A Large Hadrosauroid Skull from the Late Cretaceous Bayanshiree Formation of the Gobi Desert (Mongolia)

Takasaki, R., Evans, D.C., Tsogtbaatar, K., Chiba, K., Chiarenza, A.A., Hirasawa, T., Saneyoshi, M., Chinzorig, T., Badamkhatan, Z.

The 6th International Symposium on Asian Dinosaurs in Japan 2025

Asian Dinoaur Association

Fukui, Japan

The Upper Cretaceous terrestrial deposits of the Gobi Desert, Mongolia, are globally recognized for their well-preserved vertebrate assemblages, which have significantly contributed to our understanding of dinosaur evolution. While extensive research has targeted the younger Campanian-Maastrichtian strata (e.g., Djadokhta, Barun Goyot, Nemegt formations), the underlying Cenomanian-Santonian Baynshire Formation remains understudied, despite its importance in understanding faunal dynamics preceding the latest Cretaceous. The Baynshire Formation is geographically widespread, with key fossiliferous exposures including Baynshin Tsav in the west and the type locality, Bayan Shiree, several hundred kilometers to the east. Throughout the formation, hadrosauroids are the most abundant vertebrate fossils. To date, the hadrosaur fossil record has been attributed to a single genus, Gobihadros. Numerous well-preserved skeletons and skulls of Gobihadros are documented, primarily from Baynshin Tsav,
and a wealth of isolated elements from other localities are referred to this taxon. Despite the abundance of hadrosauroid remains, most articulated or associated cranial material has been restricted to small-bodied, presumably subadult individuals, although the existence of larger hadrosauroids had been implied by isolated, large postcranial elements. We herein report a new, large hadrosauroid specimen from the Bayan Shiree locality, consisting of a nearly complete skull, which is approximately twice as large as the largest known skull of Gobihadros. The skull exhibits a combination of plesiomorphic and derived characters. Plesiomorphic features include the long and slender nasal lacking evidence of an expanded external naris; the parietal strip exposed on the posterior margin of the skull table; anteroposteriorly short mandibular condyles of the quadrate, whose medial condyle is only slightly taller than the lateral one; and a broadly exposed ascending process of the surangular. At the same time, the specimen also displays a suite of derived characters absent in Gobihadros. Most notably, the medial process of the squamosal is straight, a condition observed in taxa such as Hadrosauridae. This is complemented by other derived features, such as the lateral expansion of the dentary at the base of the coronoid process, and the occlusal plane of the dentary dentition being parallel to the lateral margin of the bone. The discovery of the large skull from the same formation as Gobihadros warrants a careful evaluation of ontogeny as a
potential source of morphological variation. However, several features suggest a taxonomic distinction between Gobihadros and the new skull. An important point of comparison involves its close relative, Bactrosaurus, from the Iren Dabasu Formation. Notably, the derived features the new specimen (e.g., a straight medial process of the squamosal, expanded dentary at the base of the coronoid process) are absent in Bactrosaurus at any known ontogenetic stage, including the comparably large skull. This suggests that the derived characters are taxonomically informative and relatively stable throughout ontogeny, rather than being simple artifacts of
maturity, at least among the closely related hadrosauroids. The unique combination of morphological traits in the new specimen is, therefore, consistent with the presence of a second hadrosauroid taxon in the Baynshire Formation. Additionally, the comparison with Plesiohadros from the slightly younger Djadokhta Formation is particularly interesting. While the new specimen shares the everted premaxillary rim with Plesiohadros, it retains a simple, generalized hyoid which lacks the uniquely robust and twisted structure. The highly specialized premaxilla of the new skull resembling Plesiohadros, yet retaining the plesiomorphic hyoid, suggests the new skull may represent an evolutionary transition within the Gobi hadrosauroid lineage. While the new skull may
suggest co-existence of two hadrosauroids in the Bayan Shiree ecosystem, such interpretation must be carefully evaluated in future since the Bayan Shiree locality specimens that were referred to Gobihadros are isolated, small materials lacking diagnostic features.