Abstract

The aquatic sloth, Thalassocnus, is one of the most intriguing lineage of mammal known from the southern pacific coast of South America during the late Neogene. It was initially recognized in Peru, but recent paleontological surveys also recorded its presence in Chile. However, the determination of how many species of Thalassocnus were actually present in Chile remains as an open question. Here, we provide a detailed morphological description of an isolated distal fragment of humerus recovered at the Mina Fosforita member (ca. 7 Ma), Bahia Inglesa Formation (Atacama Region, northern Chile), which is referred to Thalassocnus. Morphological comparisons with others forms from Chile and Peru allow us to attribute the CPUC/C/557 specimen to Thalassocnus cf. T. natans, though some degree of intraspecific variation is acknowledged. The assessment of the stratigraphic provenance of the materials with previously assigned to Thalassocnus from the Bahia Inglesa, Horcon and Coquimbo formations, permits us to propose that the taxonomic diversity of Thalassocnus in Chile is unequivocal conformed by T. carolomartini, and T. natans. We also admitted the likely presence of T. antiquus and probably T. yaucensis. Futures discoveries of more complete specimens, and/or the detailed analysis of undescribed specimens from Chile will undoubtedly contribute to illustrate the evolutionary history of the Thalassocnus radiation in Chile.