By studying fossils 160 million years ago, Chinese paleontologists proposed that the overlapping anvil-hammer joint is a key step in the separation of the middle ear ossicles and the lower jaw, unveiling the mystery of the evolution of mammalian auditory organs, confirming that the thieves and beasts of the same era as dinosaurs already have the structure of the mammalian middle ear, and solving the long-standing problems in the study of mammalian middle ear and auditory evolution. Recently, the research results were published online in the international journal Nature under the title of A monotreme-like auditory apparatus in a Middle Jurassic haramiyidan, which nature commented on as "this is a classic fairy tale of mammalian evolution."
The research results were completed by the Institute of Paleontology of Yunnan University, the Institute of Vertebrate Evolution and the Inner Mongolia Museum of Natural History. According to reports, the middle ear of living mammals, including humans, contains three ossicles of hearing - stirrup bone, anvil bone and hammer bone, which are the smallest bones in the skeletal system of living mammals, including humans, forming an auditory chain that transmits sound waves and enhances sound wave frequencies from the eardrum to the inner ear. In contrast, reptiles have only one stirrup in the middle ear, while the joint bone in their jaw and the square bone in the skull form a jaw joint, connecting the lower jaw and the skull, with the dual functions of chewing and hearing. In the process of reptile evolution into mammals, square bones and joint bones gradually evolved into anvil bones and hammer bones, forming the keen auditory structure that we mammals now have "three bones standing on". However, exactly how the square and articular bones of reptiles separated from the lower jaw and evolved into the fine and complex ossicles of mammals has been considered a central problem in the study of biological evolution for the past two hundred years.
Traditional models of middle ear evolution suggest that the lower jaw of mammalian ancestors was connected to the skull by Mychonnearthys and articular bones, and that the enlargement of the skull during mammalian evolution led to the position of the middle ear moving backwards and eventually detaching from the lower jaw. Recent studies have proposed the "motor function drive theory", which believes that the behavior of the jaw moving backwards when polyodonts chew is the main reason why the middle ear gradually detaches from the lower jaw and eventually enters the skull. However, the mandibular, which is connected to the skull by mychondrium and articular bone, does not move backwards, and the basal mammalian clades such as platypus do not move backwards when chewing, contradicting the theory of motor function drive.
The specimen from the Late Jurassic Yanliao biota (about 160 million years ago) in Qinglong County, Hebei Province, belongs to the Vilevolodon diplomylos, which has a gliding wing membrane and is a species of thief beast. Very rarely, the sides of the specimen preserve intact ossicles and joint structures in situ, in which the anvil is only about 1 mm long. Through the study of the fine morphology and joint structure of the ossicles, it was found that the ossicles of the thief were obviously separated from the lower jaw, and there was no mychondrium attached, which belonged to the typical mammalian middle ear. The two ossicles, anvils, and hammer bones are the same as living platypus, and are superimposed. It is this overlapping connection that allows for small movements between the anvil and the hammer bone, thus providing space for the movement of the lower jaw relative to the skull, which ultimately leads to the complete separation of the ossicles from the lower jaw.
This way of connecting the upper and lower ossicles of hearing, first appeared in the early members of various branches of Mesozoic mammals, and was also seen in the early stages of the ontology of living platypus (single-hole), placental and marsupials, and was a key part of the transition of the middle ear ossicles from having the dual functions of chewing and hearing to a single auditory function. This superimposed connection, further in the Cretaceous true trilobites, polyodonts, and paraodonts, moved the anvil backwards relative to the position of the hammer bone to form a partial overlap. In the long years that followed, these two small ossicles were completely separated from the lower jaw and continued to shrink into the middle ear and full-time hearing, becoming true mammalian ossicles. It can be seen that it is natural selection, not the jaw-chewing function, that determines the evolution of the middle ear.
This study synthesizes evidence of fossil and emerging ontogenesis to give us a clearer understanding of the evolution of mammalian unique hearing organs. The evolution of the mammalian middle ear involves complex details and is the best example of the extended adaptation and re-action of existing structures (anvil, hammer bone). (CCTV reporter Chen Jian Photo: Institute of Paleontology/ Institute of Vertebrate Evolution, Yunnan University)
(Edited by Wang Hexiang)