Two Jurassic "mice" can rewrite the structure of mammalian evolution

Researchers from the Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences, and other institutions, through the study of two fossil specimens unearthed from Daohugou, Ningcheng, Inner Mongolia, dating back 168-164 million years, established a new genus of Shu beast, Zhou's Proximatus. At the same time, they also studied the evolution of mammalian teeth, proposed a new view on the origin of monotremes (representatives of living animals such as platypus, etc.), and rewrote the structure of the early evolution of the mammalian tree of life. The relevant research results were published online in the journal Nature on April 3.

Ecological reconstruction of Pericytonus zhoui and Dianacerodon yangi

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

How did teeth and ears evolve in early mammals?

As more and more ancient fossils are unearthed, paleontologists have discovered that the earliest mammals appeared as early as the Jurassic period 200 million years ago. In the middle and late Triassic period, reptiles began to evolve towards mammals. During this evolutionary process, there were two most important morphological and functional changes, which were manifested in the teeth changing from simple to complex to increase the ability to take in food, and the mandibular skull joint changing to a combination of middle ear ossicles to improve hearing effectiveness.

Dental characteristics are key elements in identifying the homology of mammals, that is, mammal "recognition of relatives".

In 1982, Zhou Mingzhen, an academician of the Chinese Academy of Sciences, and Thomas Ritchie, a senior researcher at the Museum Victoria of Australia, collaborated to report a mandibular specimen with cheek teeth from the Jurassic system in Zigong, Sichuan, and established the Dong Shu beast.

Shu beast has special lower molars, with the heel seat located at the front end of the triangular seat instead of the back end. This special pseudo-wedge-shaped tooth bites the upper proto-tip into the lower heel seat during the biting process, and the upper and lower tooth tips and tooth ridges have both shearing and extrusion and grinding functions, which is a unique and efficient way for mammals to process and digest food.

In the past 20 years, the dominant view is that Shushou, a group with pseudosphenodonts, is most closely related to Australian sphenodonts (including living monotremes). However, some scholars disagree, and they find that the teeth of Shushou are very different from those of Australian sphenodonts, and there are unexplained difficulties in terms of morphology, function and evolution.

At the same time, there is also controversy in the academic community about the homology between Shushou and Columnodon (a mammal from the Jurassic period). Some believe that the dental features of the false heel of Shushou correspond to those of columnodonts, so Shushou is classified as a columnodont. Another view is that Shushou has the same triangulum as the molar tooth, while columnodont does not have a triangulum, so the false heel of columnodont and the false heel of Shushou are of independent origin.

This time, the researchers carried out multiple scans with the help of different high-precision CT scans, and finally confirmed that two specimens from the Middle Jurassic unearthed in Daohugou, Ningcheng, Inner Mongolia were Shu beasts, and proposed a new explanation for the concept of pseudo-grinding wedge teeth.

Holotype and paratype of Peristyleodon zhoui

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

In the first paper, the researchers studied two specimens from the Jurassic period (168 million to 164 million years ago) in Daohugou, Inner Mongolia, and established a new genus and species of Shu beast, "Zhou's Proximodon", and proposed a new explanation based on a series of evidence from tooth research: the pseudo-molar teeth of Shu beast are not related to the southern molar teeth, but are homologous to the molar pattern of another extinct mammal. This study puts forward a new view on the origin of monotremes and rewrites the structure of the early evolution of the mammalian tree of life.

The second paper studied the middle ears of a second newly discovered, more primitive mammal, the "Yang's Dianchiodon" and the above-mentioned "Zhou's Proximacodon" from the Early Jurassic (201 million to 184 million years ago) in Lufeng, Yunnan. The specimens of these two new genera and species provide the most complete mandibular middle ear structure preserved in Mesozoic mammals to date, filling in the missing evolutionary evidence from the bimaxillary joint to the transitional monomaxillary joint of the middle ear.

Mao Fangyuan, a researcher at the Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences, said that the study provides a complete chain of evidence based on Chinese materials for the world's difficult problem of mammalian middle ear evolution. The results of the evolution of the middle ear and teeth will open a new chapter in the study of early mammalian evolution.

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