Thalassocnus antiquus, sp. nov., is a marine nothrothere from the late Miocene Aguada de Lomas vertebrate horizon (ca. 7 to 8 Ma) of the Pisco Formation in the Sacaco area of the southern coast of Peru. It is similar to the slightly younger latest Miocene Thalassocnus natans, but smaller and distinctly more gracile. The sloping morphology of the lateral border of the nares in T. antiquus differs from the probably plesiomorphic subvertical edge of the nares in T. natans. Parsimony analysis does not resolve the relative positions of T. antiquus and T. natans, and, therefore, does not fully confirm the possibility of a single Thalassocnus lineage, which spans over 4 Ma. However, Thalassocnus is an endemic genus and the stratigraphic distribution of its four species is well known. Furthermore, some characters indicate a continuous evolution from the oldest (T. antiquus) to the youngest species (T. carolomartini). Therefore, we prefer the hypothesis of a single Thalassocnus lineage, although a more complex evolutionary scenario is not discarded. © 2003 by the Society of Vertebrate Paleontology.