The Southernmost Zalophus in the Western North Pacific: The First Pinniped (Carnivora, Pinnipedia) fossil from Taiwan

The Taiwan waters include a notable diversity of marine mammals, but no extant pinniped populations inhabit this region. Interestingly, multiple sightings, strandings, and captures of living pinnipeds have been reported over the past century. Still, the fossil record of pinnipeds remains zero from Taiwan Island and its surrounding waters, restricting our understanding of their evolutionary and distributional histories in the western North Pacific. Here, we describe a new fossil femur from Taiwan (dredged from the sea bottom of the Taiwan Strait and broadly dated from the Middle to Late Pleistocene). This fossil femur includes diagnostic features of Zalophus: an overall broad shape, a well-developed greater trochanter projecting slightly high above the level of the femoral head, a lesser trochanter projecting medially and close to the femoral head, a shallow and relatively narrow intertrochanteric fossa, and a gently inclined intertrochanteric crest. Our discovery represents the first pinniped fossil from Taiwan and the southernmost record of Zalophus in the western North Pacific. Given this occurrence, we hypothesized that Zalophus occupied the southern region of the western North Pacific during the Pleistocene. The population from Taiwan waters likely went extinct during the Pleistocene, differing from the recent extinction in Japan during the Holocene. Our study highlights the population dynamics of Zalophus in the western North Pacific and demonstrates how fossil evidence can provide insights into extinction events. Future discoveries will clarify the historical biogeography and evolutionary history of pinnipeds in the western North Pacific and offer novel perspectives for conservation paleobiology.