Fossil sirenia of the west Atlantic and Caribbean region. V. The most primitive known sirenian, Prorastomus sirenoides Owen, 1855

Prorastomus sirenoides is both the oldest and the most primitive sirenian known; it is the only unquestioned sirenian recorded from beds of probable early Eocene age. The unique holotype skull from Jamaica is here redescribed, new material from middle Eocene beds in Jamaica is referred to this species, and two atlases from the late Eocene of Florida are referred to the family Prorastomidae. Although Prorastomus appears to have some autapomorphies that exclude it from the direct ancestry of other sirenians, it is the best available approximation to a structural ancestor of the Sirenia. Therefore, Prorastomus is more appropriate than any Recent species as a representative of the order in analyzing the relationships of sirenians to other mammals. Sirenians are cladistically related to some other primitive ungulates as follows: (Arctocyon (Phenacodus (Moeritherium, Sirenia))). The 3.1.5.3 dental formula of Prorastomus and other Eocene sirenians may be a synapomorphy of the order and a reversal of the loss of P5 and M3 postulated by McKenna (1975) to have occurred in early eutherians; or it may be simply a retention of the primitive eutherian formula. Prorastomus was probably an amphibious but mainly aquatic quadruped that inhabited coastal rivers and embayments. It was probably a selective browser on floating and emergent aquatic plants and, to a minor degree, on seagrasses.