Circular 200—Fossil molluscan faunas from four spring-related deposits in the northern Chihuahuan desert, southern New Mexico and westernmost Texas

By K. M. Ashbaugh and A. L. Metcalf, 1986, 25 pp., 4 tables, 14 figs.

Fossil molluscs from four spring-related deposits in the northern Chihuahuan Desert were examined. Current distribution and habitat preferences of molluscan species in the southwest U.S. are used to make inferences about paleohabitats and past biogeographical patterns. All of the fossil faunas studied are richer in species (approximately 52 were recovered) than extant faunas occupying the same areas today. Many of the fossil species (interpreted as being late Pleistocene-early Holocene) are currently restricted to higher elevations and/or more northern latitudes in the southwest U.S., and some species, common as fossils, are rare in the southwest fauna today. Shifts in geographic ranges and local extinctions seemingly have been influenced by climatic changes since the early Holocene.

The purposes of this study are to document and interpret the occurrence of fossil molluscan faunas from spring-related deposits at four localities in the northern Chihuahuan Desert. All localities are situated in the Lower Sonoran Life Zone of the Chihuahuan Desert, which extends from northern Mexico into Trans-Pecos Texas and southern New Mexico. Küchler noted than the semiarid climate supports vegetation consisting of shrubs, dwarf shrubs, and grasses. The value of fossil molluscs for paleohabitat reconstruction is well established. However, little work of this kind has been done concerning molluscs from Quaternary spring deposits in the Chihuahuan Desert.

The following three criteria are used for paleoecological interpretation of the fossil assemblages: (1) ecologic tolerances of individual species, (2) comparison with the extant fauna from the same area, and (3) comparison with similar living faunas. The limitations of using molluscs for paleohabitat reconstruction have been summarized by Miller, who noted that problems include unstable taxonomy, difficulties in identifying fossil shells, insufficient ecological and distributional information about modern molluscs, and possible evolutionary changes influencing ecological tolerances.