Tomonori Tanaka

Abstract The herbivorous dinosaur clade Ceratopsia flourished in the northern hemisphere during the Late Jurassic to Late Cretaceous. Previous palaeobiogeographic studies have suggested that their initial diversification occurred in Asia, with early‐branching neoceratopsians extending their geographical range to North America sometime during the Barremian to Albian. However, the specific timing and mode of their dispersal from Asia to North America remains unknown. Here we describe a new, early‐branching neoceratopsian, Sasayamagnomus saegusai gen. et sp. nov., from the Albian Ohyamashimo Formation in southwestern Japan, representing the easternmost fossil record of ceratopsians in Asia. Sasayamagnomus exhibits three diagnostic features in the jugal, squamosal and coracoid, respectively, and also has a unique combination of characters in the lacrimal. Our phylogenetic analysis indicates that Sasayamagnomus forms a clade with Aquilops americanus, one of the earliest neoceratopsians from North America, followed by the sister taxon Auroraceratops rugosus from China. The present time‐calibrated phylogenetic tree indicates that the immigration of neoceratopsians from Asia to North America occurred during the latest Aptian or early Albian, refining the previously suggested timeframe. This aligns with fragmentary neoceratopsian fossil records from the Lower Cretaceous of North America and the initial formation of the Bering land bridge. Furthermore, the simultaneous occurrence of global warming (which enabled the development of extensive forests in the Arctic region) and the emergence of the Bering land bridge during the Aptian–Albian, probably played a crucial role in facilitating the immigration of neoceratopsians from Asia to North America.