Ecological restoration map of Polistodon chuannanensis Photo: Courtesy of the research team
Chinese paleontologists have made breakthroughs in the study of the evolution of the mammalian jaw joint by re-examining two museum-hold fossils. Their work has helped establish the first evolutionary framework for primary and secondary jaw joint systems, providing a valuable case for understanding the mechanisms of vertebrate form–function evolution.
The structure of the mammalian jaw and its connection to the skull, which transited from that of reptiles, is considered a key – or even defining – feature of mammals. However, due to the scarcity of fossil materials, scientists’ understanding of the evolutionary process of the secondary jaw joint has been limited.
The research team led by Mao Fangyuan, a research fellow from the Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences, used high-resolution CT scanning to re-examine two fossils – one Polistodon chuannanensis from Zigong, Southwest China’s Sichuan Province and the other Camurocondylus lufengensis from Lufeng in Southwest China’s Yunnan Province – achieving breakthrough results, which were published in Nature magazine.
The new findings reveal the morphological diversity of mammalian jaw joint evolution and led researchers to propose, for the first time, a four-stage evolutionary sequence from the primary to the secondary jaw joint. The study also shows that the evolution of jaw joints occurred multiple times with independent origins and diverse types.
Based on the jaw joint polymorphism of the Lufeng mammal and the uniqueness of multi-toothed animal from Zigong, researchers further explored potential multiple evolutionary driving mechanisms.
The researchers said that phenotypic plasticity – the property of organisms to produce distinct phenotypes in response to environmental variation – may have played a key role in the diversification of secondary jaw joints in mammals. Although the idea of phenotypic plasticity as a major driver of adaptive evolution is debated, it is increasingly being supported by research.
The study of these two fossils not only expands the understanding of the evolutionary of key mammalian traits but also helps establish the first evolutionary framework for primary and secondary jaw joint systems, researchers said.
These findings highlight the complexity of evolutionary driving mechanisms. In addition to changes in body size, occlusal biomechanical reconstruction, ecological and behavioral adaptations and phenotypic plasticity together shaped the evolutionary trajectory of the jaw joint, providing a valuable case study for understanding the mechanisms of vertebrate form–function evolution.
Global Times