FOSSIL BATS OF THE AMERICAS
Three Tertiary localities in Florida have produced molossid bats, the late Oligocene Brooksville 2 LF, the early Miocene Thomas Farm LF, and the late Pliocene Macasphalt Shell Pit LF (Czaplewski et al. 2003b). A single isolated and waterworn upper molar of a molossid is known from Brooksville 2. This tooth is about the same size as the upper M1 in the living species Nyctinomops macrotis, but differs from that genus in the lack of a metaloph and lingual cingulum. The Brooksville tooth is larger than the two molossid upper molars from Thomas Farm, and further differs from them in lacking a metaloph, lingual cingulum, and talon cingulum, among other characters. The Brooksville specimen probably represents a new genus and species of Molossidae, but the single available tooth is not sufficient for the description of a new taxon. Two isolated well-preserved upper molars from Thomas Farm most closely resemble Tadarida but are also similar to Mormopterus. It is not possible to distinguish between these two genera based on the currently available fossil teeth from Thomas Farm. The single molossid fossil from the Macasphalt Shell Pit is a distal humerus. This fossil represents a species of Tadarida larger than T. brasiliensis, and is about the same size as the humerus of the extinct Pleistocene species T. constantinei first described from Slaughter Canyon Cave (=New Cave) in New Mexico (Lawrence, 1960), and also known from early Pleistocene (Irvingtonian) deposits in Mammoth Cave in Kentucky (Jegla and Hall, 1962) and Hamilton Cave in West Virginia (Repenning and Grady 1988; Winkler and Grady 1990). The Macasphalt humerus is not referred to T. constantinei because of the paucity of Blancan material available, several proportional differences in the humeri, and the disparity in age.
The Molossidae occur worldwide in tropical regions, with a few species found in temperate latitudes on each of the continents (Hill and Smith 1984; Koopman 1993). Legendre (1984a; 1984b; 1985) reviewed the systematics and historical biogeography of Cenozoic Molossidae, including the sparse Tertiary record of this family in North America. Czaplewski et al. (2003b) recently reviewed and updated the Tertiary history of the New World Molossidae. The earliest North American molossid is the extinct genus and species Wallia scalopidens from the middle Eocene (Uintan NALMA) of Saskatchewan, Canada (Storer 1984). The next youngest North American records of this family are the Florida fossils from Brooksville 2 and Thomas Farm. The Florida Oligocene and Miocene molossids, consisting of three isolated teeth, cannot be identified to genus pending recovery of additional material. An isolated upper tooth of Eumops cf. E. perotis from the late early Blancan McRae Wash fauna of Arizona (Czaplewski 1993a) and the distal humerus of Tadarida from the late Blancan Macasphalt Shell Pit (Morgan 1991) are the only Pliocene records of molossids from North America.
The Molossidae are one of only three or four families of bats so far known to be shared by North America and South America before the Great American Biotic Interchange, the others being Emballonuridae, Vespertilionidae, and possibly Phyllostomidae (Czaplewski 1997; Czaplewski et al. 2003b). Despite their ability to fly, bats rarely cross wide oceanic water barriers and as such most bats probably did not cross the Bolivar Trough separating North and South America prior to the connection of these continents at the Panamanian Isthmus in the Pliocene. Bats have only rarely been included in discussions of the Interchange but there is some evidence that Eumops may have evolved in South America and immigrated to North America in the Pliocene as a member of the Interchange fauna. Czaplewski (1997) identified Eumops in the middle Miocene La Venta fauna in Colombia, whereas this genus is unknown in pre-Interchange sites in North America. The origin of Tadarida in the Western Hemisphere is more problematic. The presence of Tadarida-like teeth in the early Miocene of Florida and definite fossils of Tadarida from the late Pliocene of Florida and the early to medial Pleistocene of Kentucky, New Mexico, and West Virginia, suggest that the New World representatives of this genus may have originated in North America. Tadarida, Mormopterus, or a closely related genus may have immigrated to North America from Eurasia across the Bering isthmus in the early Miocene or before. Several species of Tadarida and Mormopterus are known from Oligocene and Miocene sites in Europe and Asia (Legendre 1984b). It seems likely that Tadarida immigrated from Eurasia to North America in the Miocene, and then immigrated to South America in the late Pliocene after the onset of the Interchange.