New research on Jurassic mammaliaforms from China exposes essential evolutionary advancements in oral structures and mandibular middle ears, clarifying the transition from reptilian to mammalian functions and improving our understanding of mammalian advancement. Reconstruction of Feredocodon chowi (right) and Dianoconodon youngi (left). Credit: IVPPMammaliaforms encompass both living and extinct types that share a close relationship with mammals. Research on mammaliaforms help researchers in figuring out the evolutionary advancements responsible for various mammalian features.In two consecutive research studies in Nature, Dr. Mao Fangyuan and Dr. Zhang Chi from the Institute of Vertebrate Paleontology and Paleoanthropology (IVPP) of the Chinese Academy of Sciences, together with colleagues from Australia and the United States, recently reported 2 Jurassic mammaliaforms from China, revealing the earliest dental diversification, mandibular middle ears, and articular-quadrate joint improvement of mammaliaforms.The research studies offer crucial information about the evolutionary shift from reptilian jaw bones to early mammalian middle ear ossicles, presenting new point of views on the early variety of mammaliaforms and improving the early dental evolutionshuotheriids and mammalian phylogeny.shuotheriids were Jurassic mammaliaforms with pseudotribosphenic molars that included a “pseudotalonid” (a basin-like structure) in front of the trigonid in the lower molars. In contrast, molars in living mammals feature a tribosphenic pattern where the talonid is located behind the trigonid and “receives,” i.e., interlocks with, the protocone of the upper molar for food processing/mastication. Traditionally, shuotheriids have been phylogenetically grouped with “australosphenidans” (including the living monotremes), however this relationship is controversial and leaves some confusing morphological, paleogeographical, and functional issues unusual in mammalian forms.In the very first paper, the researchers took a look at 2 specimens from the Middle Jurassic Daohugou region in Inner Mongolia and developed a new genus and species of shuotheriid, Feredocodon chowi.Primary tooth patterns of mammaliaforms within the phylogenetic frame. Credit: IVPPBased on the evidence of the complete dentitions, occlusal relationships, and the serial homology of the teeth, the researchers proposed a new analysis: The pseudotribosphenic molars are actually homologous to the molar pattern of docodontans.The results of phylogenetic analyses reconstructed from the revised dental characters suggest that a Morganucodon-like forefather separately generated three major groups of mammaliaforms: Docodontiformes (Docodonta and Shuotheridia), Allotheria, and Holotheria (symmetrodontans, therians, and kin). The crucial feature of the tooth evolution in early mammaliaforms is that the molars, which developed from the ancestral triconodont pattern as in Morganucodon, ended up being broader and more intricate to accommodate more efficient food processing. The evolutionary processes of the 3 groups took location in different directions.Mandibular Middle Ears EvolutionIn the 2nd paper, the researchers reported the mandibular middle ears (MdME) of 2 species– one being the shuotheriid Feredocodon described above and the other being a Morganucodon-like animal from the Early Jurassic Lufeng Biota, called Dianoconodon youngi.The two types showed some new morphological functions that support the evolutionary shift from jaw joint bones to middle ear ossicles in early mammals. The mandibular features recommend that one of the dual jaw joints in the ancestral Morganucodon, the articular-quadrate joint, lost its load-bearing function in Dianoconodon, while the mandibular middle ear was better adapted for hearing. The postdentary bones of the shuotheriid species are more advanced, revealing qualities ideal for a simply auditory function.Mandibular middle ears and transformation of the articular-quadrate joints in mammaliaforms. Credit: IVPPThe new proof offers insight into how the ossified Meckels cartilage operated as a supporting mechanism and exposes that the medial displacement of the quadrate relative to the articular bone played a vital function in the improvement from a load-bearing jaw joint to the middle ear structures.This research strongly boosts the view and supports that the progressive evolution of the mammalian middle ear is a classic example of vertebrate evolution.Feredocodon chowi and Dianoconodon youngi are named in honor of Professors Minchen Chow (Zhou Minchen) and Chung-Chien Young (Yang Zhongjian), respectively.References: “Jurassic shuotheriids reveal earliest oral diversification of mammaliaforms” by Fangyuan Mao, Zhiyu Li, Zhili Wang, Chi Zhang, Thomas Rich, Patricia Vickers-Rich and Jin Meng, 3 April 2024, Nature.DOI: 10.1038/ s41586-024-07258-7″Fossils record evolutionary changes of jaw joint to mammalian middle ear” by Fangyuan Mao, Chi Zhang, Jicheng Ren, Tao Wang, Guofu Wang, Fakui Zhang, Thomas Rich, Patricia Vickers-Rich and Jin Meng, 3 April 2024, Nature.DOI: 10.1038/ s41586-024-07235-0These studies were supported by the National Natural Science Foundation of China and the Youth Innovation Promotion Association of CAS.
