Researchers from the Institute of Vertebrate Paleontology and Paleoanthropology of the Chinese Academy of Sciences and other institutions have established a new genus and species of mammal, the Zhou's Near-Zhucheng Beast, based on two fossil specimens unearthed from the Dahugou locality in Ningcheng, Inner Mongolia, dating back approximately 168 to 164 million years.
In addition, they have examined the evolution of mammalian teeth, proposing new insights into the origins of monotremes (represented by modern animals such as the platypus), thereby reshaping the early evolutionary structure of the mammalian tree of life. The findings were published online on April 3rd in the journal "Nature."
Early mammalian teeth and ears evolved through significant changes as revealed by the increasing number of ancient fossils unearthed. As early as 200 million years ago in the Jurassic period, the earliest mammals had already emerged. During the later stages of the Triassic period, reptiles began evolving towards mammals, exhibiting two crucial morphological and functional changes: the development of more complex teeth to enhance food intake capabilities and the transformation of the lower jaw joint into the middle ear ossicles to improve auditory efficiency.
The characteristics of teeth serve as key elements in identifying the evolutionary relationships among mammals, crucial for determining mammalian "kinship." In 1982, Chinese Academy of Sciences academician Zhou Mingzhen and senior researcher Thomas Rich from the Victoria Museum of Australia collaborated to report a lower jaw specimen with cheek teeth from the Jurassic strata in Zigong, Sichuan, establishing the Szechuanodon.
Szechuanodon possessed distinctive posterior molars located at the front of the jaw rather than the rear. This unique pseudo-tribosphenic tooth morphology allowed for both shearing and grinding functions between the upper cusps and lower molars during occlusion, representing an efficient method for mammals to process and digest food.
Over the past two decades, the prevailing viewpoint suggests that taxa like Szechuanodon, possessing pseudo-tribosphenic teeth, are most closely related to Australian monotremes, including extant monotremes. However, some scholars dissent, noting significant morphological, functional, and evolutionary disparities between Szechuanodon teeth and Australian monotremes.
Simultaneously, there is controversy in the academic community regarding the phylogenetic relationship between Szechuanodon and docodonts, a group of Jurassic mammals. Some argue that the pseudo-tribosphenic tooth features of Szechuanodon correspond to docodonts, thus placing Szechuanodon within the Docodonta.
Others contend that since docodonts lack tribosphenic molars while Szechuanodon possesses them, the pseudo-tribosphenic molars of docodonts and Szechuanodon have independently originated. Researchers utilized multiple high-precision CT scans to examine two specimens from the Middle Jurassic of Daohugou, Ningcheng, Inner Mongolia, confirming them as Szechuanodon and proposing a novel interpretation of pseudo-tribosphenic teeth.
In the first paper, researchers delved into two specimens from the Jurassic period (approximately 168 to 164 million years ago) unearthed in Dahugou, Inner Mongolia. They established a new genus and species called "Zhoushi Paracimex" and proposed a fresh interpretation based on a series of dental studies: the pseudotribosphenic molars of Paracimex are not related to southern tribosphenic mammals but share a common ancestry with another extinct mammal's molar pattern. This study presents a novel perspective on the origin of monotremes, reshaping the early evolution of mammals' family tree.
The second paper investigated the middle ear of the newly discovered and more primitive mammal, "Yangshi Dianping Toothed Beast," from the early Jurassic period (201 to 184 million years ago) in Lufeng, Yunnan, along with the aforementioned "Zhoushi Paracimex."
The specimens of these two new genera provide the most complete preserved mandibular middle ear structures among Mesozoic mammals to date, filling the evolutionary gap between the double jaw joint and transitional monojaw joint.
According to Dr. Mao Fangyuan, a researcher at the Institute of Vertebrate Paleontology and Paleoanthropology of the Chinese Academy of Sciences, this research offers a comprehensive evidence chain based on Chinese materials for the perplexing global issue of mammalian middle ear evolution. The findings in middle ear and dental evolution are expected to propel a new chapter in early mammal evolution research.
