What, if Anything, Can We Learn from the Fossil Record about Speciation in Marine Gastropods? Biological and Geological Considerations*

Abstract

Using fossils to study speciation requires careful analysis of the potential and limits of both biological and geological data. The most important biological data for a particular taxon includes to what degree species can be distinguished based only on hard-part morphology, what processes lead to speciation in that group today, and at what rates speciation occurs. Among the most important geological considerations is whether the incompleteness of the fossil record makes it possible to specify to within a reasonable degree of confidence when and where a putative speciation event took place. In benthic marine macroinvertebrates, the latter analysis is complicated by the “Common Cause” phenomenon: sea level change is both a major potential cause of gaps in the record and an important potential cause of evolutionary change. We consider the potential and limitations of fossil data for providing unique insight into the patterns and processes of speciation in marine shelled gastropods. A review of biological data shows considerable variation in mode of speciation and correlation between shell morphology and genetic divergence (cryptic species). Through examination of the fossil records of turritelline gastropods (Cerithioidea, Turritellidae) from Cenozoic deposits of the U.S. Gulf and Atlantic Coastal Plains and New Zealand, we propose simple tests for judging whether a particular fossil record is adequate for drawing conclusions about pattern and process of speciation. We conclude that the fossil record of marine gastropods can provide valuable information for studying species and speciation, but only if great care is used in evaluation of the data.