Abstract

The genus Trichechus (Mammalia, Sirenia, Trichechidae) harbors a modest diversity, comprising only three living species of manatees, and no species currently recognized as fossil. Herein, we report a new extinct species of manatee from the late Pleistocene of the Brazilian Amazonia, Trichechus hesperamazonicus, sp. nov. It comes from the alluvial deposits of the Rio Madeira Formation along the Madeira River, state of Rondônia, western Brazil, and is represented by a partial palate with both molar series and two partial right dentaries. The new taxon shows a mosaic of characteristics resembling those of other manatee species, as well as some unique characters. It differs from all other Trichechus species by possessing a wide space between the posterior lower tooth row and ascending ramus of dentary, and by having the anterior border of the ascending ramus covering the posterior end of the tooth row in lateral view. The results of morphometric analyses (principal component analysis and discriminant analysis) further support the distinctiveness of the new species. A phylogenetic analysis recovers the new species in a polytomy with T. inunguis and the clade formed by T. senegalensis and T. manatus. The levels from which the remains were recovered produced a radiocarbon date of 44,710 ± 880 years before present; together with molecular clock estimates, this date suggests that until recently at least two species of manatees coexisted in the fluvial systems of western Amazonia.