A species of the rudist bivalve genus MexicaprinaCoogan, 1973, from the mid-Cretaceous Mal Paso Formation of the Guerrero terrane, southwestern Mexico, is described as new: M. alata. At the type locality of this new species, a 6.5 m thick Mexicaprina-rich floatstone bed conformably overlies a taxonomically diverse build-up of reef corals. This type of facies relationship and implied faunal association has not been observed previously at any other site. Specimens of the type species, M. cornutaCoogan, 1973, also were found at the same locality, but 90 m stratigraphically below the lowest occurrence of M. alata. Compilation and detailed biostratigraphic analysis of the reported occurrences of Mexicaprina demonstrate that the age of the genus ranges from late Albian to early Cenomanian. However, most occurrences are late Albian and the Cenomanian age determinations are questionable and possibly late Albian also. The occurrence of species of Mexicaprina in the allochthonous Guerrero terrane, together with the relatively limited geographic distribution of the genus in the late Albian and early Cenomanian of the Gulf Coast of Mexico, Texas, and Honduras, indicates that this region of the Guerrero terrane was part of the Caribbean paleobiogeographic province during late Albian to early Cenomanian time. This occurrence also suggests that this portion of the Guerrero terrane was very close, possibly accreted, to the southwestern margin of North America by the close of the Early Cretaceous.

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