Fossil Sirenia of the West Atlantic and Caribbean region. I.Metaxytherium floridanumHay, 1922

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Abstract
The vast majority of the abundant fossil sirenian remains from the Bone Valley phosphate mining district (chiefly in Polk and Hillsborough counties, Florida) represent a single species of dugongid, Metaxytherium floridanum Hay, 1922. The osteology of this species is described here in detail. Most or all of these specimens apparently come from Units 4–5 of the lower Bone Valley Formation and are late middle Miocene (early Clarendonian) in age. M. ossivallense (Simpson, 1932) is a junior subjective synonym of M. floridanum. M. calvertense Kellogg, 1966, and “Hesperosiren” Simpson, 1932, are not represented in the Bone Valley Formation. Specimens are also reported here from elsewhere in Florida, mainly Alachua County. Some or all of these are later than the typical Bone Valley form, being of late Clarendonian or early Hemphillian age, and some of them, here termed Metaxytherium, cf. M. floridanum, seem to represent a slightly more advanced stage of evolution. M. floridanum had a strongly deflected rostrum adapted to bottom-feeding and very small, subconical, enameled tusks. It is practically indistinguishable from the contemporaneous M. medium of western Europe; however, the recency of their common ancestry remains to be clarified. Apparently M. floridanum was endemic to the West Atlantic and became extinct without issue by the end of the Miocene.

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