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Present address: Department of Earth Sciences Denver Museum of Natural History, 2001 Colorado Boulevard, Denver, Colorado 80205.

The Buck Spring Quarries, located in the southern part of the type area of the Lost Cabin Member of the Wind River Formation, Wind River Basin, Wyoming, provide one of the richest assemblages of fossil vertebrates known from the latest Wasatchian Land-Mammal Age (ca. 50.5 Ma, Lostcabinian, early Eocene) of North America. More than 100 species of mammals, reptiles, birds, amphibians, and fishes are known. The quarries uniquely preserve associated skeletal remains, and complete skulls and dentitions of a large percentage of the vertebrates. The fossils come from a 2-m-thick sequence, which is composed primarily of mudstones, bioturbated limestone lenses, and laminated limestone/mudstone couplets. These sediments were deposited in a well-drained swamp or ponded area between 250 and 600 m away from a low-sinuosity stream. The fossils are especially common in limestones, where they accumulated as a result of (1) natural death (articulated specimens), (2) predator activity (coprolites and kill sites), and (3) very limited hydraulic transport of smaller bones.

Rarefaction estimates from surface and quarry collections suggest similar patterns of species richness, which are among the highest known for the Paleogene and compare favorably with penecontemporaneous Lostcabinian assemblages and slightly younger Gardnerbuttean (early Bridgerian) ones. The mammalian assemblage is dominated by small species of mammals, and has a body-size distribution and species diversity similar to modern tropical communities. The abundance of arboreal mammals indicates that a multistoried woodland habitat was in close proximity to the quarry area.

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