New papers:
Microdocodon gracilis gen. et sp. nov.
Chang-Fu Zhou, Bhart-Anjan S. Bhullar, April I. Neander, Thomas Martin & Zhe-Xi Luo (2019)
New Jurassic mammaliaform sheds light on early evolution of mammal-like hyoid bones.
Science 365(6450): 276-279
DOI: 10.1126/science.aau9345
https://science.sciencemag.org/content/365/6450/276
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.
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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The early embryos of mammals and other vertebrates typically have six pharyngeal arches, tissue bands under the early brain that develop into structures of the head and neck. The first arch gives rise to the mammalian malleus and incus (middle ear bones), mandible (part of the lower jaw), and tympanic bone (which supports the ear drum); the second, the stapes (middle ear bone) and part of the hyoid bone (a horseshoe-shaped structure in the neck); and the third, the remainder of the hyoid. Although the evolutionary transition of the first pharyngeal arch is well documented by fossil evidence, those of the second and third arches have received little attention in the developmental and paleontological literature. On page 276 of this issue, Zhou et al. (1) report on a newly discovered 165-million-year-old fossil from China in which the bones of the first three pharyngeal arches are preserved. The fossil defines a new taxon named Microdocodon.
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News:
Jurassic fossil shows how early mammals could swallow like their modern descendants