Re-Evaluation of the Phylogeny and Stratigraphic Distribution of Acarinina Subsphaerica and Related High-Spired Paleocene–eocene Planktonic Foraminifera Species

This study re-evaluates the Paleogene planktonic foraminiferal genus Acarinina focusing on the species A. subsphaerica, A. pseudosubsphaerica, A. aquiensis, A. mckannai, and A. alticonica. Detailed morphological analyses, including SEM imaging and stratigraphic data from deep-sea drilling sites clarify taxonomic ambiguities and ancestor-descendant relationships. Results indicate that A. pseudosubsphaerica is conspecific with A. subsphaerica, representing a morphological continuum from the Paleocene to the middle Eocene. The holotype of A. aquiensis is synonymized with A. subsphaerica, and a new species, Acarinina neoaquiensis n. sp., is designated for taxonomic stability. Acarinina alticonica evolved from A. subsphaerica during the late Paleocene by reducing the number of chambers and modifications in the umbilical area, indicating direct ancestry. This study resolves longstanding taxonomic ambiguities and underscores the critical role of re-evaluating type specimens, historical illustrations, and original descriptions in taxonomic revisions.