Researchers identify four-step process of mammalian jaw joint evolution

(Press-News.org) During the course of evolution, the mammalian cranio-mandibular secondary joint—formed by the dentary condyle and the squamosal glenoid fossa, which replaced the reptilian articular–quadrate joint—represents an innovative structure in vertebrate evolution. By CT-scanning two classic fossils, Chinese researchers found previously unknown jaw joints and proposed a clear, four-step sequence showing how chewing and hearing functions were gradually split between jaw and ear.

The research was led by Prof. MAO Fangyuan from the Institute of Vertebrate Paleontology and Paleoanthropology of the Chinese Academy of Sciences. The team's findings were published in Nature on Sept. 24.

As cynodonts evolved into mammals, various rudimentary secondary joints emerged, such as surangular–squamosal and dentary–squamosal contacts. These structures helped support the weakening primitive joint during chewing. Advanced stem mammaliaforms, like Dianoconodon, developed a dual-jaw joint, with the lateral dentary condyle–squamosal fossa joint becoming the main load-bearing structure. However, the scarcity of fossils limited understanding of this process.

To address this knowledge gap, the researchers re-examined two old fossil specimens using high-resolution CT scans.

One specimen, Polistodon chuannanensis—a Middle Jurassic tritylodontid from Zigong, Sichuan—was first described in 1984. The new study reveals it had a unique dentary condyle–jugal fossa secondary jaw joint, the first of its kind identified in tetrapods, challenging previous ideas about secondary joint morphology.

The second specimen, from the Lower Jurassic of Lufeng, Yunnan, was a morganucodontan specimen identified as a new genus and species: Camurocondylus lufengensis. Its simple dentary condyle, formed by the upward bending of the posterior end of the dentary lateral ridge, provides evidence supporting the hypothesis that mammalian dentary condyles evolved from this ridge structure.

These discoveries allowed the researchers to propose the first four-stage sequence for the evolution of the primitive and secondary joints: Stage one is the articular–quadrate joint (reptiles); stage two is the primitive joint dominant, with secondary contact points (advanced cynodonts); stage three is the load-bearing secondary joint dominant, with primitive joint aiding sound transmission (stem mammaliaforms); and stage four is the fully developed dentary–squamosal joint, with the primitive joint transformed into a middle ear ossicle joint (mammals, docodonts, haramiyidans).

The findings suggest multiple independent origins for jaw joint types. While the dentary–squamosal joint isn't unique to mammals, a load-bearing version is characteristic of mammaliaforms. The dentary–zygomatic (jugal) joint of Polistodon represents a unique adaptation to herbivorous, fossorial lifestyles, while Camurocondylus shows features consistent with gradual evolution leading to mammalian diversification.

Furthermore, the researchers explored possible diverse evolutionary driving mechanisms. The currently prevalent "miniaturization drive hypothesis" applies to small insectivorous groups like Camurocondylus but seems less plausible for the larger-bodied Polistodon. The latter was herbivorous and showed adaptations to a specific ecological environment, which also aligns with the discovery of possible tritylodontid burrow systems at the Polistodon specimen locality.

Based on these observations, the researchers propose that phenotypic plasticity—environmentally induced variation—may have played a significant role in the diversification of secondary joints in late cynodonts.

This study expands our understanding of mammalian evolution and provides a window into the complex interplay of developmental, functional, and environmental factors shaping vertebrate morphology.

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