Early mammal fossil reveals the evolutionary origins of having a loose tongue

July 18, 2019

Our highly mobile mammalian tongues, which allow us to swallow chewed food and suckle milk as babies, may have evolutionary origins in some of our most early mammalioform ancestors, according to a new study, which finds remarkably complex and modern mammal-like hyoid bones in a newly discovered 165-million-year-old mammaliaform species. Unlike the wide, gulping mouths and non-muscular throats of reptiles or amphibians, the complex hyoid of mammals - a small, yet critical bone in the throat - enables the dexterous tongue and complex throat movements required for swallowing chewed food and suckling liquids like milk. However, while advanced food and liquid swallowing are characteristic traits in modern mammals, the evolutionary origin of the hyolingual structures that enables them is relatively unknown. Docodonts, an early branch of the mammaliaform ancestral tree, are important in understanding the early divergence and evolution of mammaliaform traits. Chang-Fu Zhou and colleagues report on the discovery of a new Jurassic-age docodontan fossil in China, which included an exceptionally well preserved and nearly intact hyoid apparatus, which was complex and very similar to those in modern mammals. Using the mammal-like morphology of the previously undescribed Microdocodon gracilis fossil, the authors were able to identify complex hyoid structures in several other early mammaliaforms. According to the results, the development of a complex and saddle-shaped mammal-like hyoid predates the separation of the middle ear from the mandible - a key evolutionary step among early diverging mammaliaforms. And, since Microdocodon is a basal species in the docodontan clade, the authors suggest that complex hyolingual structures may have been in place even before mammals. In a related Perspective, Simone Hoffman and David Krause discuss the study in more detail.

American Association for the Advancement of Science

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