Oxygen and carbon isotopic analyses have been conducted on Miocene to Pliocene (6.0 to 2.9 Ma) members of the gradually evolving, deep-dwelling planktonic foraminiferal clade, Globoconella, in temperate waters of the southwest Pacific, Deep Sea Drilling Project (DSDP) Site 593. In the late Miocene the clade began with Globoconella conoidea, and continued through G. conomiozea, G. sphericomiozea, and G. puncticulata to the extant form G. inflata. Isotopic analyses were performed on ancestor-descendant species within the clade to determine if isotopic differences exist between these species which would, in turn, suggest depth and/or seasonal habitat differences and perhaps segregation, as well as ecological changes in the clade. Isotopic analyses were also conducted on the relatively shallow-dwelling planktonic foraminifer Orbulina universa and the benthic form Cibicidoides wuellerstorfi to determine if any relationships exist between the evolution of Globoconella and paleoceanographic/paleoclimatic change.
Small (usually 0.1–0.15 ‰ but up to 0.3 ‰) oxygen isotopic differences exist between ancestor and descendant forms that we believe represent small (∼1°) temperature differences. These temperature differences are inferred to indicate depth and/or seasonal habitat differences and possible segregation between the species during the gradual evolution. The largest differences in oxygen isotopic values occur between ancestor and descendant forms during the most conspicuous morphological transition within the clade near the Miocene/Pliocene boundary. During this interval, the clade underwent a transformation from conical to spherical forms and there was a loss of the keel. No consistent differences were observed between ancestor and descendant carbon isotopic values.
Both morphological and ecological evolution appear to have been associated with paleoceanographic/paleoclimatic changes. Intervals marked by warming of surface-to-upper intermediate waters are associated with evolution of forms with a spherical test and inferred adaptation to cooler waters relative to ancestral forms. We propose two alternative models for the evolution of the Globoconella clade. In the first model, we assume that depth and/or seasonal segregation between ancestor and descendant forms provided a partial barrier to gene flow and that evolution resulted from genetic drift and/or differential selective pressures acting on each morphotype. In the second model, we assume that coeval members of Globoconella formed a vertical and/or seasonal morphocline in the water column and that segregation did not provide an effective barrier to gene flow. Evolution proceeded as directional selection acted on morphotypes of Globoconella inhabiting selectively advantageous positions in the water column.