A Jurassic Mammal-like Fossil Is Changing Our Understanding of Hyoid Bone Evolution

Mammals ability to swallow chewed-food and suckle as young may have evolved much earlier than previously thought.

A team of researchers has found some tantalizing fossil evidence that suggests mammalian hyoid bones were in place well before the middle ear separated from the mandible.

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What is the hyoid bone?

The hyoid bone, AKA tongue-bone, is a horseshoe-shaped bone that can be found in the anterior midline (front middle) of the neck in many extinct and extant vertebrate animals. It tends to sit between the chin and the cartilage of the thyroid. 

"Unlike other bones, the hyoid is only distantly articulated to other bones by muscles or ligaments. The hyoid is anchored by muscles from the anterior, posterior and inferior directions, and aids in tongue movement and swallowing. The hyoid bone provides attachment to the muscles of the floor of the mouth and the tongue above, the larynx below, and the epiglottis and pharynx behind," according to Wikipedia. 

In evolutionary terms, it is thought to have developed from the lower half of the second gill arch of fish. Forms of it are present in many vertebrate animals including mammals and birds. 

In mammals, however, it has evolved to provide a wide range of tongue,  pharyngeal and laryngeal movements compared to other animals. In mammals, the bone has become highly complex and is now a small, yet critical bone in the throat.

This diminutive bone enables the dexterous tongue and complex throat movements required for swallowing chewed-food and suckling liquids like milk.

The team found a fascinating fossil

The team studied a newly discovered 165-million-year-old Jurassic mammaliaform species. This creature was more similar to modern mammals than its contemporaneous reptiles and amphibians, who all hade gulping mouths and non-muscular throats. 

For mammals today, and their ancestors, advanced food and liquid swallowing processes were once characteristic of the class. Yet, interestingly, the development and origin of the hyolingual structures are not very well known or understood.

However, there are some tantalizing scraps of evidence from the fossil record. An early of mammaliaform animals, called docodonts, are a vital linchpin in our understanding of the development of the hyoid bone.

Study lead Chang-Fu Zhou and colleagues have recently produced a report on the discovery of a new Jurassic-age docodontan fossil in China. This rare fossil was found to have an exceptionally well preserved, and nearly intact hyoid apparatus, which was complex and very similar to those in modern mammals.

Their findings are significant

This is interesting in and of itself, but the researchers went a step further. By 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.

Their results are astounding. It appears that the saddle-shaped mammal-like hyoid bone seems to predate the separation of the middle ear from the mandible. This is widely considered a key evolutionary step among early-diverging mammaliaforms.

This means that as Microdocodon g. is a basal species in the docodontan clade, the authors believe that this suggests that complex hyolingual structures may have been in place even before mammals.

If true, their findings could push back the current understanding of this important bone's first development.