Morphology and systematic position of <i>Shaninopsis</i> Isberg, 1934 (Bivalvia: Cryptodonta), from the Boda Limestone (Upper Ordovician), Sweden

ABSTRACT In a classic study of bivalves from the Upper Ordovician (Katian) Boda Limestone, Sweden, Isberg (1934) named and described 18 species of the unusual bivalve Shaninopsis, all from the same locality. Our study of the type material indicates that only three species at most are present: Shaninopsis prona Isberg, 1934, Shaninopsis radiata Isberg, 1934, and an unnamed species. Shaninopsis features a thick, prosocline, tear-drop-shaped shell with an opisthogyrous larval shell. Anterodorsally, the shell outline appears truncated to strongly concave owing to an inset sical surface that frames a conspicuous pedal gape. Above the gape, the lunule accreted posteriorly (retrusive growth—new term) and abuts the anterior part of the ligament area, perhaps compensating for anterior splitting of the ligament during growth. The hinge plate is edentulous, with a broad, opisthodetic, monovincular ligament. Muscle scars visible include only the posterior adductor, which is set near the posteroventral shell margin. Relict calcite prismatic texture preserved on the ligament area indicates the external prismatic calcite shell layer underlaid the ligament as in cardiolid praecardioids and some inoceramiformians. Evidence for a retractable non-biomineralized sheath-like structure extending from the pedal gape is presented. Shaninopsis is reconstructed as an orthothetic, epifaunal or shallow semi-infaunal bivalve capable of deeply probing underlying sediment for H2S uptake for chemosynthesis. Shaninopsis is assigned to a new subfamily Shaninopsiinae in the family Lunulacardiidae (infraclass Cryptodonta), a group otherwise unknown in pre-upper Silurian rocks. Remarkable similarities of Shaninopsis with the Permian bivalve Eurydesma suggest a mutual though geochronologically distant phylogenetic relationship.