FOSSIL BATS OF THE AMERICAS
SUMMARY OF FLORIDA TERTIARY BATS
During the Oligocene, bats in the Neotropical families Emballonuridae and Mormoopidae dominated Florida chiropteran faunas, in particular I-75 and Brooksville 2. Other families of bats are present in Florida Oligocene faunas, including the principally Neotropical families ?Phyllostomidae, Natalidae, and Molossidae, as well as the Vespertilionidae; however, fossils of these four families are rare. By the early Miocene (~18 Ma), emballonurids had become a minor component of the Florida chiropteran fauna and mormoopids were absent. One emballonurid, one natalid, and two molossids occur at Thomas Farm, but these families constitute a small percentage of the chiropteran fauna from this site, which is primarily composed of vespertilionids, including five species. Although nearly half of the bat species from Thomas Farm (4 of 9 species) belong to Neotropical families, they comprise fewer than 10% of the total number of bat fossils from this site, which is dominated by the vespertilionid Suaptenos whitei.
The discovery of bats belonging to the Mormoopidae and Natalidae from the Oligocene and early Miocene of Florida adds an important dimension to the evolutionary history and historical biogeography of these two chiropteran families. Previously, neither mormoopids nor natalids had a pre-Pleistocene fossil record. The Florida fossils occur in what is now the Nearctic Region, whereas neither family is now found outside of the Neotropical Region. Living species of both families are found at the northern limits of the Neotropics in northern Mexico and Cuba.
Emballonurid, mormoopid, and natalid bats are unknown from Florida and elsewhere in temperate North America after the early Miocene. Phyllostomids are absent from Florida between the late Oligocene and the late Pliocene. The vampire bat Desmodus is known from one latest Pliocene site and several early Pleistocene sites in Florida. Desmodus is a desmodontine phyllostomid that almost certainly evolved in South America and then immigrated northward following the onset of the Great American Biotic Interchange at about 2.5 Ma; however, the Florida Pliocene and early Pleistocene records are the earliest known vampire bats (Morgan 1991). All other fossil records of Desmodus from North and South America are from the late Pleistocene (Morgan et al. 1988). Two additional genera of phyllostomids (Macrotus and Leptonycteris) occur in late Pleistocene cave sites in temperate North America. Molossids are unknown in Florida between the early Miocene and the late Pliocene. Tadarida occurs in the late Pliocene Macasphalt Shell Pit LF in southern Florida, along with several Interchange mammals (Morgan 1991). However, it has yet to be determined if the New World representatives of Tadarida originated in North or South America, although a North American origin by way of Eurasia appears to be the most plausible biogeographic scenario (Czaplewski et al. 2003b). The large molossid Eumops first appears in the middle Miocene of Colombia (Czaplewski 1997) and then appears in North America in the late Pliocene (about 3 Ma) of Arizona, possibly as an early participant in the Interchange (Czaplewski 1993a).
Species in the Emballonuridae, Mormoopidae, Natalidae, and Molossidae presumably survived in tropical North America (southern Mexico, Central America, and the West Indies) during the remainder of the Neogene, as all four families currently inhabit this region, but they have no pre-Pleistocene fossil record there. The Emballonuridae and Molossidae are known from the middle Miocene La Venta Fauna from Colombia in northern South America (Czaplewski 1997). Chiropteran fossils from middle Miocene through late Pliocene (16.5-3.0 Ma) sites in Florida and elsewhere in North America consist entirely of vespertilionids, the predominant family in the current chiropteran fauna of temperate North America.
The disappearance of the Neotropical bat fauna in Florida in the early to middle Miocene (~15-18 Ma) is probably related to climatic changes at this time that led to the expansion of grassland and savanna habitats in temperate North America and the reduction or disappearance of tropical forests in the southeastern United States. At about this same time, several other vertebrate groups in Florida experienced major evolutionary and biogeographic changes, also possibly related to climatic change. The early Miocene Thomas Farm fauna documents this transitional period, not only between earlier and later bat faunas, but also several other vertebrate groups. The horse Parahippus leonensis, the most common species at Thomas Farm, is close to the group that gave rise to the explosive radiation of advanced three-toed (hipparionine) horses in the late early Miocene (MacFadden and Hulbert 1988). Thomas Farm also appears to represent the time period between older snake faunas dominated primarily by boids, and younger faunas containing primarily colubrids and viperids (Auffenberg 1963). Florida Oligocene snake faunas are dominated by primitive henophidians, principally the tropical family Boidae, whereas middle Miocene and younger snake faunas are mostly composed of more advanced caenophidians, in particular Colubridae and Viperidae, the most common snake families in the modern temperate fauna of North America. Both boid and colubrid snakes occur at Thomas Farm. Thomas Farm is also one of the earliest faunas in which passeriform perching birds (Order Passeriformes) occur. Passerines are the dominant group of land birds in Pleistocene and Recent faunas (Becker 1987).
Florida Oligocene and early Miocene bat faunas provide important information on the community structure of middle Cenozoic chiropteran faunas that is available nowhere else in North America. However, these Florida faunas were almost certainly not typical of the continent as a whole because bats of similar age from the western United States consist exclusively of vespertilionids (Galbreath 1962; Czaplewski et al. 1999). Fossil bats from Florida suggest that prior to the middle Miocene, the state enjoyed a tropical climate, that, coupled with the widespread occurrence of caves in the northern half of the peninsula, may have permitted the occurrence of tropical bats that could not survive elsewhere in North America where the climate was more temperate. Three of the four tropical bat families that occur in Florida middle Cenozoic sites, Emballonuridae, Mormoopidae, Natalidae, are unknown from temperate North America after the early Miocene. Presumably, their disappearance from Florida relates to the continent-wide climatic changes that led to overall drier habitats and the widespread formation of grassland savannas, beginning in the early to middle Miocene and prevailing throughout the remainder of the Cenozoic (Webb 1977).