Jurassic Docodont Had Complex, Modern Mammal-Like Hyoid Bones

Our highly mobile tongues, which allow us to swallow chewed food and suckle milk as babies, may have evolutionary origins in some of our most early mammal ancestors, according to an analysis of hyoid bones in Microdocodon gracilis, a newly-discovered docodontan mammaliaform that lived 166 million years ago (Jurassic period) in what is now China.

An artist’s reconstruction of Microdocodon gracilis as a nocturnal animal active on a bennettitalean tree of the Jurassic. Image credit: April I. Neander.

Microdocodon gracilis was a docodont, from an extinct lineage of near relatives of mammals from the Mesozoic Era called mammaliaforms.

The ancient animal had a skull length of 3/4 inch (2 cm), a head-body length about 2 inches (6 cm), and a long tail about 3 inches (8 cm) in length. It likely weighed between 5 grams to 9 gram (less than 1/3 of an ounce).

The slender and gracile skeletal elements of Microdocodon gracilis suggest that it was an agile and active animal living on the tree. Its teeth were for insectivorous diet.

The creature co-existed with other docodonts like the semiaquatic Castorocauda, the subterranean Docofossor, the tree-dwelling Agilodocodon, as well as some mammaliaform gliders.

Its fossilized remains with delicate hyoid bones were found at the famous Jurassic Daohugou site near the Wuhua village in Inner Mongolia, China.

“It is a pristine, beautiful fossil. I was amazed by the exquisite preservation of this tiny fossil at the first sight,” said University of Chicago’s Professor Zhe-Xi Luo, senior author of the study.

“We got a sense that it was unusual, but we were puzzled about what was unusual about it.”

“After taking detailed photographs and examining the fossil under a microscope, it dawned on us that this Jurassic animal has tiny hyoid bones much like those of modern mammals.”

The fossil of Microdocodon gracilis is preserved in two rock slabs, and consists of a main part (left) and a counter part. Image credit: Zhe-Xi Luo.

Mammalian hyoid apparatus is mobile and allows the throat muscles to control the intricate functions to transport and swallow chewed food or drink fluids.

Other vertebrates also have hyoid bones, but their hyoids are simple and rod-like, without mobile joints between segments. They can only swallow food whole or in large chunks.

When and how this unique hyoid structure first appeared in mammals, however, has long been in question among paleontologists.

“Now we are able for the first time to address how the crucial function for swallowing evolved among early mammals from the fossil record,” Professor Luo said.

“The tiny hyoids of Microdocodon gracilis are a big milestone for interpreting the evolution of mammalian feeding function.”

The findings appear in the journal Science.

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Chang-Fu Zhou et al. 2019. New Jurassic mammaliaform sheds light on early evolution of mammal-like hyoid bones. Science 365 (6450): 276-279; doi: 10.1126/science.aau9345