The newly discovered Archæocursor asiaticus is Asia's earliest-known ornithischian

A team of Chinese paleontologists discovered a new dinosaur species, Archaeocursor asiaticus, in southwestern China, offering fresh insights into the early evolution of ornithischian dinosaurs in Asia. The fossil, dating back approximately 193 million years to the Early Jurassic period, could represent the oldest known representative of the Ornithischia group in the region.

Led by Xi Yao from Yunnan University, the researchers unearthed a nearly complete left femur measuring 93 millimeters in length in the Ziliujing Formation, about two kilometers north of Central Park in the Yubei District of Chongqing. According to SciencePost, the femur reveals what appears to be the earliest-diverging ornithischian dinosaur in Asia.

Archaeocursor asiaticus, estimated to have measured around one meter in length, was a herbivorous species. Ornithischian dinosaurs, including this new species, were a group of herbivorous dinosaurs with diverse lifestyles, ranging from armored ankylosaurs to more modern ceratopsians and hadrosaurs.

High-resolution CT scans allowed the team to make detailed morphological observations of distinct features. Phys.org reports that the femur exhibits a ridge on the proximal surface, a broad fourth trochanter, and a uniquely shaped medial condyle, distinguishing it from contemporaneous ornithischians like Yuxisaurus kopchicki. Osteohistological analysis revealed parallel-fibred bone and tightly packed lines of arrested growth, indicating that the specimen was a young adult at the time of death. This provided crucial information about growth rates and skeletal maturity.

The authors of the study note that during the Early Jurassic period, ornithischian fossils were abundant and diverse in the southern supercontinent Gondwana, which comprised South America, Africa, India, Madagascar, Australia, and Antarctica. In contrast, the northern supercontinent Laurasia—consisting of North America, northern Asia, Greenland, and Europe—had a very scarce fossil record of ornithischians, mainly limited to armored dinosaurs. This disparity suggests that ornithischians originated in Gondwana.

The discovery of Archaeocursor asiaticus suggests that an early wave of ornithischian dinosaurs migrated to East Asia during the Pliensbachian age. SciencePost notes that this indicates a previously unknown migration of ornithischians to East Asia at an early stage of their evolution, independent of the migration of armored dinosaurs. The affinities between Archaeocursor asiaticus and the Gondwanan species Eocursor parvus suggest that a migration northward occurred well before the appearance of armored dinosaurs in Asia.

Attempts to reconstruct the evolutionary relationship placed Archaeocursor asiaticus at a primitive position within Ornithischia. Phys.org states that these findings reinforce hypotheses of a rapid diversification during the Early Jurassic. "The new phylogenetic topology proposed in the study suggests an additional independent dispersal of ornithischian dinosaurs in eastern Asia during the Early Jurassic," the researchers stated, according to El Tiempo. "However, due to the fragmentary nature of the holotype Archaeocursor asiaticus, efforts to support this clade are ongoing, pending new fossil discoveries," they cautioned.

While dinosaur evolution in Gondwana is well documented, fossils found in Laurasia, especially in China, depict a more complex picture. SciencePost mentions that the fossils from Laurasia indicate earlier migrations and diversifications than previously thought. The discovery provides new elements to the theory of dinosaur dispersion, suggesting that ornithischians had already begun to colonize regions further north, such as East Asia, at a time when other groups, like armored dinosaurs, were not yet present.

This offers a new perspective on the geographical distribution of Ornithischia and allows for a rethinking of the models of dispersion and diversification of dinosaur groups, particularly in East Asia. The discovery fills an important gap in the Asian fossil record and is considered valuable to the field of paleontology.

Due to the fragmentary nature of the fossil, researchers emphasize that these hypotheses must be confirmed by future discoveries. "Efforts to support this clade are ongoing, pending new fossil discoveries," reports Phys.org.

The research on Archaeocursor asiaticus was published in the journal iScience.

The article was written with the assistance of a news analysis system.