Wallerconcha Valentich-Scott & C. L. Powell

Valentich-Scott, Paul, Powell, Charles L., II,, Lorenson, Thomas D. & Edwards, Brian E., 2014, A new genus and species of Thyasiridae (Mollusca, Bivalvia) from deep-water, Beaufort Sea, northern Alaska, ZooKeys 462, pp. 11-26 : 14

Taxon classification Animalia Lucinoida Thyasiridae

Wallerconcha Valentich-Scott & C. L. Powell gen. n. Figures 3 A–H, 7B

Type species.

Wallerconcha sarae Valentich-Scott & C.L. Powell, new species herein (Figures 3 A–H, 7B). No other species are currently included in the genus.

Description.

Shell moderate in size (length to 24 mm), subtrigonal, subequilateral, strongly inflated; beaks broad, strongly prosogyrous; posterior radial sulcus shallow; sculpture of moderate to strong, uneven commarginal ribs and striae; periostracum thick, dehiscent, medium to dark brown, wrinkled, without micro-spines; lunule absent; escutcheon long, moderately narrow, moderately impressed; ligament large, long, deeply sunken on a stout nymph; hinge edentulous or with minute tubercles; hinge fig well defined and strongly thickened posteriorly; anterior adductor muscle scar wide, long.

Etymology.

The genus is named in honor of Thomas R. Waller (Smithsonian Institution) for his significant contributions to our understanding of the evolution, biogeography and systematics of fossil and modern marine bivalves.

Comparisons.

Wallerconcha differs from all other members of the Thyasiridae by the combination of four primary shell characteristics: 1) a well-defined hinge fig; 2) a heavy, deeply sunken nymph; and 3) a broad, elongate anterior adductor muscle scar that is not divided into two sections; 4) a dark, thick, wrinkled periostracum, without micro-spines.

Wallerconcha is similar to the deep-water genus Spinaxinus Oliver & Holmes, 2006 (type species, Spinaxinus sentosus Oliver & Holmes, 2006) (Figure 4 A–D). The latter genus has a thin, translucent, minutely spinose periostracum (Figure 4D), whereas the periostracum of Wallerconcha is thick and wrinkled but lacks periostracal spines (Figure 3D). In addition, Wallerconcha has a much longer and wider anterior adductor muscle scar, and a longer and deeper nymph.

Another similar genus is Axinus G.B. Sowerby I, 1821 (type species Axinus angulatus G.B. Sowerby I, 1821). Oliver and Holmes (2007a) reviewed several members of this genus and concluded that it has a large lunule, a moderate to strong posterior radial sulcus, a thin hinge fig, and lacks a heavy nymph, all features which separate it from Wallerconcha .

Parathyasira Iredale, 1930 (type species Parathyasira resupina Iredale, 1930) has an external sculpture of minute rows of spines, and a distinct radial sulcus. It also has a thin hinge fig and weak nymph, which are less robust than Wallerconcha . Both genera have an elongate anterior adductor muscle scar, whereas in Parathyasira the scar is usually divided into several sections, Wallerconcha has a single, broad scar.

Maorithyas marama Fleming, 1950, the type species of the genus, has a very thin hinge fig, lacks a heavy nymph, and has a shorter anterior adductor muscle scar (Figures 5 A–G, 7A) when compared to Wallerconcha .

Okutani et al. (1999) placed their new, deep-water Japanese thyasirid species into the shallow-water genus Maorithyas Fleming, 1950. They chose the generic placement of Maorithyas hadalis Okutani et al., 1999 based on the shallow posterior radial sulcus, and relatively heavy sculpture. The internal shell characteristics of Maorithyas hadalis (holotype, NSMT 71431), namely the periostracum, hinge fig, nymph and anterior adductor muscle scar place it outside of Maorithyas or Wallerconcha (Figure 6 A–H). It potentially belongs in a new genus, but that description is outside the scope of this paper.