New Plio-Pleistocene fossils from the eastern Pacific shed light on the early evolution of otariids (Carnivora: Pinnipedia) in the Southern Hemisphere

Abstract Colonization of the Southern Hemisphere by sea lions and fur seals (Otariidae) represents a crucial yet poorly understood chapter in pinniped evolution. Here, we describe exceptionally preserved Plio-Pleistocene (2.7–1.4 Mya) fossils of sea lions from Peru, including a new taxon, Otaria josefinae sp. nov., offering insights into otariid southward diversification. Total-evidence phylogeny reveals O. josefinae as sister to the extant South American sea lion (Otaria byronia), together with Hydrarctos lomasiensis forming a robust clade. This grouping and the clade leading to Arctocephalus, Neophoca, and Phocarctos define the Southern Hemisphere otariid clade we name Australotariia. Otaria josefinae displays moderate size, pronounced sexual dimorphism, and a partially specialized palate, reflecting an incipient stage in the anatomical evolution of O. byronia associated with polygyny and versatile foraging. These changes coincided with Plio-Pleistocene climatic shifts favouring otariid dominance over phocids in productive upwelling ecosystems. Our results support an Early Pliocene (∼4.8 Mya) divergence between northern and southern otariids tied to a period of marine megafaunal turnover. These fossil findings show Otaria occupied the southeastern Pacific before Arctocephalus, evolving locally since the Pliocene. Arctocephalus might have arrived very recently, but the timing and mode of its diversification in the Southern Hemisphere ecosystems remain obscure.