The Pisco Formation (Miocene–Pliocene) of Peru contains a diverse marine vertebrate fauna. Among the more unusual members of the fauna is the semi-aquatic to aquatic nothrothere sloth, Thalassocnus. Continued fieldwork indicates that, in addition to the late Miocene type species, Thalassocnus natans, the genus is also represented by two additional species in higher horizons of the Pisco Formation. The skull morphology of T. natans is described along with that of Thalassocnus littoralis, sp. nov. and Thalassocnus carolomartini, sp. nov. from the earliest and late early to early late Pliocene, respectively. Many aspects of the cranium of the oldest species of Thalassocnus, T. natans, compare well to those of terrestrial nothrotheres. The terminal species, T. carolomartini, has the most derived features of the skull, absent in all other nothrotheres, that are related to feeding in a marine environment.
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Vol. 22 • No. 2