CVEB12001 skull and right jaw. (Courtesy of the research team)
Yunnan Lufeng is the world-famous early Jurassic dinosaur fossil production area, and also produced a large number of basal mammal fossils of the same period. Tridents belong to the mammaliamorpha, a “sister group” of mammalian ancestors, named because each of its upper canine teeth (the teeth after the front teeth) has three longitudinal tips. During the long geological history period (about 200 million to 120 million years), the triangular toothed animal species gradually flourished to 29 species, which were widely distributed around the world, of which 6 species were in Lufeng, Yunnan. However, due to poor fossil preservation and limited research methods, there are differences in the identification of the three-column dentates at the genus level.
Recently, the vertebrate evolution research team of the School of Life Sciences of Yunnan University published the latest results on the research of tridents in The History Biology Online. The study provided a detailed analysis of a fossil trident skull from the Shawan section of the Lower Jurassic Lufeng Formation, one of the smallest known triangular tooth skulls.
The researchers conducted three-dimensional reconstruction of the teeth of the specimen through microscopic CT scanning, and carefully compared and analyzed with other three-column tooth fossils produced by Lufeng, the results showed that the specimen should be classified into the delicate Lufeng beast, and the new material provided more information on the morphological variation of the three-column dentate, indicating that the previously named “Long-snouted Dianzhong” and “Little Bian” were actually the same genus as the delicate Lufeng.
The three columns of Lufeng toothed animal fossils are all from the Lower Jurassic Lufeng Formation, which consists of the lower Shawan section (dark purple layer) and the upper Zhangjiawa section (crimson layer). The main taxa of the three-column toothed beasts in the Shawan section is the Yunnan Bian beast, while the zhangjiawa section is dominated by the delicate Lufeng beast. The delicate Lufeng is a small and medium-sized three-column toothed beast with a skull length of 31–75 mm, two to three pairs of incisors, with a proximity of the length and width of the upper canine teeth, a tip of 2-3-3, and a smaller posterior tongue tip (L3). The fossil record of the delicate Lufeng beast found in the Shawan section may indicate that the geological age difference between the Shawan section and the Zhangjiawa section is not large.
Most vertebrate teeth are replaced longitudinally, while three-row toothed mammals employ a transverse tooth replacement strategy: new teeth germinate from the back of the dentition and then move forward. Through high-precision micro-CT scan and three-dimensional reconstruction, the researchers found that the specimen was also transversely realented, and the occlusal position and root morphology of the upper and lower teeth of the specimen suggested that the replacement method and rate of the upper and lower teeth of the tripartite may be different.
The teeth of some living mammals are also replaced laterally, such as elephants, manatees, frost rats, etc., and the number of teeth in these animals is constant, and the age of individuals can be determined according to the replacement rate. If the rate of tooth replacement in the trident can be quantified, it would be a good breakthrough point to judge the ontogenesis of the fossil.
The research is supported by the “Double First-Class” joint project of Yunnan Provincial Science and Technology Department and Yunnan University, the State Key Laboratory of Paleontology and Stratigraphy of Nanjing Institute of Geology and Palaeontology of the Chinese Academy of Sciences, and the Sino-Myanmar Joint Laboratory of Eco-environmental Protection. (Source: China Science Daily Cui Xueqin)
Related paper information:https://doi.org/10.1080/08912963.2022.2104643