MOSAIC EVOLUTION IN THE MIDDLE MIOCENE PLANKTONIC FORAMINIFERA GLOBOROTALIA FOHSI LINEAGE

Recent studies have shown that modes of evolution, including directional trend, random walk, and stasis, vary across morphologic traits and over geographic ranges (Grey et al., 2008; Hopkins & Lidgard, 2012, Hunt et al., 2015). If so, is it possible that our interpretation of evolutionary modes is actually driven by our selection of traits? In an attempt to answer this question, we restudied the Middle Miocene planktonic foraminifera (PF) Globorotalia fohsi lineage, an iconic example of gradual evolution (Bolli et al., 1985). In contrast to previous studies that focused on the gross morphology of specimens, we analyzed multiple phenotypic traits of which the biologic and ecologic significance is well understood in extant PF populations. We find that traits in this lineage did not evolve in concert. The timing and geographic pattern of changes in shape, coiling direction, size, and ecology were different. The evolution of the G. fohsi lineage is a mosaic combination of different evolutionary modes of different traits. The interpretation of gradual evolution in the fohsellid lineage is a biased reading of fossil records due to 1) overemphasis of the importance of type specimens as reference points of the various stages of the evolution of the G. fohsi lineage; 2) biased selection of the examined trait; 3) biased geographic coverage. Gradual evolution significantly underestimates the dynamic evolutionary history of the fohsellids.