The First Dinosaur from the Lower Cretaceous Bayan Gobi Formation in the Western Yin'gen-Ejinaqi Basin of Inner Mongolia, China

The Bayan Gobi Formation is an important Early Cretaceous dinosaur fossil-bearing horizon, although the number of known dinosaur taxa remains relatively limited. Here, a new dinosaur material, primarily comprising proximal caudal vertebrae, is reported from the Bayan Gobi Formation. The specimen marks the first published dinosaur record from the Bayan Gobi Formation in the western Yin'gen-Ejinaqi Basin, further extending the geographical range of dinosaur fossils in this horizon. The specimen was discovered at a depth of 473.2 m in a drill core and displays several features indicative of its ornithischian affinity: slender, distally tapering transverse processes near the neurocentral suture, positioned well below the level of the zygapophyses, and slender ossified tendons on the dorsal side of the neural arch. The proportions among the caudal vertebral elements closely resemble those of Psittacosaurus , and principal component analysis based on geometric morphometrics supports this similarity. However, given the limited material available, assignment to a specific genus or species remains uncertain. Notably, the dorsolateral surface of the 2nd to 4th caudal centra shows regularly arranged nutrient foramina, a feature not previously reported in other ornithischians, thereby providing new morphological data for dinosaurs from the Bayan Gobi Formation. However, since the new specimen may represent an immature individual and the available material is limited, it is difficult to determine whether this feature reflects interspecific, interindividual, or ontogenetic variation.