Speciation and structural evolution in the Palaeocene Morozovella lineage (planktonic Foraminiferida)

Abstract. The Palaeocene members of the planktonic foraminiferal genus Morozovella evolve through a series of intermediate morphologies from the globigerine ancestral species Subbotina pseudobulloides to the angulo-conical globorotaliid species Morozovella velascoensis. This example of structural evolution is investigated using eigenshape analysis on 41 samples from DSDP Site 577 (Shatsky Rise, North-West Pacific). The results indicate that the initial members of this lineage (S. pseudobulloides to M. angulata forma protocarina) exhibit a directional shape trend whereas, subsequent to the evolution of M. velascoensis, shape fluctuations are random. The interval of maximum shape change in this lineage is accompanied by an increase in the mean size of its members. Both the shape and size change occur during the peak in speciation of the Palaeocene morozovellids (at 63Ma), a phenomenon that we interpret as an example of species selection. This interval is also characterised by high levels of marine productivity, a factor considered instrumental in the control of the evolution of the Palaeocene Morozovella species.

The potential of eigenshape analysis as a taxonomic aid is also investigated. Specimens mounted in side view are insignificantly different until the evolution of the late stage morozovellids M. angulata forma protocarina, M. velascoensis, M. occlusa and M. edgari in the Late Palaeocene. This suggests that the shape of the edge view of Palaeocene morozovellids is not a good measure of real species differences.