A TEST OF THE CORRESPONDENCE BETWEEN PALEONTOLOGICAL AND BIOLOGICAL SPECIES IN LATE CRETACEOUS OSTRACODES

Although analysis of higher taxa such as genera is common in evolutionary paleobiology, it is species that are interacting evolutionary units, and thus they should provide more direct information about processes such as extinction, speciation and phyletic evolution. These interpretations, however, require that paleontological species largely correspond to real, species-level biological lineages. Molecular work on living species has demonstrated the existence of many morphologically cryptic species. This problem of morphologically cryptic species must be more severe in paleontology, where only the subset of features reflected in mineralized skeletons are available to diagnose species. To date, however, this question has been hard to address with paleontological data.

Here we assess this question using a very large dataset of observations from cytheroid ostracodes from the Late Cretaceous of the U.S. Coastal Plain. As part of a previous study on sexual dimorphism, we broadly characterized size and shape in over 100 nominal species from this fauna, representing about 60% of its known diversity. When examining sexual dimorphism in individuals from what we initially thought were a single population, in about 10% of cases we encountered the presence of morphological clustering indicative of two, previously undetected, biological species in the sample. In at least some of the cases, we were able to discover additional, independent characters that corroborate the statistical clustering, supporting this interpretation of taxonomically unrecognized diversity. This 10% incidence of cryptic diversity must be an underestimate because some kinds of cryptic variation will not be detectable by our approach. However, the incidence of cryptic diversity is not random – it preferentially occurs in taxa that are abundant and feature-poor, especially the genus Brachycythere.