Marginulinopsis costata

Marginulinopsis costata (Batsch, 1791) Pl. 2, figs. 10–12

Nautilus (Orthoceras) costatus Batsch, 1791, p. 2, pl. 1, fig. 1a–g.

Marginulina raphanus d’Orbigny 1826, p. 258, pl. X, fig. 7–8.

Marginulina costata Brady, 1884, p. 528, pl. 65, fig. 10–13; Bagg, 1912, p. 62, pl. XVIII, fig. 4; Cushman, 1921, p. 256, pl. 41, fig. 5–8; Heron-Allen & Earland, 1922, p. 176; Cushman, 1923a, p. 132, pl. 37, fig. 2; Jones, 1994, p. 77, pl. 65, fig. 13; Milker & Schmiedl, 2012, p. 74, fig. 18.26; Obaje & Okosun, 2013, p. 360, pl. 1.18.

Vaginulinopsis bradyi Jones, 1994, p. 77, pl. 65, figs. 11–12.

Description: The test wall is calcareous and finely perforate. The test is elongate, uniserial and circular in crosssection. The initial portion is coiled and round. Up to nine chambers are visible along the exterior. The chambers are rectilinearly arranged, separated by depressed sutures. The test is covered with thick longitudinal costae. The costae may extend from the initial to the terminal chamber and are slightly curved. Many of the specimens do not have the costae covering the terminal chambers. The final chamber is globular in shape, with a terminal and radiate aperture at the dorsal angle of a pronounced neck.

Remarks: The relative abundance of M. costata is generally low, forming trace components (<1%) in some of the samples of core 2670. There is variation in the size of the tests. Smaller specimens measure up to 0.2 mm in cross section diameter and 1 mm in length, while some tests that are ornamented throughout, are large, measuring up to 0.4 mm in cross section and 2 mm in length.

The tests show a variable degree of size and ornamentation. Similarly, there appears to be variation in the ornamentation within this species itself and among the synonymies. For example, the test in pl. 2, fig. 10 closely resembles the sketched imaged of Marginulina bifurcata in Fornasini (1902), with the terminal chamber being just as strongly costate as the preceding chambers. Marginulina bifurcata has, however, been synonymised into M. costata. The specimens in figures 11 and 12 resemble Marginulina raphanus (d’Orbigny, 1826), but that species has a pointed initial end, whereas specimens from this study area is rounded along the initial portion. There has also been confusion in the identification of this taxon. Barker (1960) re-identified this species as Marginulinopsis bradyi, whereas Jones (1994) split the figured identifications of M. costata in Brady (1884) into Vaginulinopsis bradyi and M. costata. The original specimen of Marginulina bradyi appears to be more initially coiled and flatter, broadening towards the terminal end, whereas M. costata does not show the same initial portion and later stages. In this study, the figured specimens have been identified as Marginulinopsis costata. The figured specimens 11 and 12 also resemble that of Marginulina sendaiensis in Asano (1937, 1949), possessing less or no ornamentation on their terminal chamber and are more abundant (up to 5% in some of the samples) than those having tests completely covered in costae.The difference between Marginulina sendaiensis and M. costata is that the aperture of Marginulina sendaisensis is centrally located on the terminal chamber, but the aperture of M. costata is located excentrically.

Life strategy: Species of this genus are generally shallow-infaunal in environments with variable conditions of low to high oxygen (Milker & Schmiedl, 2012).

Global stratigraphic range: This species occurs from the Jurassic to Recent (Bagg, 1912).

Regional occurrence: M. costata is documented to occur in middle Miocene sediments on the Namibian outer continental shelf, south of the Kunene River mouth (this study).