Diversity of late Cenozoic gastropods on the Southern High Plains

Gastropods are often the only fossils found in late Cenozoic sediments on the Southern High Plains. By virtue of their conservatism and low rate of mobility, gastropods should be sensitive indicators of their environment, and the pluvial cycles of the late Cenozoic should have resulted in repeated local invasions and extinctions of species not presently living in this area. Recent advances in absolute age chronology through radiometric dating of volcanic ash beds have resulted in revision of the late Pliocene and early Pleistocene stratigraphic sequence throughout the Great Plains. Although intensively studied in other areas, earlier studies of the fossil gastropods of this area have been limited in scope, and conducted under the assumption of a single Pearlette ash datum.

A detailed study of the fossil gastropod assemblages, collected by bulk sampling techniques, with concomitant study of the sedimentology and stratigraphy of selected localities was undertaken. It was found that for most stages of the late Cenozoic, index species and/or diagnostic assemblages existed. During the Wisconsinan stage, several stades could be identified by assemblages. Those stages in which diagnostic assemblages did not occur could be identified with reasonable certainty by a combination of sedimentology, stratigraphy and environmenta1 interpretations from the fossil assemblage. Statistical coefficients of correlation are used, but found to be more sensitive to similarity of environment than to relative age. The phylogeny of certain species are of use in determining the relative age of the assemblage.

As environmental indicators, terrestrial gastropods are found to be useful and sensitive. Aquatic gastropods, insulated from the outside climate, are less sensitive but useful in confirmation of interpretations. One tolerant species, Pupoides albilabris, faithfully records long range climatic conditions in shell morphology. Environmental interpretations, based on the fossil gastropod assemblages, suggest revision of the commonly accepted late Cenozoic climatic cycles. The late Pliocene was slightly cooler and moister than the present, not warm and semiarid. The Yarmouthian stage was similar to the present, but somewhat moister, while the Illinoian pluvial was cool and differed from other pluvials in being dry and probably of short duration. At no time during the late Cenozoic was the Southern High Plains extensively forested, although riparian woodland, including scattered conifers, probably occurred during most pluvial maxima.