Fossils of extinct fur seals and walruses (Carnivora: Pinnipedia) occur within rich vertebrate fossil assemblages recovered from the shallow marine Mio-Pliocene Purisima Formation, central California. Two isolated postcranial bones—a humerus and a radius—belonging to a juvenile fur seal (Pinnipedia: Otariidae) exhibit circular depressions. These bone modifications are associated with radial and circular fractures, and are characterized by inward displacement of the cortex. These depressions lack features typical of erosive invertebrate borings, trampling damage from media ( = substrate) interaction, puncturing by another object during diagenetic compaction, such as a clast embedded or associated with the modification, or pathologic bone modification. These features are best interpreted as tooth marks. These tooth marks lack certain characteristics of commonly reported marks inflicted by shark teeth, such as linear gouges and subparallel scrapes formed by xiphodont and serrated teeth. These bone modifications instead exhibit a circular shape and inward displacement of the cortex, consistent with puncturing by a conical mammal tooth. The size and distribution of the tooth marks, in concert with the known vertebrate assemblage from the Purisima Formation, indicate several possible producers of the bone modifications: a pilot whale or beluga-like cetacean, a terrestrial carnivore, a dusignathine or odobenine walrus, or a case of infanticide by a conspecific otariid.