PALEOLIMNOLOGICAL SIGNIFICANCE OF SPINOSE AND NON-SPINOSE MORPHS OF PYRGOPHORUS-HIBBARDI (LEONARD AND FRANZEN, 1944)

The basal portion of thr Ogallala Formation (= 'Laverne Formation') (L ower Pliocene) Beaver County, Oklahoma, contains an interesting assemb lage of non-marine fossil molluscs that include both spinose and non-s pinose forms of the aquatic gastropod species Pyrgophorus hibbardi. Th e origin and paleolimnological significance of the spinose morph has b een a source of much conjecture that has influenced environmental reco nstructions of this assemblage. In one hypothesis the spinose forms of P. hibbardi are assumed to be associated with brackish water conditio ns by analogy with some populations of a related hydrobiid Potamopyrgu s jenkinsi. To test the hypothesis that the spinose forms lived under different water conditions than the non-spinose morphs, we analyzed 10 specimens each of the two varieties for stable oxygen and carbon isot ope ratios in the shell aragonite. The mean isotope ratios for the smo oth and spinose morphs show no significant difference (oxygen: t = 0.2 8, df = 18, P (T less than or equal to t) 0.78 n.s.; carbon: t = 0.96, df = 18, P (T less than or equal to t) 0.35 n.s). We conclude that th e lack of a statistically significant difference between the means of the oxygen and carbon isotope values for the smooth and spinose morphs suggests that the two forms lived in waters having similar isotope si gnatures. The considerable range in oxygen isotope values recorded by both morphs of P. hibbardi, including values as high as 5-6 parts per thousand, suggest that both morphs were associated with waters which w ere periodically evaporatively enriched in O-18.