a. The middle ear connected to the lower jaw by the bone behind the mandibular; b. the middle ear connected to the lower jaw by mychondr; c. the middle ear separated from the lower jaw. (Photo courtesy of Bi Shundong) Photo courtesy of Bi Shundong
Beijing, January 28 (China News Network) (Reporter Sun Zifa) How is hearing formed? What is the significance of the evolution of the middle ear for hearing? The study of the formation and evolution of hearing in mammals, including humans, has been a matter of great concern.
Through the in-depth study of a fossil specimen of a double-bowled toothed toothed beast in the Yanliao biota 160 million years ago, combined with evidence of the development of living organisms, researchers from the Inner Mongolia Museum of Natural History and Yunnan University proposed that the overlapping anvil-hammer bone joint is a key step in the separation of the middle ear ossicles and the mandibular in the middle ear, based on the new discovery of the connection between the middle ear and auditory ossicles in mammals, and solved the long-standing problem in the study of mammalian middle ear and auditory evolution.
Mammalian anvil-hammer joint evolutionary diagram. (Photo courtesy of Bi Shundong) Photo courtesy of Bi Shundong
This paper on the important evolution of mammals was published online on the 28th beijing time by the internationally renowned academic journal Nature under the title of "Platypus Middle Ear Reveals the Evolution Mechanism of Mammalian Hearing and Middle Ear".
How the mammalian "three-boned" auditory structure is formed
Professor Bi Shundong of Yunnan University, the corresponding author of the paper, said in an interview through the Internet on the same day that the middle ear contains three small 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 of mammals that are now "three bones". 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.
The skull and middle ear of the rhinoceros. (Photo courtesy of Bi Shundong) Photo courtesy of Bi Shundong
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 anvil-hammer bone overlap relationship solves the problem of jaw movement mechanism
Bi Shundong pointed out that the latest research of the cooperative team has discovered the theory of anvil-hammer bone superposition relationship, thus solving the problem of the jaw movement mechanism. The specimen from the mid- and late-Jurassic Yanliao biota dating back about 160 million years in Qinglong County, Hebei Province, belongs to the double-bowl xiang tooth beast, with gliding wing membrane, which is a kind of thief beast. Very rarely, the sides of this 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, the anvil bone and the hammer bone, like the living platypus, 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.
Restoration of the Toothed Beast. (Photo courtesy of Bi Shundong) Photo courtesy of Bi Shundong
The research team believes that this way of connecting the upper and lower layers of ossicles 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.
Bi Shundong said that the newly published study synthesizes evidence of fossil and ontological ontogenesis, giving the academic community a clearer understanding of the evolution of mammalian unique auditory 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 bones, hammer bones)". (End)
Source: China News Network