“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,” said Zhe-Xi Luo, a professor of organismal biology and anatomy at the University of Chicago and the senior author of a new study of the fossil, published this week in Science.
“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,” said Luo, a leading scholar who has shaped our understanding the origins of mammals.
‘A pristine, beautiful fossil’
This 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. They can only swallow food whole or in large chunks.
Courtesy of Chang-Fu Zhou et al.
When and how this unique hyoid structure first appeared in mammals, however, has long been in question among paleontologists. In 2014, Chang-Fu Zhou from the Paleontological Museum of Liaoning in China, the lead author of the new study, found a new fossil of Microdocodon preserved with delicate hyoid bones in the famous Jurassic Daohugou site of northeastern China. Soon afterwards, Luo and Thomas Martin from the University of Bonn, Germany, met up with Zhou in China to study the fossil.
“It is a pristine, beautiful fossil. I was amazed by the exquisite preservation of this tiny fossil at the first sight. We got a sense that it was unusual, but we were puzzled about what was unusual about it,” Luo said. “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.”
This new insight gave 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, paleontologists anticipated that hyoids like this had to be there in all of these early mammals, but it was difficult to identify the delicate bones. After finding them in Microdocodon, Luo and his collaborators have since found similar fossilized hyoid structures in other Mesozoic mammals.
“Now we are able, for the first time, to address how the crucial function for swallowing evolved among early mammals from the fossil record,” Luo said. “The tiny hyoids of Microdocodon are a big milestone for interpreting the evolution of mammalian feeding function.”
New insights on mammal evolution as a whole
Luo also worked with postdoctoral scholar Bhart-Anjan Bhullar, now on the faculty at Yale University, and April Neander, a scientific artist and expert on CT visualization of fossils at UChicago, to study casts of Microdocodon and reconstruct how it lived.
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. Meanwhile, the hyoids 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.