PHYLOGENETIC AND STRATIGRAPHIC ANALYSIS OF EARLIEST PALEOCENE BISERIAL AND TRISERIAL PLANKTONIC-FORAMINIFERA

Several different phylogenetic schemes have been proposed for the early Paleocene planktonic foraminiferal genera Guembelitria, Woodringina, and Chiloguembelina. Evolutionary relationships within and between these taxa have been difficult to ascertain because of differing taxonomic concepts, the possibility of morphologic convergence, and a general absence of high resolution stratigraphic records. This study uses phylogenetic analysis of morphologic data in order to test the most parsimonious cladograms corresponding to previously suggested phylogenies for these groups. Results indicate that these genera constitute a monophyletic or paraphyletic group, with biserial Woodringina spp. and Chiloguembelina spp. more closely related to triserial Guembelitria spp. than to biserial Heterohelix spp. In addition, it appears that Chiloguembelina spp. are more closely related to Woodringina spp. than to Guembelitria spp. Comparison of each cladogram to detailed biostratigraphic datums does not challenge this phylogenetic interpretation because the stratigraphic record of the analyzed character states is consistent with all examined phylogenetic hypotheses. The presence of an apparent character state reversal in the most parsimonious cladogram may be interpreted as evidence for convergent evolution of adult biseriality in planktonic foraminifera. Alternatively, this apparent reversal may indicate either that more one character state is coded as biserial within the context of this study or that several other character states are incorrectly coded, forcing an incorrect cladogram topology. Within this guembelitriid/chiloguembelinid group, origination rates and taxonomic richness peak immediately following the Cretaceous-Tertiary boundary. Attempts to classify the examined taxa or to model Paleocene foraminiferal diversity should explicity recognize the earliest Paleocene species richness of this group and the probable derivation of chiloguembelinid species from a guembelitriid ancestor.