Fossil Sirenia of the West Atlantic and Caribbean region. VIII. Nanosiren garciae, gen. et sp nov and Nanosiren sanchezi, sp nov.

摘要

The new dugongine dugongid Nanosiren garciae is the third species of fossil sirenian to be described from the Bone Valley phosphate deposits of Florida. It is found in the Upper Bone Valley Formation and is of latest Hemphillian (early Pliocene) age. Nanosiren sanchezi is described from the late Miocene upper member of the Urumaco Formation, Falcon State, Venezuela. It appears to be directly ancestral to N. garciae, and was preceded in the West Atlantic, Caribbean, and East Pacific region by other, undescribed members of the same generic lineage dating back at least to the early Miocene. The Nanosiren lineage is the most basal clade of the Subfamily Dugonginae after Crenatosiren olseni, and is the sister group to all remaining dugongines. It comprises the smallest known post-Eocene sirenians, with adult body lengths and weights probably around 2 m and 150 kg. Newborn animals may have had body masses little greater than the theoretical limit of 6.8 kg for a viable aquatic endotherm. In addition to a shallow draft, Nanosiren spp. had small conical tusks and strong rostral deflections. These characters possibly indicate that they foraged in shallower water than the larger dugongids with which they were sympatric, such as Metaxytherium (a rib from Urumaco probably referable to this genus is also recorded here). Nanosiren probably fed in nearshore marine waters on small seagrasses such as Halodule and Halophila.