Cytheropterina crassicostata sp. nov.

Fig . 10: 10-13

Etymology .

crassus (lat.) = coarse, fat; costatus (lat.) = ribbed; referring to the broad rounded ribs.

Holotype.

Right valve, figured on Fig. 10: 10, SMNS 70521/71.

Paratypes.

two left valves, one right valve, figured on Fig. 10: 11-13, SMNS 70521/72-74.

Type locality.

Heiligenbach valley near Hechingen-Beuren (SW Germany).

Type horizon.

Achdorf Formation, claystone above the Calceolabank, bed no. 46 in Fig. 4; Concavum Zone.

Material.

11 RV, 11 LV in samples He19-26-27, Mue19-1 and Ro19-4-5.

Diagnosis.

Medium size. Subtriangular outline, with broad, rounded irregular ribs; the underside of the alate extension being smooth except for its posterior part.

Description.

Carapace medium-sized, subtriangular. Right and left valve (identical in outline and ornamentation): Anterior margin symmetrically rounded. The dorsal margin is straight, medianly slightly concave. The ventral margin converges, partly straight or in a slight curve, to the acuminate posterior end. The ventral margin is obscured by a broad triangular wing. The anterior and posterior marginal zones are covered with roundish pits. The anterior marginal zone is offset with a small, asymmetrically rounded step against the lateral surface. A broadly rounded, mediodorsally-anteroventrally running rib forms a second step running diagonally over the anterior half of the valve. From this rib another broad rib branches off slightly below the middle, running subvertically towards the edge of the wing. The adjoining surface, which ends in the symmetrically rounded tip of the broad wing, is characterised by three wide, flat, bulbous elevations separated by wide furrows and pits. The furrows and pits as well as the ribs and depressions are covered with roundish secondary pits. Posteriorly, this area is bounded by a slightly curved, subvertical, rounded rib that tapers from dorsal to ventral. The posterior marginal zone is offset with a further, almost straight, subvertical step. Marginal zone wide anteriorly (Fig. 10: 13). The underside of the wing, which in ventral view forms a rectangular triangle, is smooth, except for its posterior third. A triangular area is covered by small grooves which are most prominent along the posterior margin of the wing. Hinge and other internal details are not observed.

Comparisons.

Cytheropterina crassicostata resembles in outline Cytheropterina bicuneata (Braun) as reported by Franz et al. (2018) from the Aalenian and Lower Bajocian of SW Germany, and Cytheropterina cribra Fischer, 1962 (p. 339, Fig. 25: 8-11) from the Toarcian to Bajocian. The main differences are the coarse, softly rounded ornamentation, the smooth underside of the wing and the bigger size.

Occurrence.

Upper Aalenian, Bradfordensis Zone (Gigantea Subzone) to Concavum Zone; SW Germany.