Hearing the Voices of Our Cretaceous Ancestors from Our Middle Ears

Humans can have such sensitive hearing because of the existence of the middle ear, which is located between the inner and outer ears and is the main part of sound wave transmission. "Hearing" is one of the most wonderful things in the process of biological evolution, and the evolution and development of the middle ear in nature has always attracted much attention.

Wang Haibing, an associate researcher at the Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences, and Wang Yuanqing, a researcher, discovered a new species of early eutherian, the "Dream Beast", in the Jehol Biota. Based on the study of Dream Beast, they revealed the middle ear structure of the eutherians in the Early Cretaceous for the first time, and combined the evidence of paleontology and developmental biology to discuss the evolutionary innovation of the middle ear of therian mammals. The relevant research results were published online in Nature Communications, which is the first study on the middle ear structure of eutherians in the Early Cretaceous.

The holotype specimen and restoration diagram of the fantasy beast (skeleton restoration diagram drawn by Chen Yu)

Image source: Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences

The "dream beast" about 120 million years ago,

Shows typical features of the middle ear

The mammalian middle ear has three auditory ossicles - the stapes, incus and malleus. They are the smallest bones in the mammalian skeletal system, important structures in the auditory chain, and the key to mammals' hearing.

The mammalian middle ear is a classic example of gradual evolution, but due to the difficulty in preserving fossils, the discussion of the early evolution of the mammalian middle ear has always lacked direct fossil evidence . According to the research team of Wang Yuanqing, the fossil of the true beast was discovered in the Jiufotang Formation of the Lower Cretaceous in Yangjiaogou, Kazuo, Liaoning (about 120 million years ago) - a "little mouse" about 12 cm long. After a long period of repair, analysis and research, it was finally determined that the fossil represented a new species of early true beasts, and it was found that its middle ear bone morphology included the external tympanic bone, hammer bone, incus and stapes, and it was named "Microtherulum oneirodes". The Meckel's cartilage groove of its mandible has completely degenerated, and the middle ear ossicles have completely detached from the mandible, with the characteristics of a typical mammalian middle ear . Wang Yuanqing said that this means that in the early stages of mammal evolution, even in species with close phylogenetic relationships, there is a certain degree of variation in the middle ear morphology.

Dreamlike Little Beast Ecological Restoration Map (drawn by Zhao Chuang)

Image source: Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences

One of the most important findings of the study is that the "Dream Beast" preserves a complete, saddle-shaped hammer-incus joint. The results of the study confirm that the middle ear structure of modern mammals began to evolve as early as the Cretaceous .

Combined with the bulbous process of the hammer bone, the research team classified the middle ear of the dream beast as a "small middle ear". Living mammals with "small middle ears" are all adapted to high-frequency hearing, which shows that early eutherians may have had more sensitive hearing than other mammals of the same period.

After comprehensive analysis, the research team believes that the divergent evolution of the auditory organs and feeding organs of mammals occurred independently.

Comprehensive sources: Xinhua News Agency, Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences, Science and Technology Daily, China Science Daily, etc.