Island-hopping across the Wallace Line: A new Pleistocene Stegodon fossil skull from Luzon (Philippines) reveals dispersal links to Wallacea

Southeast Asia is renowned for its exceptional biodiversity and complex biogeography, shaped by major faunal boundaries such as the Wallace and Huxley Lines. Among the most iconic Pleistocene megafauna in the region is Stegodon, an extinct proboscidean relative of modern elephants, commonly represented in fossil assemblages across various Southeast Asian islands, including Luzon, Philippines. However, the evolutionary history and dispersal of the Luzon Stegodon remain poorly resolved due to the scarcity of diagnostic cranial material. Here, we present the first formal description of a Stegodon skull from the Philippines, recovered from Lannig, Solana, Cagayan (northeastern Luzon), along the northwestern flank of the Enrile Anticline, and likely originating from the lower Awidon Mesa Formation, estimated to be of late Early Pleistocene age. The specimen (CM-B-1-2021) is fragmentary and deformed, preserving a complete right upper cheek tooth, interpreted as the first molar (M¹), and the proximal sections of two small tusks. Morphological and morphometric comparisons indicate it belonged to a late juvenile (approaching subadult) intermediate-sized individual with affinities to the S. trigonocephalus group. Notably, the molar's narrow, nearly subhypsodont morphology closely resembles that of S. f. florensis from Flores, Indonesia (late Early to Middle Pleistocene), suggesting a possible north–south faunal connection between the Philippines and Wallacea, island-hopping across the Wallace Line. This study provides new insights into Stegodon dispersal dynamics across Southeast Asia and highlights the need for a comprehensive reexamination of other Luzon Stegodon specimens to refine their taxonomy and clarify broader evolutionary patterns in the region.