The Origin of Phocid Seals and Evolution of Key Behavioral Character Traits

The earliest phocid seals evolved 30–24 million years ago (Ma) in the western North Atlantic. There remains uncertainty in evolutionary relationships, with molecular data positioning phocids as the sister group to all other pinnipeds (fur seals, sea lions, and walruses), while morphology places them as sister to the extinct Desmatophocidae. The two major subfamilies of extant seals, Monachinae (southern seals) and Phocinae (northern seals) diverged 18 Ma and diversified in the North Atlantic and Paratethyan region. Both clades colonized the southern hemisphere, although southern hemisphere phocines quickly went extinct, while monachine seals declined in diversity in the North Atlantic. Monachines and phocines exhibited opposing body size trends with monachines increasing and phocines decreasing in size. Phocids are the only pinnipeds to display four feeding strategies: pierce, grip and tear, suction, and filter-feeding. Phocids were ancestrally shallow water divers with increased diving capabilities evolving among some large-bodied taxa. In terms of mating systems, ice breeding appears to have independently evolved in both clades. Although faunal dynamics are inadequately studied there is evidence during the Plio-Pleistocene for the replacement of desmatophocids and walruses by otariids and phocids and for the replacement of otariids by phocids in South America.