- GeoRef, Copyright 2004, American Geological Institute.
The warm-water planktonic foraminiferal Globorotalia tumida lineage has been studied in a 10-Myr-long stratigraphic sequence (late Miocene through Recent) from the Indian Ocean to determine long-term evolutionary patterns through the lineage's history, and particularly to study in great detail the evolutionary transition from G. plesiotumida to G. tumida across the Miocene/Pliocene boundary. Sampling resolution was very good, between 5 X 10 3 and 15 X 10 3 yr across the Miocene/Pliocene boundary about 2 X 10 5 yr otherwise. The test shape was analyzed in edge view, permitting determinations of variation in inflation and elongation of the test. Shape was analyzed quantitatively using eigenshape analysis. This method represents the greatest proportion of variation observed among a collection of shapes by the least number of different shapes. The late Miocene (10.4-5.6 Myr B.P.) populations exhibited only minor fluctuations in shape that did not result in any net phyletic change. This period of stasis was followed by an 0.6-Myr-long period (between 5.6 and 5.0 Myr B.P.) of gradual transformation of the late Miocene morphotype (G. plesiotumida) into the early Pliocene morphotype (G. tumida). The populations were again more or less in stasis in the Pliocene and Pleistocene (5.0 Myr to the present day), so that no major modifications of the newly evolved early Pliocene morphotype occurred during these 5 Myr. Thus it would appear that the G. tumida lineage, while remaining in relative stasis over a considerable part of its total duration underwent periodic, relatively rapid, morphologic change that did not lead to lineage branching. For this evolutionary modality we propose the term "punctuated gradualism" and suggest that this may be a common norm for evolution--at least within the planktonic foraminifera.