PALEOECOLOGICAL SIGNIFICANCE OF A TURRITELLINE GASTROPOD-DOMINATED LIMESTONE IN THE LOWER CRETACEOUS OF TEXAS

Turritelline gastropods are one of the most abundant and diverse macrofossil groups of the past 100 million years. Understanding the environmental context of their evolutionary history may offer general insights into patterns of diversity change during this time. A facies of the Lower Cretaceous (Albian) Whitestone Limestone Member of the Walnut Formation in south-central Texas contains abundant molds and casts of a turritelline gastropod, Turritella cf. seriatim- granulata Roemer, and can be designated a “turritelline-dominated assemblage” (TDA). This occurrence is the first recognized Cretaceous TDA from a calcarenitic limestone (as opposed to a calcareous sandstone). It highlights the peculiar facies distribution of TDAs through the Cretaceous and Cenozoic, which may indicate a complex history of environmental evolution in turritellines. Although they are widespread in siliciclastic facies from Cretaceous to Recent, TDAs in carbonate facies occur almost exclusively in the Paleogene. This distribution may indicate any or all of the following: 1) some Paleogene carbonate environments were cooler or more nutrient-rich; 2) secular changes occurred in the environmental tolerances of turritellines over the group’s history; 3) particular clades of turritellines invaded carbonate environments; or 4) the observed facies distribution of turritellines in North America is not representative of the global distribution of the group. Further research is needed to evaluate these hypotheses.