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New Jurassic mammaliaform sheds light on early evolution of mammal-like hyoid bones

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Science  19 Jul 2019:
Vol. 365, Issue 6450, pp. 276-279
DOI: 10.1126/science.aau9345

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Early suckler?

One trait that is unique to mammals is milk suckling. Suckling requires the presence of stability and motion in the throat, both of which require a complex hyoid apparatus. Zhou et al. describe a mammaliform docodontan fossil from the Jurassic that was preserved with a nearly intact hyoid (see the Perspective by Hoffmann and Krause). The structure is complex and saddle shaped, like that seen in modern mammals, suggesting that a muscularized throat was present before the development of mammals.

Science, this issue p. 276; see also p. 222

Abstract

We report a new Jurassic docodontan mammaliaform found in China that is preserved with the hyoid bones. Its basihyal, ceratohyal, epihyal, and thyrohyal bones have mobile joints and are arranged in a saddle-shaped configuration, as in the mobile linkage of the hyoid apparatus of extant mammals. These are fundamentally different from the simple hyoid rods of nonmammaliaform cynodonts, which were likely associated with a wide, nonmuscularized throat, as seen in extant reptiles. The hyoid apparatus provides a framework for the larynx and for the constricted, muscularized esophagus, crucial for transport and powered swallowing of the masticated food and liquid in extant mammals. These derived structural components of hyoids evolved among early diverging mammaliaforms, before the disconnection of the middle ear from the mandible in crown mammals.

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