Why do mammals, including humans, hear sounds?How did the middle ear, the most crucial of all, evolve?These mysteries in the history of mammalian evolution have always been the tireless pursuit of scientists.
Found a "Dream Beast".
On November 27, the reporter learned from the Institute of Vertebrate Paleontology and Paleoanthropology of the Chinese Academy of Sciences that the team led by associate researcher Wang Haibing and researcher Wang Yuanqing of the institute found a true mammal fossil belonging to the Cretaceous period in the Rehe biota, which was named "Dream Beast". Based on the study of this "dream beast", they revealed for the first time the structure of the middle ear of the Early Cretaceous eumammal, and discussed the evolutionary innovation of the middle ear of mammals in combination with paleontological and developmental biology evidence. The findings** were published in the journal Nature Communications. 
"Dream Beast" ecological restoration map. Zhao Chuang and Wang Haibing said that the middle ear has always been a classic case of gradual evolution in mammals. For a long time, there has been a lack of direct fossil evidence for the early evolution of the mammalian middle ear due to fossil preservation issues. "Studies have shown that the ossicles of the mammalian middle ear have undergone two evolutionary stages: the mandibular middle ear and the transitional middle ear, and finally completely separated from the mandibular and entered the middle ear cavity, an evolutionary event that occurred many times in different branches of mammals during the Mesozoic era. However, these findings are concentrated in the basal taxa, and direct evidence of the evolution of the middle ear is limited in our mammal ancestors, which are closely related to our human evolution. ”
Until a few years ago, Wang Yuanqing's team found this piece in the Cretaceous Jiufotang Formation under the Yangjiaogou in Kazuo, Liaoning Province200 million years of "dream beasts". After extensive in-house repairs, high-precision CT scans, 3D reconstructions, and new phylogenetic studies, the research team determined that the fossil represents a new species of early eumammals.
Fill the gap in the early evolution of the middle ear in mammals
Wang Yuanqing said that compared with the discovery of new species with strange shapes and beyond imagination, the "dream beast" is the kind of fossil that has been awaited for a long time and is expected. "The fantastic thing about this little beast is that, as one of the first eu-mammals, it shows the typical characteristics of the middle ear of modern mammals, filling in the gaps in the early evolution of mammals. ”
Through high-precision CT scans of the type specimens, the research team reconstructed the skull morphology of the "Dream Beast" and reported for the first time the middle ear bone morphology of the Early Cretaceous true mammals, including the outer tympanic bone, malleus bone, incus and stapes. Wang Haibing said that the cartilage groove of the mandible of the "dream beast" has completely degenerated, indicating that the ossicles of the middle ear have been completely detached from the lower jaw and have the characteristics of a typical mammalian middle ear. "The results differ from the recently published Nine Buddha Hall Legend, which still retains the more slender ossified Mai cartilage. This suggests that in the early stages of mammal evolution, there was a certain degree of variation in the morphology of the middle ear, even in species with close phylogigenesis. ”
One of the most important findings of this study is that the intact, saddle-shaped malleus-anus joint is preserved in the "fantastic little beast", which is clearly distinguished from the currently known basal taxa and consistent with modern mammals. This finding provides an important reference and new thinking for the discussion of the transformation of the malleus-anus joint in early mammals. Wang Haibing said.
In addition, the team discussed the evolutionary trends of the hearing and feeding organs of early mammals, and proposed that the evolution of the hearing and feeding organs of mammals occurred independently. "The evolution and innovation of the hearing organs of the early Cretaceous eumammals and the improvement of the wedge-type tooth structure have laid an important foundation for the improvement of the hearing ability and the expansion of ecological space in modern mammals. Wang Haibing said.
*: Popular Science Times.
Author: Chen Jie.
Editor: Wu Tong.
Review: Wang Fei.
Final review: Yin Chuanhong.