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Integrated hearing and chewing modules decoupled in a Cretaceous stem therian mammal

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Science  17 Jan 2020:
Vol. 367, Issue 6475, pp. 305-308
DOI: 10.1126/science.aay9220

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Making a mammalian ear

Mammals have keen hearing owing to their complex inner ear. In our vertebrate ancestors, as in extant reptiles, the three bones that make up the inner ear were instead part of the jaw. Understanding the functional transition of these bones is challenging given their small and delicate nature. Mao et al. describe a new genus and species of stem therian mammal represented by six well-preserved specimens, seemingly caught as they slept huddled together (see the Perspective by Schultz). The unprecedented preservation reveals a clear transitional stage between the two very different functions of the bones.

Science, this issue p. 305; see also p. 244

Abstract

On the basis of multiple skeletal specimens from Liaoning, China, we report a new genus and species of Cretaceous stem therian mammal that displays decoupling of hearing and chewing apparatuses and functions. The auditory bones, including the surangular, have no bone contact with the ossified Meckel’s cartilage; the latter is loosely lodged on the medial rear of the dentary. This configuration probably represents the initial morphological stage of the definitive mammalian middle ear. Evidence shows that hearing and chewing apparatuses have evolved in a modular fashion. Starting as an integrated complex in non-mammaliaform cynodonts, the two modules, regulated by similar developmental and genetic mechanisms, eventually decoupled during the evolution of mammals, allowing further improvement for more efficient hearing and mastication.

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