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In western Colombia, between the Cordilleras Central and Oriental of the Andes Mountains, the Río Magdalena flows through the scenic, arid “desierto de La Tatacoa” [image ] [image ]. There, in the upper valley of the Magdalena north of Neiva, Huila Department, are extensive exposures of the Honda Group [image ]. Vertebrate fossils of middle Miocene age are abundant in fluvial sedimentary deposits of Honda Group in the Villavieja and La Victoria formations. The La Venta fauna originating from these beds is very significant as one of the few diverse Tertiary vertebrate faunas in tropical South America. The La Venta fauna consists of approximately 155 kinds of fossil animals in 71 families, including a crab, a cartilaginous fish, about 25 bony fishes, 2 amphibians, 32 reptiles, 5 birds, and about 89 mammals (Kay et al. 1997; Kay and Madden 1997; Villarroel 1998, Villarroel 2000). Madden et al. (1997) used the mammals to define the Laventan land mammal age. In addition to the lowland meandering river system indicated by the geology, the vertebrate fauna furthers the interpretation of the paleoenvironment. For example, diverse and abundant fossils of freshwater fishes, turtles and crocodilians indicate aquatic habitats, fossils of several kinds of arboreal monkeys indicate the presence of tropical rain forest, and various other kinds of mammals seem to indicate the presence of forests, forest/grassland mosaics, and open grasslands (Kay and Madden 1997).

The fossil mammals are particularly diverse and abundant at La Venta, consisting of marsupials, xenarthrans, rodents, astrapotheres, litopterns, toxodonts, typotheres, primates, a sirenian, and bats. Several kinds of primates have been found especially in a unit known as the Monkey Beds at the base of the Villavieja Formation. Bats make up a fair portion of the fauna, but the 14 kinds presently known as fossils at La Venta is probably only a fraction of the bat fauna that existed there in the Miocene. A few kinds of large bats were found by surface prospecting [image ], but most species are small and their remains have been recovered by screenwashing of sediments dug from the beds [image Photo by Anne H. Walton.]. One locality in the western part of the basin near San Nicolás has produced most of the bats. Surface prospecting and screenwashing of sediments from the Monkey Beds near San Nicolás [image Photo by Anne H. Walton.] yielded the bats Diclidurus sp., an unidentified emballonurid, Noctilio albiventris, Notonycteris magdalenensis, Tonatia or Lophostoma sp., Palynephyllum antimaster, Thyroptera lavali, Thyroptera cf. T. tricolor, and an unidentified molossid. A second species of Notonycteris, N. sucharadeus, and the molossid Eumops sp. were found in the Las Lajas Member of the Villavieja Formation near Cuzco. Two kinds of molossids, Mormopterus colombiensis and Eumops sp., and an unidentified possible vespertilionid were found in a thin unit known as the Fish Bed. One other bat, the large molossid Potamops mascahehenes, was found about 60 m above the Chunchullo Sandstone Beds near the top of La Victoria Formation (Czaplewski 1997; Czaplewski, Takai, et al. 2003c). The nectar- and pollen-feeding bat Palynephyllum antimaster is the only known Tertiary representative of the phyllostomid subfamily Glossophaginae.