ivy TRANSACTIONS OF THE SAN DIEGO SOCIETY OF NATURAL HISTORY Volume 21 Number 6 pp. 89-110 5 December 1986 Holocene terrestrial gastropod faunas from Isla Santa Cruz and Isla Floreana, Galapagos: evidence for late Holocene declines Steven M. Chambers '-^Sf^/\p^ Office of Endangered Species, U.S. Fish and Wildlife Service, Washington, D.C. 20240 USA and Department of Biology, George Mason University, Fairfax, Virginia 22030 USA i _ -'^ 7986 David W. Steadman' Department of Vertebrate Zoology, National Museum of Natural History, Smithsonian Institution, Washington, D.C. 20560 USA ^N: Abstract. We report 1 6 species of land snails from late Holocene cave deposits and nearby surface areas on Isla Santa Cruz and Isla Roreana, Galapagos Islands. These snails are associated with vertebrate fossils that accumulated in lava tubes or fissures largely as the regurgitated items of bam owls (Tyto punctatissima). Most or possibly all of the snails, however, probably do not represent prey remains, but entered the cave mainly with infilling sediment. The fossil snails represent a much less complete sample of each island's historic fauna than do the vertebrate fossils from the same deposits. Radiocarbon ages on these faunas range from 2420 ± 25 years BP to Modem. Lectotypes and paralectotypes are designated for Naesiotus nesioticus and Naesiotus reibischi. Naesiotus galapaganus, which is rare in historic collections, occurred abundantly in the fossil deposits on Floreana. Five species from these collections on Santa Cruz had not been previously reported from this island (Gastrocopta duncana, Succinea corbis, Guppya bauri, and two species of Naesiotus that have recently been described else-where). As with vertebrates, certain species of snails have suffered considerable late Holocene declines in range and abundance, and possibly have become extinct in Galapagos. The causes of gastropod declines are not clear, but are likely related to human impact of the past century. The most likely cause is habitat destruction by introduced goats, donkeys, and pigs, although predation by introduced rodents may also be important. Introduction Fossils have traditionally played a minor role in studies of the evolution and biogeography in the Galapagos Islands. This is largely because few scientists have looked for them and have regarded the volcanic terrain of Galapagos (Fig. 1 ) to be a poor environment for the deposition and preservation of fossils. Invertebrate fossils have been reported from localized marine sedimentary rocks in Galapagos (Hertlein 1972 and references therein), but these studies were based upon field work done decades before absolute dating of volcanic rocks was possible through potassium-argon and paleomagnetic determinations. Renewed interest in the Quaternary marine fossils of Galapagos (James 1984, Hickman and Lipps 1985) has resulted from new collections from deposits stratigraphically related to lava flows whose ages are fairly well known. Until the past several years, the paleontology of terrestrial organisms in Galapagos had received even less attention than that of marine organisms. Again, much of this lack of attention was due to the axiom of geology that states the rarity or lack of fossils in volcanic rocks. Biologists (e.g.. Lack 1960) lamented that Galapagos was, unfortu-nately, an area that had yielded no paleontological clues about the history of the birds ' Present address: Biological Survey, New York State Museum, The State Education Department, Albany, New York 12230 USA.