Chongqing — A collaborative Sino-American study has revealed astonishing diversity in jaw joint evolution among mammalian ancestors and their close relatives. A fossil from Yunyang, Chongqing, provides decisive evidence confirming a significant evolutionary innovation.
The study "Convergent evolution of diverse jaw joints in mammaliamorphs" was published in Nature on September 24, 2025.
The study "Convergent evolution of diverse jaw joints in mammaliamorphs", published in Nature on September 24, 2025, analyzed precious mammaliaform fossils from Sichuan Zigong, Yunnan Lufeng, and Chongqing Yunyang, demonstrating that evolution experimented with diverse jaw joint structures before settling on the modern mammalian form.
The dentary-squamosal jaw joint (where the dentary bone connects with the squamosal bone) is a key characteristic of mammals. This revolutionary structure enables complex jaw movements in multiple directions, allowing efficient chewing that contributed significantly to mammalian evolutionary success.
When researchers examined Polistodon chuannanensis, a herbivorous tritylodontid from the Middle Jurassic (approximately 168-160 million years ago) discovered in Dashanpu, Zigong, they found something extraordinary. This creature's jaw joint differed markedly from the "double joint" (comprising both dentary-squamosal and quadrate-articular bones) of early Jurassic Morganucodon-like mammals found in Lufeng, Yunnan, and also from the primitive "quadrate-articular" joint typical of reptiles like dinosaurs.
Polistodon evolved an entirely new type of secondary jaw joint—the "dentary-jugal joint." Instead of connecting to the squamosal bone as in modern mammals, its dentary bone extended upward to form a dentary condyle that articulated with a socket in the zygomatic bone (commonly known as the "cheekbone"), creating a "pulley-like" structure.
Mandibular joint characteristics of Polysphenodon chuannanensis. (a, Internal view of the zygomatic arch and glenoid fossa; b, Lateral view of the dentary condyle; c, Internal view of the mandible and zygomatic arch.)
Comparison of joint characteristics among Polysphenodon chuannanensis (b), dinosaurs (a), mammaliaforms (c), and extant mammals (d)
Researchers speculate this unique adaptation might have supported an extended tooth row and accommodated a burrowing lifestyle, representing an important structural-functional innovation in tritylodontid evolution.
The study also reported a new genus and species from the Early Jurassic Lufeng - Camurocondylus lufengensis (approximately 201-184 million years ago). This small insectivorous mammal possessed a primitive dentary-squamosal joint where the dentary condyle was formed simply by the curved end of an external ridge, not yet expanded into a ball shape. This morphology is more primitive than the ball-like dentary condyles of early mammaliaforms, providing evidence for how the mammalian jaw joint formed.
Schematic diagram of the mandibular joint features of Polysphenodon chuannanensis and Lufengocynodon qukuaiensis.
The discovery raised an important question: was this a unique evolutionary innovation or merely a pathological anomaly in a single specimen?
To answer this, the research team turned to tritylodont specimens discovered in the Chongqing Yunyang dinosaur fauna. The jaw structure of Yunyang's Bienotheroides species represents a transitional state for its group: the posterior dentary has converging ridges forming a sharp articular process lacking a distinct condyle, with a relatively flat jaw joint surface, while the zygomatic bone shows strong downward expansion, providing the foundation for joint formation.
In contrast, the Zigong "maverick" Polistodon has a fully separated and independent dentary condyle with a well-developed articular fossa in the zygomatic bone.
Critically, both fossils were found in Middle Jurassic strata in the Sichuan Basin, with nearly identical geological ages and locations. This proximity made the Yunyang Bienotheroides specimen crucial evidence, convincingly demonstrating that Polistodon's jaw joint represented a genuine evolutionary innovation rather than a pathological anomaly.
Comparison of skull characteristics between Polysphenodon chuannanensis and the Bienotheroides sp. from Yunyang (Middle Jurassic Lower Shaximiao Formation). (a–c, Ventral view of the skull of P. chuannanensis (a) and Bienotheroides sp. (b, specimen 16YPDC-01; c, specimen 19YP-67). d–f, Lateral view of the same specimens in a–c, respectively. g–i, Internal view of the zygomatic arch formed by the jugal and squamosal bones in the same specimens.)
Comparison of the mandibular joint characteristics between Polysphenodon chuannanensis (top) and the Bienotheroides sp. from Yunyang (bottom).
This research reveals that evolution follows anything but a straight path, instead resembling a process of repeated trial and error. Faced with different survival pressures - such as varying feeding requirements (herbivory, insectivory) across ancient environments in Yunyang, Dashanpu, and Lufeng - closely related species could evolve completely different solutions.
The Yunyang dinosaur fauna contains not only numerous dinosaur fossils but also critical mammaliaform fossils that have become essential resources for deciphering Mesozoic life evolution puzzles. The pivotal role of Yunyang specimens in this study demonstrates how each fossil site represents a unique piece of the puzzle, collectively working to reconstruct the grand epic of life's history with its countless contingencies and evolutionary innovations.
Restoration of Polysphenodon chuannanensis (Artwork by Zhao Chuang / © PMSO).
The collaborative research involved multiple institutions, including the Institute of Vertebrate Paleontology and Paleoanthropology (Chinese Academy of Sciences), Chongqing Institute of Paleontology, Zigong Dinosaur Museum, Lufeng Natural Resources Bureau, Chuxiong Yi Autonomous Prefecture Fossil Research Center, and the American Museum of Natural History.
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