Sphaeroidinellopsis seminulina

Sphaeroidinellopsis seminulina (Schwager 1866)

1866 Globigerina seminulina Schwager: fig. 142.

1959 Sphaeroidinella dehiscens subdehiscens Blow: pl. 12, fig. 71a–c.

1960 non Sphaeroidinellopsis disjuncta Banner & Blow: pl. 7, fig. 2.

2009 Sphaeroidinellopsis seminulina Hokuto, Ayayu, Toshiaki, Hayashi & Tanaka: 549, pl. 3, fig. 2a–c.

Test morphology. Low trochospiral, compact, equatorial periphery broadly ovate to slightly trilobulate. The ultimate whorl presents three subglobular chambers, with sutures obscured by a heavy cortex. In umbilical view, the umbilicus is narrow with a low umbilical aperture bordered by a thickened crenulated rim (Kennett & Srinivasan 1983). In spiral view, previous chambers’ whorls tend to be hidden under the cortex. When visible, the sutures are straight and incised. In edge view, rounded margin and compact outline with a flat trochospire.

Range. Tortonian (Zone M13)–Piacenzian (Zone PL 3). According to Kennett & Srinivasan (1983) this taxon appeared in the early Miocene in Zone N7 (= M4), but this datum might be affected by the misconception caused by the neotype erected by Banner & Blow (1960).

Remarks. This species can be distinguished from S. disjuncta by having a fully developed cortex, giving this species the typical glossy appearance shared with its descendant in the Sphaeroidinella lineage. Sphaeroidinellopsis seminulina is distinguished from S. kochi by its higher trochospire and having three chambers in the ultimate whorl and a fully developed cortex, often absent in S. kochi. Possible synonymies requiring further investigation are Prosphaeroidinella valleriae Bronniman, Whittaker & Parisi 1988 and Sphaeroidinella spinulosa Subbotina in Bykova 1958. Sphaeroidinellopsis seminulina can be distinguished from Globoturborotalita connecta by its fully developed cortex, or in uncovered specimens (lacking a cortex) by its coarser honeycomb wall texture. It differs from G. woodi in having three chambers in the ultimate whorl, a more compact coiling, the lower aperture, the narrower umbilicus and the secondary crystallization of the cortex. It can be distinguished from Sphaeroidinella dehiscens due to the lack of a supplementary aperture on the spiral side and the trilobate overall shape, compared with the bilobate appeareance of S. dehiscens.

Taxonomic history. Schwager (1866) described this species from the lower Pliocene of Car Nicobar Island (Srinivasan & Sharma 1974), but the holotype and primary paratypes have been lost, requiring the definition of neotypes. Banner & Blow (1960) indicated that S. disjuncta was a junior synonym of S. seminulina, an argument strongly debated by Jenkins (1971) and Kennett & Srinivasan (1983). This synonymy is not retained as valid today. The absence of the holotype motivated Banner & Blow (1960) to re-examine the metatype material at the Natural History Museum, London, describing and illustrating a controversial neotype. Despite the majority of the metatypes having three chambers in the ultimate whorl, as did the holotype illustrated by Schwager (1866), Banner & Blow (1960) selected an atypical four-chambered specimen as the neotype. This has since given the impression that S. seminulina is typically a four-chambered taxon (Stainforth et al. 1975). In Blow (1959), a new species was erected, S. subdehiscens, considered different from S. seminulina due to having three chambers in the ultimate whorl. Srinivasan & Kennett (1981) considered S. subdehiscens a junior synonym of S. seminulina and consequently re-designated S. seminulina (Schwager 1866) to be the genotype of Sphaeroidinellopsis (Kennett & Srinivasan 1983), after comparing the original topotypes of S. seminulina from Car Nicobar Island with S. subdehiscens (Blow 1959). No significant differences in terms of chamber number, structure of the cortex or in the apertural characters were identified by Srinivasan & Kennett (1981).

In the original description of Schwager (1866), Sphaeroidinellopsis seminulina is described as having three and seldom four chambers in the ultimate whorl, while the cortex may fully or partially cover the test, but no supplementary apertures are present. Sphaeroidinellopsis seminulina is retained as the ancestor of S. paenedehiscens, which led to the origin of Sphaeroidinella dehiscens, adding supplementary apertures and developing a full cortex (Blow & Banner 1962; Kennett & Srinivasan 1983; Kŭcera 1998; Aze et al. 2011). The relationship between S. seminulina and S. disjuncta is still unclear, and some morphological overlap between these taxa is common. Lam & Leckie (2020) reported transitional specimens between these two species in the north-west Pacific Ocean.