Evolution of manatees: a speculative history.

Abstract

The earliest known probable trichechid, Potamosiren from Colombia, is Middle Miocene in age. 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 Andaean orogeny dumped large quantities of silt and dissolved nutrients into many South American rivers, stimulating growth of aquatic macrophytes. 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. 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 T. manatus remaining in coastal waters. In the Indopacific, Dugong evolved root hypsodonty. In the Caribbean, however, immediate competition from the already 'hypsodont' manatees forestalled any such adaptation in the dugongids of that region, which became extinct and were replaced by Ribodon and, later in the Pliocene, Trichechus. The apparent competitive exclusion between T. manatus and T. inunguis is discussed.-from Author