Clinal variation in aperture shape in Southwest Pacific Globorotalia partimlabiata and its ancestor about the time of the Middle Miocene Climatic Transition

To clarify the taxonomy and biostratigraphy of Southwest Pacific populations currently identified as Fohsella peripheroronda a morphometric study is reported on the shape of its aperture in collections from DSDP Sites 588A and 593, and from the stratotype of the New Zealand Lillburnian Stage. This material is compared with Globorotalia partimlabiata from Serravallian strata in Sicily. Shape analyses use 50 equally-spaced coordinates along the margin of the apertural re-entrant of the last-preserved chamber. Data are aligned by the Procrustes algorithm and projected onto principal component axes. Canonical discriminant analysis of these scores shows that convexity of the margin increases clinally. The cline is interpreted as primary evidence for a previously unrecognized lineage for which a provisional taxonomy is advanced. Globorotalia partimlabiata, which has individuals with strongly convex margins, represents the climax of the cline. The highest occurrence of its ancestor, recognized here informally as Globorotalia cf. peripheroronda, is determined by the highest sample whose mean shape does not differ significantly from its earliest representative at Site 593. There and at Site 588A that datum is close to, or within, Miocene isotope event Mi-3b. The architectural history of the lineage differs from the tropical fohsellids and its relation to them is unresolved.