According to recent analyses of fossils from two mammalian forms, the development of jaws in modern mammals proves to be more intricate than previously understood. (i) Polystodon chuananensis, a mid-Jurassic herbivorous tritylodont known for its relatively large size and possibly fossilized lifestyle, and (ii) Camulochondylus rufengensis, a newly identified Morganucodontan from the Early Jurassic.

Reconstructed illustration of Polystodon chuananensis. Image credit: Chuang Zhao / PNSO.

“In mammals, the joint connecting the skull to the lower jaw consists of two bones: the squamous bone and the dentary bone, where the lower jaw teeth are situated,” stated Dr. Jin Meng, a curator at the American Museum of Natural History and a researcher at the City University of New York, along with colleagues.

“This configuration replaced the older temporomandibular joint seen in reptiles, which is composed of two different bones: the quadrate and the articular bone.”

“As organisms transitioned from early mammal-like reptiles to true mammals, various ‘experimental’ versions of this new temporomandibular joint arose to withstand the forces of mastication.”

“Ultimately, this culminated in the formation of the bimandibular joint, with the new dentosquamous joint handling most of the chewing pressure, maintaining the joint functionality in reptiles, and establishing an initial system for detecting airborne sounds.”

“Over time, the dentate squamosal became the sole joint, transforming the quadratoid joint into a diminutive bone within the mammal’s middle ear, a critical feature that aids in hearing.”

However, scientists still lack a comprehensive understanding of how this new temporomandibular joint evolved, primarily due to the scarcity of fossils from this era.

“The evolution of the mammalian temporomandibular joint represents one of the most fascinating yet incomplete chapters in vertebrate history, with gaps in fossil records obscuring significant transitions,” remarked Dr. Meng.

Researchers utilized high-resolution computed tomography scans from the Institute of Vertebrate Paleontology and Paleoanthropology at the Chinese Academy of Sciences.

One of these species, Polystodon chuananensis, is an opossum-sized creature featuring “horns” potentially used for digging.

The other, Camulochondylus rufengensis, is a newly described squirrel-sized animal that lived during the Early Jurassic, approximately 174 to 201 million years ago.

Paleontologists identified new jaw structures in both ancient species.

In Polystodon chuananensis, they discovered a uniquely formed secondary temporomandibular joint located between the zygoma and dentary. This marks the first identification of this joint structure in a tetrapod.

In Camulochondylus rufengensis, they characterized a simple articular head of the dentary bone that likely indicates an evolutionary step towards a morphology adaptable to the new temporomandibular joint socket.

“These discoveries enhance the diversity of temporomandibular joints in mammalian evolution and broaden our comprehension of the evolutionary lineage of key mammalian features crucial for understanding how mammals process food and perceive airborne sounds,” the authors concluded.

Their study was published in the journal Nature in September 2025.

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F. Mao et al. Convergent evolution of diverse temporomandibular joints in mammals. Nature published online on September 24, 2025. doi: 10.1038/s41586-025-09572-0