Sphaeroidinellopsis and Sphaeroidinella (planktonic foraminifera): morphological variability, wall texture and taxonomic implications

We used biometry, multifocal optical and scanning electron microscopy to investigate the morphological variability and wall texture of the planktonic foraminifera genera Sphaeroidinella and Sphaeroidinellopsis. Our study focuses on the late Miocene to Pleistocene (Zone M11 to Subzone PT1b) from the west Pacific within the ‘warm pool’ (International Ocean Discovery Program Site U1490) and from the north-west Australian margin (International Ocean Discovery Program Site U1482). Our observations indicate a new type of wall texture that we describe and name the disjuncta-type, after the first morphospecies to bear it (Sphaeroidinellopsis disjuncta). The disjuncta-type wall texture is restricted to the genera Sphaeroidinellopsis and Sphaeroidinella, and it is defined by a spinose wall with extremely large pores and the ability to overgrow a thick imperforate secondary layer of calcite, known as a cortex. This type of wall is similar to the sacculifer-type texture, but with a larger pore size and smaller pore number per unit area. Biometric analysis of the maximum diameter and the circularity was measured on over 1100 individuals with a sample resolution of 500 kyr, to highlight the main morphological differences exhibited along this evolving lineage and to document the local phenotypic variability of both genera. We reassess the taxonomy of Sphaeroidinellopsis kochi, designating a neotype to accommodate the taxonomic instability surrounding this taxon. Members of the extant Sphaeroidinella lineage, Sphaeroidinella dehiscens and Sphaeroidinella excavata, are discussed and recognized as distinct and valid morphospecies due to their biostratigraphic potential.