Science & Technology

The evolution of swallowing in mammals revealed by newly unearthed fossil

The fossilised remains of a tiny shrew-like animal sheds new light on the evolution of swallowing in mammals and the group of animals as a whole.
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The fossilised remains of a tiny shrew-like animal sheds new light on the evolution of swallowing in mammals and the group of animals as a whole.

A 165-million-year-old fossil of Microdocodon gracilis–a tiny, shrew-like animal–shows the earliest example of modern hyoid bones in mammal evolution and thus the evolution of swallowing in mammals.

Diagram of the hyoid bone (highlighted in blue) in the jaws of various mammals (April Neander, University of Chicago)

The hyoid bones are the bones which link the back of the mouth to the openings of the oesophagus and the larynx. The hyoids of modern mammals– including humans–are arranged in a “U” shape. This is reminiscent to the seat of a child’s swing, suspended by jointed segments from the skull.

It helps us transport and swallow chewed food and liquid – so is obviously a crucial function on which our lives depends.

In this respect, mammals as a whole are far more sophisticated than other living vertebrates. Whereas mammals chew up food, swallowing it one small lump at a time, other creatures such as alligators are forced to gulp down huge bites or even whole prey.

The modern 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. Hence why they can only swallow food whole or in large chunks.

Whilst this may seem superficial, Zhe-Xi Luo, PhD–a professor of organismal biology and anatomy at the University of Chicago–believes that the swallowing mechanism is responsible for the diversity of mammals across the natural world.

Luo explains: “Mammals have become so diverse today through the evolution of diverse ways to chew their food, whether it is insects, worms, meat, or plants. But no matter how differently mammals can chew, they all have to swallow in the same way.”

“Essentially, the specialized way for mammals to chew and then swallow is all made possible by the agile hyoid bones at the back of the throat.”

Microdocodon gracilis: ‘A pristine, beautiful fossil’

When and how mammals’ unique hyoid structure first appeared has long been questioned by palaeontologists. In 2014, Chang-Fu Zhou, PhD, from the Paleontological Museum of Liaoning in China, the lead author of the new study–published in the journal Science —found a new fossil of Microdocodon preserved with delicate hyoid bones in the famous Jurassic Daohugou site of northeastern China.

Shortly after this, Luo and Thomas Martin from the University of Bonn, Germany, joined Zhou in China to study the fossil.

Luo describes the find: “It is a pristine, beautiful fossil. I was amazed by the exquisite preservation of this tiny fossil at first sight.

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

The fossil of Microdocodon gracilis is preserved in two rock slabs, and consists of the main part (PMOL-AM00025A, left) and a counter part (PMOL-AM00025B, right). It was found in a site near the Wuhua village in the Daohugou area of Inner Mongolia, China, and the estimated age of the fossil site is at least 164 million years. The type specimen of this new mammaliaform is deposited in the Paleontological Museum of Liaoning (Shenyang, China) (Zhe-Xi Luo)

After taking detailed photographs and examining the fossil under a microscope, the team realised that this Jurassic animal has tiny hyoid bones much like those of modern mammals. The new insight giving Luo and his colleagues added context on how to study the new fossil. 

Microdocodon is a docodont, from an extinct lineage of near relatives of mammals from the Mesozoic Era called mammaliaforms. Previously, palaeontologists anticipated that hyoids like this would be present in these early mammals, but identifying these extremely delicate bones has proved difficult.

After finding them in Microdocodon, Luo and his collaborators have since found similar fossilized hyoid structures in other Mesozoic mammals.

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

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

Shedding light on mammalian evolution

After the presence of the hyoid was established, Luo turned to postdoctoral scholar Bhart-Anjan Bhullar, PhD, April Neander, a scientific artist and expert on CT visualization of fossils at UChicago, to study casts of Microdocodon and reconstruct how the animal may have lived.

Life reconstructions of Microdocodon gracilis – a docodont mammaliaform. Microdocodon was a tiny animal. It has a skull length of 2 cm (3/4 inch), a head-body length about 6 cm (2 inches), and a long tail about 8 cm (about 3 inches) in length. The animal likely weighed between 5 grams to 9 gram (less than 1/3 of an ounce). Its slender and gracile skeletal elements suggest that it was an agile and active animal living on the tree. Its teeth were for insectivorous diet. This reconstruction depicts Microdocodon as a nocturnal animal active on a bennettitalean tree of the Jurassic. (April Neander, University of Chicago)

The jaw and middle ear of modern mammals are developed from (or around) the first pharyngeal arch, structures in a vertebrate embryo that develop into other recognizable bones and tissues. The hyoids, however, are developed separately from the second and the third pharyngeal arches. 

Microdocodon has a primitive middle ear still attached to the jaw like that of other early mammals like cynodonts, which is unlike the ear of modern mammals.

Yet its hyoids are already like those of modern mammals.

Bhullar explains: “Hyoids and ear bones are all derivatives of the primordial vertebrate mouth and gill skeleton, with which our earliest fishlike ancestors fed and respired.

“The jointed, mobile hyoid of Microdocodon coexists with an archaic middle ear — still attached to the lower jaw. Therefore, the building of the modern mammal entailed serial repurposing of a truly ancient system.”

The tiny, shrew-like creature likely weighed only 5 to 9 grams, with a slender body, and an exceptionally long tail. The dimensions of its limb bones match up with those of modern tree-dwellers.

Neander adds: “Its limb bones are as thin as matchsticks, and yet this tiny Mesozoic mammal still lived an active life in trees.”

The fossil beds that yielded Microdocodon are dated 164 to 166 million years old. Microdocodon probably co-existed with other docodonts like the semiaquatic Castorocauda, the subterranean Docofossor, the tree-dwelling Agilodocodon, as well as some mammaliaform gliders.

Original research: https://science.sciencemag.org/content/365/6450/276

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