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A new genus and species of fossil bat, Koopmanycteris palaeomormoops, representing an ancient member of the endemic Neotropical family Mormoopidae, is described from the Oligocene of Florida. This new species is known from two paleokarst deposits in northern peninsular Florida, the early Oligocene (Whitneyan, 30-32 Ma) I-75 Local Fauna in Alachua County and the late Oligocene (early Arikareean, 26-28 Ma) Brooksville 2 Local Fauna in Hernando County. The discovery of Koopmanycteris extends the fossil record of the Mormoopidae back in time almost 30 million years. Compared to most other described taxa of North American Tertiary bats, the fossil sample of Koopmanycteris palaeomormoops is substantial, consisting of 50 specimens including: P4, all lower teeth from p3 through m3, dentary, petrosal, humerus, proximal radius, and femur. Koopmanycteris is morphologically intermediate between the two living genera of mormoopids, Mormoops and Pteronotus, but is most similar to the former taxon. Characters shared by Koopmanycteris and Mormoops include: large anterolabial basin on P4; large, double-rooted p3; secondary process ventral to angular process on dentary; absence of groove separating capitulum into medial and lateral portions on distal humerus; presence of prominent ridge on posterolateral edge of distal humeral shaft; rounded extremity on proximal radius; and presence of a prominent ridge or tubercle on posterodistal shaft of femur. All of these features appear to be relatively derived, with the exception of the large, double-rooted p3. In several mandibular characters, Koopmanycteris more closely resembles Pteronotus, including: pronounced posteroventral process on the symphysis; lack of curvature of ventral edge of horizontal ramus between posterior edge of symphysis and p4; and less pronounced dorsal upturning of ascending ramus compared to Mormoops. Koopmanycteris retains the primitive dental morphology of Mormoops, with a large double-rooted p3, and the primitive posterior mandibular morphology of Pteronotus, with the lesser degree of dorsal flexion of the ascending ramus. A phylogenetic analysis including all known extant mormoopid lineages as well as representative outgroups from other noctilionoid clades confirms that Koopmanycteris and Mormoops are sister taxa. These findings indicate that the lineages leading to Pteronotus and Mormoops were distinct by ~30 Ma. The fossil record tentatively indicates that the Mormoopidae originated in North America in the early Oligocene or earlier, and may not have reached South America until the Pliocene following the onset of the Great American Biotic Interchange. Several lines of evidence support this hypothesis: (1) the earliest fossil record of the Mormoopidae is in the early Oligocene of Florida; (2) the greatest diversity and endemism of modern mormoopid species is in the West Indies; and (3) the absence of a pre-late Pleistocene record, lack of endemism, and marginal recent distribution of mormoopids in South America. Most species of living mormoopids are obligate cave dwellers, suggesting that the occurrence of caves and paleokarst deposits has affected both their modern distribution and fossil record.