MORPHOLOGIC AND ISOTOPIC INVESTIGATION OF THE NEOGENE PLANKTONIC FORAMINIFERAL SUBGENUS GLOBOROTALIA (TRUNCOROTALIA)

In an effort to understand the isolating mechanisms that drive speciation in the plankton, morphometric and stable light isotopic analyses were employed in the investigation of the Neogene planktonic foraminiferal subgenus Globorotalia (Truncorotalia). Previous studies of morphotypes of the extant end-member of the lineage, Gr. truncatulinoides, have identified ecophenotypes and genotypes from recent or sub-recent samples from core tops and plankton tows. Our statistical analysis of morphometric measurements of over 1000 truncorotalid specimens from the South Atlantic Ocean has identified these morphotypes, and their correlation with coiling direction, thus confirming patterns known from previous studies in other areas.

This study tracks the persistence of those morphotypes temporally, concentrating on speciation events in the lineage. Some of these morphologic characters retain their correlation with coiling direction and are persistent through time and appear to be consistent with the “genetic species” found in studies of Gr. truncatulinoides. Coiling direction in Gr. truncatulinoides is known to correlate locally with the Pleistocene glacial/inter-glacial cycles. Ancestral species of the plexus also show changes in coiling direction related to oceanographic change. Additionally, the truncorotalid lineage shows a higher occurrence of kummerform specimens during times of environmental stress.

Stable light isotope analysis of individual chambers shows a difference in depth preference between the coiling types within species and between species of the lineage. This supports the idea that that the dextral and sinistral populations have different depth preferences and/or timing of the annual reproductive cycle, providing a mechanism for genetic isolation in the lineage and within each species. The significance of stable light isotopic variation among kummerform and normalform specimens will also be presented.