FOSSIL BATS OF THE AMERICAS
|Image of the site as it appeared in early summer 1984, with Gary Morgan and Robin Brown]
|Image Left to right: Steve Emslie, Ann Pratt, and Gary Morgan working at Thomas Farm in 1984]
|Image Left to right: Nick Czaplewski, Art Poyer, and Gary Morgan collecting at Thomas Farm in 2001. Photo by Erika Simons.]
The Thomas Farm site, located 12 km north of Bell in Gilchrist County in northern peninsular Florida, has produced the best known early Miocene (early Hemingfordian, ~18 Ma) vertebrate fauna in eastern North America. The Thomas Farm sediments consist of alternating layers of clay and sand filling a 30-m-deep sinkhole/cave complex developed in marine Eocene limestone [image ]. Thomas Farm was discovered in 1931, and has been excavated over the past 70 years. The Thomas Farm LF is composed of about 90 species of vertebrates, including 23 species of large mammals, the most abundant of which is the three-toed horse Parahippus leonensis. Thomas Farm has an abundant small vertebrate fauna of nearly 70 species, including taxonomically diverse samples of frogs, lizards, snakes, birds, bats, and rodents. In addition to bats, other small mammals from Thomas Farm include: the didelphid marsupial Herpetotherium; the soricid insectivore Limnoecus; three sciurid rodents, Petauristodon pattersoni, Nototamias hulberti and cf. Miospermophilus; two heteromyid rodents, Proheteromys floridanus and P. magnus; and the eomyid rodent Pseudotheridomys.
The artiodactyls, equids, carnivores, and sciurids (Maglio 1966; Forsten 1975; Tedford and Frailey 1976; Pratt and Morgan 1989) from Thomas Farm indicate an early Hemingfordian age (He1, 18-19 Ma). The early Hemingfordian is defined by the first appearance of the immigrant carnivores Amphicyon, Hemicyon (Phoberocyon), and Leptarctus, the first occurrence of the canid Tomarctus, and the camelid Floridatragulus, and the last occurrence of the amphicyonid Cynelos and the rhinocerotid Menoceras. These genera are all present at Thomas Farm. The earliest North American records for the ursid Hemicyon (Phoberocyon) and the flying squirrel Petauristodon are from Thomas Farm. The early Hemingfordian Garvin Gully fauna from the Texas Gulf Coastal Plain and Thomas Farm share the same or closely related species of the carnivores Amphicyon and Cynodesmus, the equids Anchitherium, Archaeohippus, and Parahippus, and the artiodactyls Nothokemas and Prosynthetoceras. The three equids and Amphicyon from Thomas Farm are also very similar to species of these same genera from early Hemingfordian faunas in the Runningwater Formation of Nebraska. Other correlative early Hemingfordian faunas from the western USA are the Flint Hill LF of South Dakota and the Martin Canyon LF of Colorado (Tedford et al. 1987).
The majority of bat fossils from Thomas Farm were recovered from about 2 metric tons of sediment screenwashed between 1981 and 1985 by Ann Pratt and Arthur Poyer, as part of Pratts PhD Dissertation. The greatest number of bat fossils occur in a 1-m-thick lime sand that is very rich in small vertebrate fossils [image ]. The fossil assemblage from the lime sand appears to represent a cave deposit that was formed through the natural accumulation of bat carcasses and a coprocoenosis representing the fecal remains from small carnivores or the regurgitation pellets of raptorial birds. The Thomas Farm site probably consisted of a deep, vertical-walled sinkhole containing caves, surrounded by a forested habitat (Pratt 1989, Pratt 1990).
Thomas Farm has the largest sample of bats from any pre-Pleistocene fossil deposit in North America, numbering almost 2,000 specimens. The chiropteran fauna is composed of at least nine species, including five species of Vespertilionidae, two species of Molossidae, and one species each in the Emballonuridae and Natalidae. Lawrence (1943) described two new genera and species of vespertilionids from Thomas Farm, Suaptenos whitei and Miomyotis floridanus. Czaplewski and Morgan (2000) described a third extinct genus and species of vespertilionid from Thomas Farm, Karstala silva, characterized by its large size. This site has also produced the oldest known bat in the Neotropical family Natalidae, Primonatalus prattae (Morgan and Czaplewski 2003). Czaplewski et al. (2003b) recently reported two species of Molossidae from Thomas Farm, both of which are similar to the genera Tadarida and Mormopterus. In addition to the three genera listed above, the vespertilionids include a new genus similar to Lasiurus and second new genus similar to Corynorhinus. The majority of bat fossils from Thomas Farm (~75%) represent the large vespertilionid Suaptenos whitei.