The following historical scenario is proposed to explain the available data on manatee evolution. Protosiren-like sirenians probably reached the isolated South American continent in the Eocene; this dispersal and subsequent genetic isolation marked the origin of the Trichechidae. The earliest known probable trichechid, Potamosiren from Colombia, is Middle Miocene (Friasian) in age, and lacks the horizontally-replaced supernumerary molars which characterize later manatees. Up to the Late Miocene, trichechids may have been restricted to coastal rivers and estuaries of South America, where they fed on freshwater plants, while dugongids inhabited West Atlantic and Caribbean marine waters and exploited seagrass meadows. The Mio-Pliocene Andean orogeny dumped large quantities of silt and dissolved nutrients into many South American rivers, stimulating growth of aquatic macrophytes, particularly true grasses (Gramineae). Manatees adapted to this newly abundant but abrasive food source by evolving supernumerary molars continually replaced throughout life, as in the Mio-Pliocene form Ribodon from Argentina. Further elaboration of this functional analogue of hypsodonty led to smaller but more numerous teeth (providing greater length of enamel ridge per unit of occlusal area) in the Pliocene-Recent genus Trichechus. The Late Miocene orogeny also blocked the Pacific entrance to the upper Amazon region, temporarily forming a closed basin. Manatees trapped in the Amazon basin, or entering it via stream-capture events, further adapted to feed on the gramineous "floating meadows" of Amazonian lakes and evolved the yet smaller, more complex and wear-resistant teeth of the modern T. inunguis, in contrast to the less progressive T. manatus remaining in coastal waters. Meanwhile, the Pliocene onset of continental glaciation, by lowering sea level and increasing erosion and silt runoff into nearshore seagrass beds, probably put added stress on the already marginally adequate brachyodont dentitions of marine bottom-feeding dugongids. In the Indopacific, Dugong evolved root hypsodonty, possibly in response to this source of increased tooth wear. In the Caribbean, however, immediate competition from the already "hypsodont" manatees must have forestalled any such adaptation in the dugongids of that region, which became extinct and were replaced by Ribodon and, later in the Pliocene, Trichechus. Pliocene or Pleistocene waif dispersal of primitive Trichechus to West Africa gave rise to T. senegalensis. Also discussed are the homologies of manatee cheek teeth, the adaptive significance of differing rostral deflections among Trichechus species, and apparent competitive exclusion between T. manatus and T. inunguis.
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