Parvitrigonia cooperi, Halligan, 2023

Halligan, William Keith, 2023, Washington State (USA) trigoniids (Bivalvia) from the conglomerate of Patterson Lake (Early Cretaceous), PaleoBios 40 (8), pp. 1-15 : 10-11

PARVITRIGONIA COOPERI n. sp.

Figs. 8 A-D, 9A, B

Zoobank LSID— urn:lsid:zoobank.org:act:F7F98567-F4C0-4A13-BB52-83B7B83B51ED

Diagnosis— Very small, moderately inequilateral, elongate-pyriform, and inflated. Anteroventral margin smoothly convex becoming nearly straight posteriorly. Posterodorsal margin nearly straight. Posterior terminus narrow, nearly pointed. Beaks small, pointed, opisthogyrate, and located in the anterior third of shell. Marginal angulation conspicuous, rounded, sharper near the umbo, and gently concave as it courses from beak to posterior terminus creating an angle of almost 90° between the surfaces of the flank and area. Sunken area/escutcheon forms a platform that mimics the concave curve of the marginal angulation; platform unornamented except for fine growth lines. Umbo and flank ornamented with 11-12 subcommarginal, flat-topped, non-tuberculate ribs. Ventral 5 ribs wavy and attenuate as they approach the posterior terminus.

Holotype — UWBM 112558

Paratypes — UWBM 112557 , UWBM 115724 .

Age and Occurrence— Type locality: UWBM loc. B-9486. Specimens were found in the upper part of the cPL at both the Patterson Mountain and Rendezvous Road sites ( See below), Okanogan County, Washington State ( USA). Age : middle Albian .

Material— UWBM 112557 : Paratype. A slightly compressed natural mudstone cast of a RV from UMBM loc. B-9487. It is nearly complete and shows some detail of the area/escutcheon but is missing the posterior terminus . UWBM 112558 : Holotype. A slightly crushed, mildly deformed, articulated specimen from UWBM loc. B-9486. The LV shows the flank ornamentation and the posterior terminus well, but is missing the anterodorsal margin, area, and escutcheon. The RV retains some shell material and is missing the posterior terminus, area, and escutcheon. The schizodont dentition is partially visible under magnification . UWBM 115724 : Paratype. An undistorted natural sandstone cast , RV, complete, from UWBM loc. B-9541. This cast demonstrates the external outline and general shape of the shell, but because of its sandstone matrix, lacks the ornamental detail seen in the other specimens .

Etymology— Named after Michael R. Cooper of South Africa who has worked for over 40 years to bring order to the chaotic world of global trigoniid systematics, and whose encouragement and guidance made this study possible.

Description— Very small (L = 16mm, H = 10mm), elongate-pyriform, longer than high (H/L = 0.625), inflated (W/H = 0.43) and moderately inequilateral (umbo in the anterior third). Umbo weakly prominent, opisthogyrate. Beaks small, pointed, and mildly incurved. Anterior margin convex, merging smoothly with the ventral margin. Ventral margin convex anteriorly becoming nearly straight posteriorly. Posterior end narrow and nearly pointed. Posterodorsal margin nearly straight. Marginal angulation conspicuous, rounded and gently concave as it courses from beak to posterior terminus creating an almost 90° transition between the flank and area surfaces.

Sunken area/escutcheon forms a platform that mimics the course of the concave marginal angulation and narrows posteriorly; platform rises up smoothly to form the nearly straight posterodorsal margin. Area/escutcheon unornamented except for fine, closely spaced, oblique growth lines.

Umbo and flank ornamented by 11-12 subcommarginal, flat-topped, non-tuberculate ribs. Ventral five ribs wavy and fade as they approach the posterior terminus. Interspaces and ribs equal in width in the umbo region; ribs wider than the interspaces in the mid-flank area; interspaces wider than the ribs anteriorly.

Figures

Figure 8 (left). Parvitrigonia cooperi n. gen. et sp. A, B. UWBM 112557 from UWBM loc. B-9487. Paratype. Natural mudstone cast, RV, complete except for posterior terminus (dotted area). A. Flank view showing general outline. Costae mildly eroded. B. Dorsal view showing fine growth lines on area/escutcheon. C, D. UWBM 112558 from UWBM loc. B-9486. Holotype. Slightly crushed natural mudstone cast of articulated valves. Missing area and escutcheon (dotted area). C. RV with some shell remaining, flank view, show- ing wavy costae anteriorly. D. LV, flank view, showing flattopped costae. Scale bar = 5 mm