Evolutionary consequences of the latest Paleocene thermal maximum for tropical planktonic foraminifera
Study of planktonic foraminiferal assemblages preserved in the central, equatorial Pacific (ODP Site 865) reveal that genera which inhabited the near-surface, mixed layer diversified during the latest Paleocene thermal maximum (LPTM). This transient diversification gave rise to a suite of short-lived (50 to several 100 ka), distinctive morphotypes (Morozovella allisonensis sp. nov., M. africana, and Acarinina sibaiyaensis) that are restricted to the LPTM interval. Parallel morphometric and stable isotopic data have been recorded from individual shells of members of the M. velascoensis/M. allisonensis and A. soldadoensis/A. sibaiyaensis lineages. The single-specimen isotope data confirm that M. allisonensis and A. sibaiyaensis are indeed stratigraphically restricted to the LPTM carbon isotope excursion; all specimens recorded anomalously low δ13C values. A stratigraphic succession of single-specimen isotope data from within the δ13C excursion interval was used to reconstruct the complex population dynamics which mediated the rapid (<10 ka) evolution of the LPTM taxa. These data reveal that local populations of ancestral M. velascoensis temporarily collapsed with the descendant M. allisonensis suddenly originating from a peripherally isolated population. This pattern is most consistent with peripatric speciation. Overall, the evolutionary transition from Acarinina soldadoensis to A. sibaiyaensis is most consistent with sympatric speciation, although certain elements of this transition are suggestive of a parapatric mechanism. We postulate that the rapid evolution of the LPTM taxa A. sibaiyaensis and M. allisonensis was fostered by a deepening of the nutrient-depleted, mixed layer. Alternatively, the LPTM morphotypes M. allisonensis and A. sibaiyaensis may have been extreme ecophenotypic variants arrayed along an intensified ecological gradient. Support for this latter interpretation is derived from the morphometric data which clearly show the morphological intergradation of the LPTM taxa with their respective ancestors. Thus, the morphologic diversification seen during the LPTM may represent failed speciations. Both scenarios invoke a temporary steepening of clinal gradients in response to intensified oligotrophy. The strong size-dependencies displayed by the δ18O and δ13C signatures of M. allisonensis and A. sibaiyaensis suggest that photosymbiosis facilitated the evolutionary success of the morozovellids and acarininids during the LPTM.