| Comparison of stable isotope signals (δ18O and δ 13C) of various species belonging to the planktic foraminiferal genus Morozovella (ODP Site 758, Indian Ocean) reveal subtle, yet consistent, isotopic offsets between taxa. Oxygen isotope evidence indicates that, while the morozovellids occupied a relatively shallow depth habitat, their morphological diversification during the late Paleocene was accommodated by changes in depth ecology. The taxonomic patterns of depth-habitat segregation implicate ecological divergence in the evolution of new morozovellid species.; Planktic foraminiferal assemblages preserved in the tropical Pacific (ODP Site 865) show genera that inhabited the near-surface, mixed layer diversified during the late Paleocene thermal (LPTM). This rapid (<10 kyr) diversification gave rise to a distinctive suite of transient (150–200 kyr) morphotypes (e.g., Morozovella allisonensis, M. africana, and Acarinina sibaiyaensis). Single-specimen isotope analyses confirm that M. allisonensis and A. sibaiyaensis are indeed stratigraphically restricted to the LPTM carbon isotope excursion.; A stratigraphic succession of single-specimen isotope data from within the δ13C excursion interval was used to reconstruct the population dynamics that unfolded during the LPTM. Various elements of the M. velascoensis/M. allisonensis transition (e.g., ancestor suffering populational bottleneck, ecological displacement between parent and daughter species, and subsequent sympatry) are consistent with a peripatric speciation model. Overall, the evolutionary transition from Acarinina soldadoensis to A. sibaiyaensis conforms to a sympatric speciation model, although certain aspects are suggestive of depth-parapatry. Insufficient areal coveration, however, hinders evaluation of geographic speciation models. The LPTM morphotypes may have been subspecific variants that immigrated into the study area. Deepening of the oligotrophic, mixed layer may have fostered the morozovellid and acarininid diversifications during the LPTM.; The final stages of extinction in the M. velascoensis lineage are embodied by populations of diminutive M. edgari. Eigenshape analysis of shells from size-segregated populations reveal a pattern of morphological change consistent with paedomorphosis. The pattern of the stratigraphic succession, where the final occurrences of the lineage consist of populations of dwarfed paedomorphs, conforms to a process called “terminal progenesis”. Climatic cooling in earliest Eocene (54.02 Ma) may have contributed to the demise of the M. velascoensis lineage. |
