Divergences and phyletic transformations in the history of the Globorotalia inflata lineage

Extract

To the student of evolution, Malmgren and Kennett's (1981) detailed study of the Globorotalia inflata lineage demonstrates the continuity of the stratigraphic record and the abundance of planktic microfossils in deep-sea sediments. Their work recalls the prediction (Prothero and Lazarus 1980) that planktic microfossils might serve as the laboratory animals of evolutionary studies and indicates the potential value of fossils in determining ancestry. However, for theorists to judge the evolutionary import of Malmgren and Kennett's high-resolution morphometric study, broader consideration of lineage relationships is needed. They portray the G. inflata lineage as a set of five successional taxa in simple ancestor-descendant relationship. Trends, established morphometrically and claimed to be “one of the clearest examples of gradualism in any group of fossils” (Malmgren and Kennett 1981, p. 236), are shown to progressively transform a compressed shell with peripheral keel (G. conoidea) into a globose shell (G. inflata) without a keel. While the succession of taxa given by Malmgren and Kennett has been supported in other studies of western South Pacific sequences (e.g., McInnes 1965; Walters 1965; Kennett 1966; 1973; Jenkins 1971; Scott 1979; Hornibrook 1982), it is not established that the order indicates simple phyletic transformation without branching, as Malmgren and Kennett imply. Crucial evidence concerns the history and status of keeled populations which resemble the ancestral G. conoidea but occur in the Pliocene, associated with advanced members of the G. inflata lineage. These populations are not mentioned by Malmgren and Kennett, yet, if they are also descendants of G. conoidea, their occurrence strongly suggests that lineage divergence occurred. I wish to review evidence in support of this view.