As part of ongoing revisionary work on Antillean Megalonychidae, we document four new sloth species from Quaternary cave localities in Haiti and the Dominican Republic.^* The new taxa and their tribal affiliations are as follows: Megalocnus zile (Megalocnini), Acratocnus ye (Acratocnini), and Neocnus dousman and N. toupiti (Cubanocnini). Significantly, each is closely related to species in genera long known from Cuba. This observation is of primary biogeographical importance because the most parsimonious explanation for the presence of parallel arrays of lower-level clades of sloths on opposite sides of the Windward Passage is vicariance, not a series of uncoordinated over-water dispersals. For a brief period in the late Paleogene, eastern Cuba, northern Hispaniola, Puerto Rico, and the Aves Rise formed a large positive structure (GAARlandia) that was evidently briefly continuous with northwestern South America. We infer that the later subdivision and subsidence of major portions of GAARlandia must have finely vicariated its biota (which included sloths at least as early as the Early Oligocene). On this argument, Megalocnus, Acratocnus, Neocnus, and Parocnus (= Mesocnus) must have already been in existence as independent clades at the time of origin of the Windward Passage (early Neogene), because cladistically diagnosable members of these taxa occur in Quaternary contexts in both Cuba and Hispaniola. This interpretation is consistent with several new lines of evidence concerning the paleontological and paleogeographical history of the Caribbean region.