and is one of the earliest known species of herbivorous mammals.
From the well-preserved auditory bones – malleolus, anvil, and ectotympanic bone – a small bone supporting the eardrum, paleontologists have established that the ear structure of the vilevolodon is remarkably similar to that of modern members of the order of monotremes – platypus and echidna.
Previously, it was thought that these endemic species of Australian fauna has a unique structure of the inner ear, but now it is clear that they simply have the most ancient version for mammals – in fact, transitional from early mammals to modern mammals.
The authors believe that an early variant of the structure of the entire incudomalleolar joint, which includes the auditory ossicles, persisted in mammals throughout the Mesozoic. Today, it is found in a modified variant in monotremes and also in marsupials and placentals in early ontogeny.