Malletiidae Adams & Adams 1858

Family Malletiidae Adams & Adams, 1858 gen. et sp. indet.

(Fig. 7)

Diagnosis. Subequilteral to inequilateral; usually with elongate, compressed posterior end; sculpture of commarginal striae or ribs; anterior and posterior gapes present; hinge plate weak, with fine taxodont teeth in two series, sometimes separated by plain area, without resilifer; ligament external, opisthodetic to amphidetic, weak; pallial sinus large.

Material. Three larval shells from LD-04 (NHMUK PI MB 1248–1250).

Description. Shell is equilateral, elliptical, H/L ratio of 0.7, and moderately inflated. Umbo is broad, orthogyrate, with beak positioned centrally. Shell smooth except for concentric growth lines. Hinge plate has three anterior and three posterior teeth in two series separated by a large plain area with a groove. Ligament is predominantly external, amphidetic and weak.

Remarks. Malletiidae are very similar to Nuculanidae, but the lack of a resilifer in mature adults supports their separation (Coan & Valentich-Scott 2012). These specimens do not belong to the Neilonellidae because they possess a small conspicuous gape between the valves. The Malletiidae is a long-ranging family known from the Ordovician to the Recent, and three genera (Malletia, Palaeoneilo and Taimyrodon) belonging to the family have previously been reported from the Lower Triassic (e.g. He et al. 2007; Wasmer et al. 2012). Externally, these specimens resemble the larval shells of Paleoneilo ? fortistriata figured by Wasmer et al. (2012), but they have far fewer hinge teeth. There are also equal numbers of anterior to posterior hinge teeth in these specimens, whereas in P. fortistriata there are many more posterior than anterior teeth (Wasmer et al. 2012).

Mode of life. Shallow infaunal, fully motile, slow, miner (Stanley 1968).