Jurassic fossils reveal the mechanisms of mammalian hearing and middle ear evolution

The Skull and Middle EarXiangtooth Beast Recovery Map
On January 28, Nature magazine published online the results of cooperation between the Inner Mongolia Museum of Nature and Yunnan University. Based on new findings on the connection between the small bones in the mammals' middle ear, the study suggests that the overlapping birch-hammer bone joint is a key step in the separation of the middle ear hearing bones from the jaw, solving the long-standing problems in the study of the evolution of the middle ear and hearing in mammals.
study, which took three years, combined fossil and living biological individual development evidence to help better understand the evolution of mammalian unique auditory organs. The evolution of the mammalian middle ear contains complex detailed processes and is the best example of the extended adaptation and re-action of existing structures (cheekbones, hammer bones). Bi Shundong, a professor at Yunnan University and one of the authors of the paper, told the China Science Daily.
the study found that the theory of the crucle-hammer bone overlap relationship solves the problem of the mechanism of jaw movement. The specimen of the study comes from the Yanliao biome of the mid-to-late Jurassic in Qinglong County, Hebei Province (about 160 million years from now), and belongs to the twin-toothed beast, which has a gliding wing membrane and is a species of thief beast.
" specimens are preserved in place with a complete hearing bone and joint structure, where the birch bone is only about 1 mm long, very rare. Wang Jun, the first author of the paper and a natural museum in Inner Mongolia, said.the three hearing bones in the middle ear of
-bones, cheekbones, and hammer bones - are the smallest bones in the bone system of living mammals, including humans, forming an auditory chain that transmits sound waves and enhances the frequency of sound waves from the eardrum to the inner ear. In contrast, reptiles have only onebone in their middle ear, while the joints in their jaws and squares in their skulls form jaw joints that connect the jaws and skulls, with the dual function of chewing and hearing.
reptile evolved into mammals, square and joint bones evolved into cheekbones and hammer bones, forming a keen auditory structure that mammals now "stand on three bones". However, how exactly the reptile's square and joint bones are separated from the jaws and thus evolved into delicate and complex mammalian listening bones has been considered a central problem in biological evolution studies for the past two hundred years.
traditional middle ear evolution model suggests that the jaws of mammalian ancestors were connected to the skull through Macy's cartilage and joint bones, and that the increase in the brain during mammalian evolution caused the middle ear to move back and eventually out of the jaw. Some recent studies have put forward the "motion-driven thesis", which holds that the behavior of the lower jaw movement when the polymatic toothed beast chews is the main reason that causes the middle ear to gradually disengage from the lower jaw and eventually enter the skull. However, the jaws attached to the skull through Macy's cartilage and joint bones do not move back, and base-dry mammalian branches such as platypus do not move back when chewing, contradicting the theory of motion-driven driving.
study of the subtle forms and joint structure of the hearing bone found that the listening bones of the thieves and beasts were clearly separated from the jaws, and were not connected to mace cartilage and belonged to the typical mammalian middle ear. Two small listening bones, birch bones, hammer bones and the live platypus, for the up-down overlap relationship. It is this overlapping connection that allows for small movements between the cheekbones and the hammerbones, thus providing space for the movement of the jaw relative to the skull, which ultimately leads to the complete separation of the hearing bone from the jaw.
This way of connecting the upper and lower folds of the hearing bones, first appeared in the early members of the various branches of the meso-living mammals, and was also visible in the early stages of individual development of the live platypus (single-hole), placental and bagged species, and was a key link in the transition of the hearing bones from the middle ear to the single auditory function with the dual function of chewing and hearing.
this overlapping type of connection, in the Cretaceous true three-pointed beasts, polymato-toothed beasts and pairs of toothed beasts further, the cheekbone relative to the position of the hammer bone moved back to form a partial overlay. Over the long years that followed, the two small bones were completely separated from the jaw and kept shrinking into the middle ear and full-time hearing, becoming true mammalian listening bones. Thus, it is natural selection, not the jaw chewing function, that determines the evolution of the middle ear.
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