A MORPHOMETRIC APPROACH TO REFINING SPECIES CONCEPTS FOR THE LATE CRETACEOUS PLANKTIC FORAMINIFERAL GENUS GLOBIGERINELLOIDES

Biostratigraphic, paleoceanographic and macroevolutionary studies employing fossil planktic foraminifera rely on the accuracy of their taxonomic definitions. Species concepts for this group are based on shell morphology and growth characteristics. In many cases, these concepts need further refinement using modern morphometric methods. The Late Cretaceous planispiral genus Globigerinelloides is one group that requires revision as there has been considerable confusion in taxonomic concepts used by different workers and ancestral-descendent relationships are generally poorly known. To better characterize taxonomic distinctions and the phylogenetic history of this taxon we picked all morphotypes of Globigerinelloides present in late Campanian-Maastrichtian Deep Sea Drilling Project (DSDP) and Ocean Drilling Program (ODP) sediment samples that were drilled at both high and low latitude sites in the Atlantic and Pacific oceans (sites: 327, 463, 465, 511, 690, 1049 and 1050). Species identified include G. asper, multispinus, G. prairiehillensis, G. messinae, G. subcarinatus, G. volutus, G. alvarezi, G. impensus and G. ultramicrus. Lower, middle and upper levels of the cored intervals were picked at each site to determine whether the species shows morphological stasis or gradual morphologic shifts within their evolutionary ranges. Using a scanning electron microscope (SEM), side and edge images were taken for five individuals per species group from each sample level and for the holotype and paratypes of each species. High-resolution, cross-sectional X-ray images were used to obtain measurements of the ontogenetic changes in chamber cross-sectional areas. The resulting size and size-independent morphological and qualitative observations (e.g. uni- versus bi-apertural) for this genus were then analyzed using multivariate statistical methods. Completion of these morphometric analyses will enable justification for taxonomic revision of the Late Cretaceous planispiral taxa and improved understanding of the phylogenetic history of this biostratigraphically important but underutilized taxonomic group.