MAINTENANCE OF LONG-TERM MORPHOLOGIC BIMODALITY IN THE NEOGENE TRUNCOROTALIA

Biometric analysis of the Truncorotalia lineage of the Neogene planktonic foraminifera revealed long-term patterns of morphologic bimodality associated with coiling direction changes as part of a larger study to determine the mode and tempo of speciation in the lineage. Statistical analysis of measured morphometric variables was used to determine when in the lineage morphologic bimodality appears within the species of the Truncorotalia. Coiling direction changes were tracked and compared with morphologic changes to investigate the origin of coiling direction reversals and dominance. The investigation reveals that morphologic changes, ecophenotypes, and diachroneity in the Truncorotalia are associated with latitude and water mass conditions.

Results indicate that the Truncorotalia coiling types are morphologically distinct. Patterns of morphologic and coiling direction changes were similar at the sites investigated. Coiling direction and morphologic similarity are maintained through speciation events. The statistical analyses show a sensible pattern where the species progress through morphospace in accordance with their stratigraphic appearance. At a species first appearance, coiling is either 100% sinistral or dextral. The dominant coiling type ancestor gives rise to the same coiling type descendent. The phenomenon of biogeographic coiling dominance seen in the lineage appears to be related to the mechanism of reproductive isolation leading to speciation. The patterns of morphologic and coiling direction change suggest that the evolution of the lineage occurred in environmental conditions where one coiling type was favored. Coiling changes at a site are likely the result of migrating water-mass frontal boundaries as ocean circulation reorganizes in response to climate change. Modern coiling types of Gr. truncatulinoides are genetically different and may even be distinct species. Whether or not the rest of the species coiling types within the lineage are distinct species remains unclear. Previous research combined with the results of this study suggests a depth parapatric mode and a gradual tempo of speciation in the Truncorotalia.