PHYLOGENY AND DEPTH ECOLOGY OF LATE CRETACEOUS PLANKTONIC FORAMINIFER SPECIES OF GLOBIGERINELLOIDES

At least eight species of foraminifera in the genus Globigerinelloides (G. multispinus, G. prairiehillensis, G. messinae, G. subcarinatus, G. volutus, G. alvarezi, G. aspensis, and G. impensus) are frequently identified in various studies of Late Cretaceous marine sediments, but opinions vary as to which of these species should be treated as junior synonyms. In an effort to stabilize the taxonomy of this group primary type specimens of most of these species were compared with Globigerinelloides morphotypes from the upper Campanian-Maastrichtian of ODP Site 690 (Weddell Sea), at least two Maastrichtian sample levels at DSDP, Sites 463 and 465 (tropical Pacific), and the Turonian-Maastrichtian of DSDP Site 511 (Falkland Plateau). Ontogenetic morphometric data, coiling metrics and shape analysis based on high-resolution x-ray images from umbilical and edge views were extremely useful in characterizing population variability for each of these planktonically coiled morphotypes.

Our results indicate that Late Cretaceous Globigerinelloides include two distinct lineage groups, one with 10 to 15 chambers and the other with 20 to 25 chambers in adult specimens. Differences in chamber shape, chamber size increase rates, and external shell ornamentation are used to delineate species populations within these two lineage groups. Stable isotope analyses of the biometrically differentiated Globigerinelloides taxa will be obtained to determine their relative depth ecologies. Pre-Campanian Globigerinelloides taxa will be similarly analyzed to reconstruct their Late Cretaceous phylogenetic history.