Members of the strictly stygobiont, continental subterranean amphipod family Metacrangonyctidae are reported for the first time outside the Old World. Two new species of Metacrangonyx are described from two widely separated localities in the Dominican Republic (Hispaniola), one facing the Caribbean and the other the Atlantic ocean. The discovery of metacrangonyctids in the western Atlantic suggests that they are an ancient subterranean lineage tied to the shores of the Tethys belt, and thus weakens previous biogeographic arguments raised to favour their separate and independent family status with respect to the Hadziidae. The discovery in the Mediterranean of marine populations of metacrangonyctids is reported as well, and both findings are used to test the reliability of the scenario currently held for the origin and evolution of this peculiar group of stygobiont amphipods. It is concluded that Metacrangonyx is a thalassoid lineage already present in the shores of the western Tethys before the complete aperture of the central North Atlantic (circa 110 Myr BP), and with marine populations persisting at both shores of this ocean until some time in the Quaternary, in case they have not yet disappeared. Evidence derived from Hispaniolan and Balearic Metacrangonyx does not support the correspondence between species-groups and the time at which precise waves of colonization of continental ground waters took place (after Turonian and Senonian marine regressions, respectively) as is assumed to occur for Old World taxa.