The evolution of the modern mammal jaw is more complex than previously thought, according to a new analysis of fossils belonging to two mammal species: (i) Polystodon chuannanensisa Middle Jurassic tritylodontid herbivore with a relatively large body size and a lifestyle that was probably burrowing, and (ii) Camurocondylus lufengensisa recently described morganucodontan from the Early Jurassic.
“In mammals, the joint connecting the skull to the lower jaw is composed of two bones: the squamosal and the dentary, the dentary bone of the lower jaw,” said Dr. Jin Meng, a curator at the American Museum of Natural History and a researcher at the City University of New York, and colleagues.
“This structure replaced an older jaw joint found in reptiles, which included two different bones: the quadrate and the articular.”
“As animals evolved from early mammal-like reptiles into true mammals, several 'experimental' versions of this new jaw joint evolved to help maintain chewing pressure.”
“Eventually, this led to the emergence of the bimaxillary joint, in which the new dentofacial joint carried most of the chewing load while the reptilian joint continued to function, providing the initial airborne hearing system.”
“Over time, the squamodentate joint became the only joint, and the quadrate squamosal joint became the tiny bones of the mammalian middle ear, which is key to helping mammals hear.”
But scientists still don't fully understand how this new jaw joint evolved, mainly because the fossil record from this period is very limited.
“The evolution of the mammalian jaw joint is one of the most intriguing and unfinished chapters in vertebrate history, with key transitions obscured by gaps in the fossil record,” Dr Meng said.
Researchers used high-resolution computed tomography to study two fossil samples from Institute of Vertebrate Paleontology and Paleoanthropology at the Chinese Academy of Sciences.
One of them Polystodon chuannanensisan opossum-sized animal that had a “horn” that it probably used for digging.
Second Camurocondylus lufengensisa recently described squirrel-sized animal that lived during the Early Jurassic, between approximately 174 and 201 million years ago.
In both ancient animals, paleontologists discovered new jaw formations.
IN Polystodon chuannanensisthey discovered a uniquely shaped secondary jaw joint between the jugal and dentary bones, the first discovery of its kind in any four-limbed animal.
And in Camurocondylus lufengensisthey described a simple articular head of a dentary that appears to show an evolutionary transition to a shape that fits into the socket of the original new jaw joint.
“These discoveries increase jaw joint diversity in mammalian evolution and expand our understanding of the evolutionary sequence of key mammalian characteristics that are critical to understanding how mammals process food and hear airborne sounds,” the authors concluded.
Their paper was published in September 2025 in the magazine Nature.
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F. O etc.. Convergent evolution of diverse mammalian jaw joints. Naturepublished online September 24, 2025; doi: 10.1038/s41586-025-09572-0
