Sphaeroidinellopsis seminulina

description

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 M 13) – Piacenzian (Zone PL 3). According to Kennett & Srinivasan (1983) this taxon appeared in the early Miocene in Zone N 7 (= M 4), but this datum might be affected by the misconception caused by the neotype erected by Banner & Blow (1960).

Systematic taxonomy of middle Miocene Sphaeroidinellopsis (planktonic foraminifera)

discussion

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 (1

Systematic taxonomy of middle Miocene Sphaeroidinellopsis (planktonic foraminifera)