GEOGRAPHY

Fossils of euphrozoites in the Rehe biota were the first to find ossified Mychonite cartilage


Orthotype specimen of The Beast (Courtesy of Vertebrate Paleontology and Paleoanthroptery, Chinese Academy of Sciences)

Ecological Restoration Diagram of Chuanchuan Beast (Painted by Chen Yu)

Recently, Wang Haibing and Wang Yuanqing of the Institute of Vertebrate Paleontology and Paleoanthropology of the Chinese Academy of Sciences, in collaboration with Peking University Stomatological Hospital and American counterparts, published the research results on the evolution of early euphroids in the Journal of the Natural Science Society of the Royal Society B, which for the first time reported the ossified Mysch cartilage and inner ear morphology in early euphropods, providing important evidence for the evolution of mammalian auditory organs.

Mychondrium is ubiquitous during vertebrate development, and although its morphology and function vary in different taxa, ossification of Mychondrium in adult individuals has so far only been found in fossil mammals.

Embryonic development and fossil evidence suggest that Mychondrium played an important role in mammalian evolution: during the evolution of the separation of the auditory ossicles from the mandible in the middle ear, the Myrner cartilage played a role in supporting and stabilizing the ossicles until the middle ear completely detached from the mandibular and stabilized at the base of the skull. The connection between Mychondral and the bones of the lower jaw and middle ear is the core element that defines the evolutionary stage of the middle ear in mammals, and it is also an important information reflecting the different evolutionary stages of early mammals.

At present, paleontologists have found ossified Mystenia cartilage in some primitive mammals found in Rehe organisms (such as true trilobites, Zhanghe and mammals), which are closely connected to the auditory ossicles of the middle ear, indicating that their middle ear is in the evolutionary stage of the transitional middle ear. However, as one of the most important structures in the early evolution of mammals, scientists still have limited understanding of its evolutionary process, such as whether the phenomenon of mychondrogenic ossification occurs in more mammalian taxa, and the contact relationship between Mychondrial and middle ear bones during the degradation of Mychondrium.

The new specimen reported by Wang Yuanqing’s team was found in the Nine Buddha Halls Group of the current Cretaceous Unity in Lingyuan, Liaoning Province (about 120 million years ago). After a long period of careful indoor repairs, high-precision CT scans and comparative studies, the researchers believed that the fossil represented a new species of early true mammals, and named it jiufotang chuanchuan beast, which was dedicated to Li Chuankuo, a researcher at the Institute of Vertebrate Paleontology and Paleoanthropology of the Chinese Academy of Sciences, to commemorate his contribution to the study of early mammal evolution.

The ossified Malpentilage preserved in the Passerians is slender and short, and is likely to lose substantial connection with the middle ear ossicles (and the outer drum bone), a finding that confirms that some adult individuals of early euphrasia still retained the more degenerated ossified Mychondrium. This is the first time that the existence of ossified Myrdria cartilage has been found and confirmed in eulogy, filling the gap in the evolution of ossified Myrnelloy cartilage in early mammals. At the same time, the morphology of the ossified Mai’s cartilage preserved in the beast is significantly different from that of the true trilodons and paradodonts of the same era.

The ossified Mychondromacene cartilage found in true trilobites (such as reptiles, swallow-sharp toothed beasts, and Liao-pointed toothed beasts, etc.) and pairs of toothed mammals (such as Zhanghe beasts, hairy beasts, and source skimmers) represents the transitional stage of mammalian middle ear evolution. At the same time, the new fossil evidence is the most degraded ossified Mystenia cartilage known in Mesozoic mammals, indicating that the middle ear of the passerian is likely to be completely detached from the mandibular, representing the key node of the mammalian middle ear from the lower jaw, reflecting the gradual degeneration of the ossified Malthorn cartilage of mammals; the contact relationship between the ossified Mesthral cartilage preserved in the mammals and the bones of the mandibular and middle ear bones provides a more accurate reference standard for improving the definition of the evolutionary stages of the mammalian middle ear.

Based on high-precision CT scans of specimens, researchers reconstructed for the first time the labyrinth morphology of the inner ear bone of the early Cretaceous eucalyptus. The inner ear of the beast is relatively intact, and its morphology has both primitive and progressive characteristics, filling the gap in the evolution of the inner ear of the basal mammal and the late Cretaceous true beast. The volchal coil of the heraldic beast is close to 360°, surpassing that of other groups of mammals of the same era and close to most of the Late Cretaceous true mammals.

The research was supported by the Strategic Pioneering Science and Technology Project of the Chinese Academy of Sciences (Category B), the National Natural Science Foundation of China, the Youth Innovation Promotion Association of the Chinese Academy of Sciences, and the State Key Laboratory of Modern Paleontology and Stratigraphy. (Source: China Science Daily Cui Xueqin)

Related paper information:https://doi.org/10.1098/rstb.2021.0042



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