MORPHOMETRIC AND STRATOPHENETIC STUDY OF THE ROTALIPORA LINEAGE (PLANKTIC FORAMINIFERA) DURING THE MIDDLE AND LATE CENOMANIAN

Planktic foraminifera are an excellent organismal group in which to study morphological evolution; they have a global distribution and a nearly complete fossil record, small samples can contain thousands of specimens, and their taxonomy is relatively well understood. Their evolutionary history has been influenced by paleoceanographic, climatic, and extra-terrestrial events and has also resulted from random walks that are unassociated with any discernable extrinsic cause. The Cenomanian is an intriguing time in which global warmth may have led to increased planktic foraminiferal speciation. However, evolutionary relationships between various taxa are tangled in the literature, impeding further understanding. A study of the genus Rotalipora (focusing on cushmani, praemontsalvensis, and montsalvensis) was conducted on well preserved samples from Ocean Drilling Program Site 1050 (Blake Nose, subtropical North Atlantic Ocean). Spiral and edge view orientations were imaged and digitized to obtain a suite of size and size-independent measurements across the stratigraphic ranges of each species. Primary type material was included in the study for comparison, as were other published images from the literature. The rotaliporid morphospace is defined by both a mixture of qualitative descriptive terms used in Cretaceous planktic foraminiferal taxonomy (e.g., accessory aperture position and types of sutures, 11 in total) as well as 14 quantitative parameters constructed by ~40 measures. With these data, it is possible to characterize the population variability and evolutionary transition from the ancestral to descendent morphologies. A variety of statistical methodologies are employed to define the morphospace including hierarchical cluster and principle components analyses. Study of the Rotalipora lineage will lead to a better understanding of its position within the greater Cenomanian planktic foraminiferal morphospace and determination whether the ancestor-descendent transitions were marked by morphologic discontinuities or gradual shifts in population morphospace.