Abstract:
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 ( approximately 10 m.y.) of gradual morphological evolution is, however, punctuated by an abrupt ( approximately 50 k.y.) deepening in habitat depth that is roughly coincident with the FAD of G. (Fohsella) fohsi at approximately 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 ( approximately 35% sinistrally-coiled), but switch to predominantly sinistral coiling ( approximately 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 (delta (super 18) O, delta (super 13) C) 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.