Similar to the T. sacculifer plexus, G. fistulosa has also undergone taxonomic change following Spezzaferri et al. (2015). Globigerinoidesella fistulosa had previously been termed Globigerinoides fistulosus under the traditional concept of Globigerinoides. However, Spezzaferri et al. (2015) re-introduced the genus Globigerinoidesella El-Naggar, 1971, to separate forms with extended projections from Trilobatus.
Globigerinoidesella fistulosa is a short-ranging morphospecies (mid-Pliocene to early Pleistocene). The restricted biostratigraphical range of G. fistulosa makes it a useful zone fossil in tropical assemblages worldwide, and its last appearance is used to define the base of Zone PT1 (Wade et al. 2011). Its ancestral stock is the T. sacculifer plexus and it is inferred to have arisen gradually from T. sacculifer (e.g. Parker 1967; Kennett & Srinivasan 1983; Chaisson & Pearson 1997). Globigerinoidesella fistulosa sensu stricto is characterized by its ornate, digitate morphology, often with numerous protuberances on a single chamber. It is therefore easy to identify and distinguish from other taxa, including its ancestor T. sacculifer. Trilobatus sacculifer regularly possesses an extended final chamber, but is typically not digitate. Trilobatus sacculifer sensu stricto is indeed strikingly disparate from G. fistulosa sensu stricto; the latter possesses one or more elongated projections from at least one of the final chambers. However, there exists a complete morphological intergradation between the T. sacculifer plexus and G. fistulosa sensu stricto, impeding objective delimitation and thus constraining its first (and last) stratigraphical occurrences. The presence of such intermediate or ‘transitional’ forms with progressively more developed protuberances towards the rise of G. fistulosa sensu stricto has been observed by Chaisson & Pearson (1997) from Ocean Drilling Program (ODP) Site 925 (Ceara Rise, western equatorial Atlantic Ocean).
Transitional forms are most common during the stratigraphical range of G. fistulosa sensu stricto, which also co-exists with all morphospecies of the T. sacculifer plexus. In other words, G. fistulosa does not replace T. sacculifer, but co-exists with it until the last stratigraphical occurrence of G. fistulosa in the early Pleistocene. Following the last occurrence of G. fistulosa, all morphospecies of the T. sacculifer plexus persist to the present day. However, it is demonstrated here that transitional forms with incipient protuberances also range to the Recent. It is unsatisfactory for these forms to be termed G. fistulosa, as then all biostratigraphical utility is lost. They are typically regarded as extreme morphological variants of T. sacculifer. In this study, we use the occurrence of such transitional forms in Pleistocene to Recent cores from the GLOW cruise (Kroon & Scientific Participants 2010) to form a morphological basis for delimiting the gradual transition between T. sacculifer and G. fistulosa in the mid-Pliocene to early Pleistocene. This delimitation is used to refine the morphological concept of G. fistulosa in a systematic taxonomic appraisal presented herein.