ITERATIVE PATTERNS OF WALL TEXTURE EVOLUTION AND ITS SIGNIFICANCE FOR PHYLOGENETIC RECONSTRUCTION AMONG CENOZOIC PLANKTONIC FORAMINIFERA

Patterns of wall texture on the surfaces of planktonic foraminiferal tests have been shown to be important characters for reconstructing evolutionary lineages. Test wall texture is typically a conservative character that exhibits little change for prolonged periods throughout a lineage’s history. Exceptions to this conservatism occur during the early evolutionary stages of several key Cenozoic lineages. It is during these early, critical stages of morphological evolution that wall textures can exhibit significant degrees of modification prior to becoming fully established within a lineage. For example, the evolution of Praemurica uncinata into Morozovella angulata during the late Danian entailed a transformation from a coarsely cancellate, “neogloboquadrinid” ancestral texture to a smooth-walled, heavily pustulose (muricate) descendant texture. This evolutionary shift in wall texture took ~200 k.y., and is coupled to profound changes in test morphology and paleoecology. Subsequent to the P. uncinata/M. angulata transition, the muricate wall texture of the morozovellids remained a relatively fixed character for nearly 23 m.y. Hence, significant degrees of evolutionary change are concentrated within brief intervals that are confined to the early stages of diversification. Scanning electron microscopy is used to demonstrate that patterns of wall texture evolution within the non-spinose, Neogene Globorotalia (Fohsella) and Globorotalia (Globoconella) lineages are grossly similar to those seen in the Paleogene morozovellids. The primitive wall texture of ancestral G. (Fohsella) peripheroronda is distinctly coarser and more deeply pitted than that of its smooth walled descendants, G. (Fohsella) peripheroacuta and G. (Fohsella) fohsi. Likewise, an early ancestor of the G. (Globoconella) lineage – G. (Globoconella) incognita – possesses a coarse, deeply-pitted wall texture, while its descendants G. (Globoconella) miozea and G. (Globoconella) conoidea have smooth, pustulose wall textures. The patterns of wall texture evolution seen in these phylogenetically removed Paleogene and Neogene lineages qualify as an example of iterative evolution, and indicate that wall texture alone should not be used as an uncorroborated diagnostic tool for retracing the early, ancestral roots of lineages.