Mulinoidesplenicostatus

DeVries, Thomas J., 2019, Early Paleogene brackish-water molluscs from the Caballas Formation of the East Pisco Basin (Southern Peru), Journal of Natural History 53 (25), pp. 1533-1584 : 1551-1552

Mulinoidesplenicostatus sp. nov.

( Figure 3 (r – t))

Diagnosis

Highly arched valves, barely elongated posteriorly; sharply angled posteroventral and anteroventral umbonal ridges. Well defined and regular commarginal ribs across most of specimens.

Description

Shell about 30 mm long, trapezoidal, subequilateral, inflated medially, posteroventrally truncate; for adult specimens, H:L = 0.9, W:H = 0.7. Beaks low, prominent, prosogyrate, extending nearly to hingeline. Small lunular-like area with change in sculpture; posterior counterpart not visible. Commarginal sculpture of low, bevelled, flattened, narrow ribs, rarely bifurcating, with interspaces narrower than commarginal ribs; ribs extending at least three-quarters distance from ventral margin to beak. Anteroventral umbonal ridge angular; commarginal ribs on anterodorsal area strong, half as numerous as across middle of valve. Posteroventral umbonal ridge strongly angular with broad and shallow anteriorly adjacent sulcus. Hinge and interior not visible.

Remarks

Specimens of Mulinoidesplenicostatus sp. nov. are more highly arched and have a more centrally located beak than specimens of the type species, M. chilca , from the upper Maastrichtian ‘ Baculites ’ beds of the Tortuga Formation in the Sechura Basin of northern Peru ( Olsson 1944). In these characters, the Caballas Formation species greatly resembles M. chicama Olsson, 1944 , from the upper Campanian portion of the Tortuga Formation ( Olsson 1944). Specimens of M. plenicostatus differ from the figured specimen of M. chicama by having a truncate posterior margin and commarginal ribs developed over most, if not all, the area between the two umbonal ridges.

Etymology

‘ Plenicostatus ’, Latin for ‘ fully ribbed ’, referring to commarginal ribs crossing the entire valve of this species.

Material

UWBM 107571 , holotype, B8772 (type locality), L 28.1, H 25.7, W 17.2 (pair) ; UWBM 107572 , paratype, left valve, B8769, L (18.2), H (15.9), W (8.2) ; MUSM INV 244 , paratype, B8772, L 31.7, H 29.0, W 19.8 (pair

).

Occurrence

Lower Paleogene, Cuenca Member, Caballas Formation, East Pisco Basin, southern Peru.

Figures

Figure 3. Bivalves from the Caballas Formation. (a, b) Twig moulds on attachment scars of Ambigostrea sechura. (a) UWBM 107557, latex cast. (b) UWBM 107564, oyster length 11 mm. (c–e) Carolia (Parinomya) parinensis Olsson, 1928, on oyster attachment scars. (c) MUSM INV 243, latex cast of external mould on Ambigostrea sechura (MUSM INV 237), Carolia H 11.7 mm (top to bottom). (d) UWBM 107569, external mould on Ambigostrea sechura (UWBM 107559), Carolia H 22.9 mm (left to right). (e) UWBM 107570, external mould on Ambigostrea sechura (UWBM 107560), Carolia H 15.3 mm (left to right). (f–k) Incatina collinsae sp. nov., B8772. (f) UWBM 107637, holotype, L 13.7 mm. Right valve of pair, exterior. (g) UWBM 107637, left valve of pair, exterior. (h) UWBM 107637, paired valves, dorsal view. (i) UWBM 107638, paratype, L 10.0 mm. Right valve of pair. (j) UWBM 107639, paratype, L 16.4 mm. Right valve of pair. (k) UWBM 107638, left valve of pair. (l–q) Pleuriocardia (Incacardium) schneideri sp. nov. (l) UWBM 107575, paratype, B8769, L 13.7 mm. Paired valves, dorsal view, anterior to right. (m) UWBM 107574, paratype, B8770, L 25.0 mm. Right valve, exterior. (n) MUSM INV 247, paratype, B8772, L 21.1 mm. Right valve, exterior. (o) UWBM 107576, paratype, B8769, L 25.5 mm. External mould of left valve, oblique view. (p) MUSM INV 246, paratype, B8772, L 20.4 mm. External mould of right valve. (q) 107573, holotype, B8772, L 21.5 mm. Left valve, exterior. (r–t) Mulinoides plenicostatus sp. nov., UWBM 107571, holotype, B8769, L 28.1 mm. (r) Left valve, exterior. (s) Dorsal view, anterior on right. (t) Right valve, exterior.