The Maastrichtian-Danian radiation of triserial and biserial planktic foraminifera: Testing phylogenetic and adaptational hypotheses in the (micro)fossil record

Abstract The earliest Tertiary planktic foraminiferal taxonomic diversification has long been regarded as one of the best examples of adaptive radiation within the microfossil record. However, the morphologic, phylogenetic, and paleoecologic data required to validate this interpretation have never been adequately synthesized. In order to determine whether or not the Maastrichtian through Danian evolution of the triserial and biserial planktic foraminiferal genera Guembelitria, Woodringina , and Chiloguembelina can be identified as an example of adaptive radiation, both morphologic and paleoecologic data were gathered. Phylogenetic analyses of twelve species based on 52 morphologic characters representing all ontogenetic stages indicate that all three genera belong to a single monophyletic clade. Within this clade, Guembelitria appears to constitute a monophyletic sister group of Woodringina-Chiloguembelina . Although there is compelling evidence for recognizing Chiloguembelina as a subsidiary monophyletic group, both Fitch and Wagner parsimony analyses suggest that Woodringina most likely represents a morphologic grade within a larger Woodringina-Chiloguembelina clade. Ecologic diversification within this monophyletic group was assessed by tracing species-specific changes in preferred depth habitat as reflected by variations in the δ 18 O and δ 13 C isotopic composition of populations from Nye Klov (Denmark), Brazos River (Texas), and ODP Site 738C (Kerguelen Plateau, Australian-Antarctic Basin), along with addition data from the published literature. Results indicate that Guembelitria cretacea, Guembelitria trifolia, Woodringina hornerstownensis , and Chiloguembelina morsei occupied near surface (mixed layer) planktic habitats whereas Guembelitria danica, Chiloguembelina crinita, Chiloguembelina midwayensis , and Chiloguembelina waiparaensis preferred intermediate depth habitats; probably near the base of the thermocline. Since no isotopic data were used in the phylogenetic analyses, direct comparison of these paleoecologic and phylogenetic data permits the reconstruction of certain aspects of this clade's ecologic history. While these patterns of morphologic change and habitat shift are congruent, instances of speciation appear to have occurred much more frequently than instances of habitat diversification. This finding is contrary to the “newly vacated ecospace” model of adaptive radiation, but is consistent with adaptive radiation via the “taxon pulse” model. Moreover, since there seems to be no evidence indicating that habitat diversification within this clade took place at an atypically rapid pace during the earliest Danian, these data also suggest that intermediate-depth planktic foraminiferal niches may have been occupied by K/T survivor populations during the interval from Danian planktic foraminiferal biozones P0 to P1a—a position supported by increasing body of independent evidence. By combining morphologic, stratigraphic, and ecologic studies within an explicitly phylogenetic framework (as illustrated by this study), micropaleontologists can develop tools for probing many different types of macroevolutionary phenomena.