Globigerinoides subquadratus

description

Description: The wall surface is macroperforate and cancellate-spinose. The test is trochospiral, sub-globular to sub-quadrate in shape and semi-circular in cross section. The chambers are sub-rounded to sub-quadrate in shape, with two basal chambers on the umbilical side, separated by a straight deep incised suture, which is overarched by a large chamber. The final chamber comprises approximately half the size of the test. The primary aperture is large and forms an arch symmetrically above the two basal chambers in apertural view. Two supplementary apertures form on the spiral side of the test along sutures of earlier chambers.

Taxonomy of Middle Miocene foraminifera from the northern Namibian continental shelf

discussion

Remarks: The relative abundances of Gs. subquadratus are major (<25 %) in samples along the Mio-Pliocene contact of all three cores, increasing in abundance in the deeper cores. The tests of Gs. subquadratus are relatively small, measuring 0.3 mm in diameter. This species is very similar to Globigerinoides ruber, but are stratigraphically separate and can be morphologically differentiated. Gs. subquadratus is identified in the Miocene, whereas Gs. ruber is extant and occurs in younger strata. Morphologically, the shape and outline of the test and chambers of Gs. subquadratus is more quadrate in shape compared to Gs. ruber (Bolli et al., 1985). Environmental preferences: Gs. subquadratus is associated with Neogene tropical environments (Brönnimann et al., 1971; Wade et al., 2011). Global stratigraphic range: This species occurs in Miocene strata from the Aquitanian to Tortonion (Kennett & Srinivasan, 1983; Wade et al., 2011). Regional occurrence: Gs. subquadratus has been identified in middle Miocene strata from the Congo Basin (Kender et al., 2008) and the northern Namibian outer continental shelf, south of the Kunene River mouth (this study).

Taxonomy of Middle Miocene foraminifera from the northern Namibian continental shelf