Section 1.1: The Role of Hyoid Bones

Hyoid bones play a vital role by connecting the back of the mouth to the openings of the esophagus and larynx. In modern mammals, including humans, these bones form a “U” shape, similar to the seat of a child's swing, suspended from the skull. This structure is essential for effectively transporting and swallowing food and liquids, making it crucial for survival.

Mammals exhibit a remarkable level of sophistication compared to other vertebrates. Unlike alligators, which must swallow prey in large chunks, mammals can chew their food into smaller pieces, allowing for a more refined swallowing process. The mobility of the hyoid apparatus enables the throat muscles to manage complex swallowing actions, a feature that other vertebrates lack, as their hyoids tend to be simple and rod-like, limiting them to gulping down food whole.

Zhe-Xi Luo, PhD, a professor of organismal biology and anatomy at the University of Chicago, emphasizes the significance of this swallowing mechanism in understanding mammalian diversity. He states, “Mammals have evolved an array of chewing techniques for various diets, yet the swallowing process remains fundamentally consistent across species.”

Microdocodon gracilis: A Remarkable Find

When and how the specialized hyoid structure in mammals emerged has long puzzled paleontologists. In 2014, Chang-Fu Zhou, PhD, from the Paleontological Museum of Liaoning in China, unearthed a remarkable fossil of Microdocodon at the renowned Jurassic Daohugou site.

Shortly thereafter, Zhou collaborated with Luo and Thomas Martin from the University of Bonn, Germany, to examine the fossil. Luo recalls, “It is a pristine, beautiful fossil. The exquisite preservation of this tiny specimen was astonishing at first glance, and while we sensed its uniqueness, we were initially uncertain of its significance.”

After conducting detailed analyses, the team discovered that this Jurassic creature possessed hyoid bones remarkably similar to those of modern mammals. This finding provided new context for studying the fossil and understanding the evolution of swallowing in early mammals.

Microdocodon is classified as a docodont, belonging to an extinct lineage of near-mammalian relatives from the Mesozoic Era. Although paleontologists long theorized that such hyoids existed in early mammals, confirming the presence of these delicate structures has proven challenging. Following the identification of the hyoids in Microdocodon, Luo and his team have since located similar fossilized hyoid structures in other Mesozoic mammals.

Luo continues: “This discovery allows us to explore how the essential function of swallowing evolved among early mammals through the fossil record. The tiny hyoids of Microdocodon mark a significant milestone in understanding mammalian feeding evolution.”

The first video, "Rare fossil discovery reveals mammalian evolution secrets," provides a deeper look into how these findings shed light on the evolutionary journey of mammals.

Section 1.2: Reconstructing the Life of Microdocodon

After establishing the presence of the hyoid, Luo enlisted the help of postdoctoral scholar Bhart-Anjan Bhullar, PhD, along with scientific artist April Neander, to examine casts of Microdocodon and reconstruct its potential lifestyle.

The jaw and middle ear structures of modern mammals derive from the first pharyngeal arch, while the hyoids develop from the second and third arches. Interestingly, Microdocodon retains a primitive middle ear still attached to its jaw, akin to early mammals like cynodonts, distinguishing it from modern ear structures.

Bhullar explains, “Hyoids and ear bones are all remnants of the primordial vertebrate mouth and gill skeleton, which our early fish-like ancestors utilized for feeding and respiration. The jointed, mobile hyoid of Microdocodon exists alongside an archaic middle ear, highlighting the evolutionary repurposing of an ancient system.”

This tiny creature likely weighed between 5 to 9 grams and had a slender body with an exceptionally long tail, suggesting it lived an agile life among trees. Neander remarks, “Despite its limb bones being as thin as matchsticks, this diminutive Mesozoic mammal was still active in arboreal environments.”

The fossil beds where Microdocodon was found date back to 164 to 166 million years ago, indicating it coexisted with other docodonts, including the semi-aquatic Castorocauda, the burrowing Docofossor, and the tree-dwelling Agilodocodon, as well as some gliding mammaliaforms.

The second video, "Evolution of the swallowing function displayed by newly unearthed fossil," explores how these hyoid structures influence swallowing mechanics in mammals.