Abstract

The herbivorous dinosaur clade Ceratopsia flourished in the Northern Hemisphere during the Late Jurassic to Late Cretaceous. Previous palaeobiogeographic studies suggest 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 elusive. 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 it also displays 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 of 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 to early Albian, refining the previously suggested timeframe. This aligns with fragmentary neoceratopsian fossil records from the Lower Cretaceous in 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, along with 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.