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The western Amazonian manatee (Trichechus hesperamazonicus) is an extinct species of manatee that lived about 40 thousand years ago in the Madeira River, in the Brazilian state of Rondonia. It is known from this single locality, with possible occurrences in the Brazilian state of Acre.

Description
T. hesperamazonicus is known by two fragmentary mandibles and part of the palate. Compared with other manatees, it has a wide space between the posterior lower tooth row and ascending ramus of dentary, where the buccinator muscle is located, and a wide ascending ramus of the mandible, which covers the posterior end of the tooth row. It also shares some characters of the living species, as relatively few large teeth and few mental foramina as in T. manatus and T. senegalensis (in contrast with the many smaller teeth and many mental foramina of T. inunguis) and a straight and shallow mandibular symphysis as T. inunguis and T. senegalensis (in contrast with the downturned snout and deep mandibular symphysis of T. manatus). The affinities of this species are still not fully resolved, being recovered in a polytomy with T. inunguis and a clade formed by T. manatus and T. senegalensis.

Distribution
The latin name hesperamazonicus refers to the western localization of the species, hesperos (west) and amazonicus (from the Amazon region). The fossil was found near the village of Araras, in Nova Mamoré, state of Rondonia, Brazil. The site also yielded pampatheres and a fossil species of tapir. 14C dates from associated wood samples give an age of 44,710 ± 880 years before present. At that time, the Madeira River had slow currents and the formation of marginal lakes. At the end of the Pleistocene, changes on the river regime originated several rapids, creating an environment unsuitable to manatees, that are currently absent from the region.