The late Neogene planktonic foraminifera genus Globigerinoidesella : taxonomy, biostratigraphy, evolution and palaeoecology

Planktonic foraminifera have one of the most complete fossil records of any group, and have proved invaluable in the fields of biostratigraphy, palaeoceanography and evolutionary studies. Gradual morphological change characterises their evolution, meaning that species delimitation for biostratigraphic purposes is typically artificial and arbitrary. Delimited taxa in the fossil record are ‘morphospecies’ rather than biological species. The late Neogene morphospecies Globigerinoidesella fistulosa is of considerable biostratigraphic utility because of its short stratigraphic range (mid-Pliocene to early Pleistocene) and unique morphology. It evolved distinctive protuberances on the final chamber(s), but morphospecies delimitation is problematic because of intergradation with its ancestor Trilobatus sacculifer. The fossil record of Globigerinoidesella fistulosa was investigated from multiple ocean basins, with research focused on four main areas: (1) Systematic taxonomy of G. fistulosa and the Trilobatus sacculifer plexus; (2) Biostratigraphy and biochronology of the extinction of G. fistulosa; (3) The role of heterochrony (i.e. changes in the timing and/or rate of development) in the morphological evolution of G. fistulosa; (4) Determination of the palaeoecology of G. fistulosa in relation to its ancestor T. sacculifer, to understand the mode of speciation and the potential palaeoceanographic applications of G. fistulosa. The taxonomic concepts of Globigerinoidesella fistulosa and the Trilobatus sacculifer plexus were revised, resolving numerous taxonomic issues in the group. Protuberance development was found to not only occur in Pliocene-Pleistocene T. sacculifer, but was also observed in Trilobatus immaturus, Trilobatus quadrilobatus and modern T. sacculifer specimens. New ages for the extinction of G. fistulosa and comparison with updated literature ages revealed that the currently used age of 1.88 Ma is only suitable locally and not applicable at other sites worldwide, where the extinction occurred between 1.716 and 1.74 Ma (~150,000 years later). The morphological evolution of G. fistulosa is demonstrated to be an example of the heterochronic process hypermorphosis. Stable isotope analyses (δ13C and δ18O) and palaeobiogeographic data reveal that G. fistulosa was a photosymbiotic, shallow-dwelling morphospecies with a distribution that tracked warm water oceanographic features. Geochemical depth profiles through test walls of G. fistulosa and T. sacculifer generated through laser ablation inductively coupled plasma mass spectrometry show heterogeneous Mg/Ca distribution, including cyclic high and low Mg/Ca banding and low Mg/Ca outer crusts.