SINGLE-SPECIMEN STABLE ISOTOPE ANALYSES TO INVESTIGATE EVOLUTIONARY CHANGES IN COILING RATIOS AND DEPTH ECOLOGY WITHIN THE MIDDLE MIOCENE GLOBOROTALIA (FOHSELLA) LINEAGE

Historically, the Globorotalia (Fohsella) lineage has been considered a flagship for phyletic gradualism within the planktonic foraminifera. This lineage displays gradual morphological evolution during the early-to-middle Miocene transition from G. (Fohsella) peripheroronda to G. (Fohsella) fohsi. This prolonged period (~10 m.y.) of gradual morphological evolution is, however, punctuated by an abrupt (~50 k.y.) deepening in habitat depth that is roughly coincident with the FAD of G. (Fohsella) fohsi at ~12.9 Ma. Examination of G. (Fohsella) assemblages preserved through this same stratigraphic interval at ODP Site 806 reveals that this well-documented ecological shift is paralleled by a significant change in the lineage’s preferred direction of coiling. Specifically, coiling ratios within the plexus exhibit a modest tendency to be dextrally coiled prior to 12.9 Ma (~35% sinistrally-coiled), but switch to predominantly sinistral coiling (~97%) through the stratigraphic interval in which the depth-habitat shift is recorded. Moreover, the fohsellids suffered a transient decline in overall relative abundance during this critical stage of their ecological evolution, a possible indication that environmental change temporarily perturbed local population structures and paleobiogeography. The temporal relationship between the abrupt changes in coiling preference and ecological evolution within the G. (Fohsella) lineage has been previously overlooked. Single-specimen stable isotope (d¹⁸O, d¹³C) analyses were employed to determine if subtle, depth-habitat differences could be detected between sinistrally and dextrally coiled individuals prior to the concurrent shifts seen in depth-habitat and preferred direction of coiling. The preliminary data suggests that no significant difference in preferred depth ecology existed between sinistrally and dextrally coiled individuals prior to the depth-habitat shift; it appears that both dextrally- and sinistrally-coiled subpopulations calcified their shells within the warm, mixed layer. Future study will provide additional stable isotope data which are needed to clarify the depth-habitat preferences of these fohsellid subpopulations and to refine our interpretations.