“O SOLEMYA…” LIGAMENTAL PLESIOMORPHY AND APOMORPHY IN THE ACHARAX-SOLEMYA CLADE (BIVALVIA: SOLEMYIDAE)

Although recent studies have helped clarify solemyid taxonomy, significant problems remain, especially regarding the familiar genera, Solemya and Acharax, whose intimate phyletic ties are obscured by: 1) incomplete understanding of the diagnostically important features of the ligament and its supporting structures; and 2) persistent use of problematic names, including Janeia and Janacekia.

In both Solemya and Acharax, the ligament is amphidetic: the primary ligament (PL), is opisthodetic/parivincular, supported by nymphae with an additional, partially internal anterior extension of the outer ligament layer, often attaching the valves asymmetrically, and variously marked by small internal lobes (demipads) that are used in Solemya as a subgeneric taxobasis. The principal difference between Acharax and Solemya concerns the PL and nymphae: external (plesiomorphic) in Acharax, but submarginal (apomorphic) in Solemya, depressed just below the hinge line and overgrown by a thin outer prismatic layer so as to partly occlude the posterior adductor muscle (PA). Submarginal nymphae in Solemya are unsuitably termed chondrophores, thus obscuring their homological relation to external nymphae. A further difference concerns the thin buttress bordering each PA where, in Acharax, it plesiomorphically functions for reinforcement for PA attachment, whereas, by exaptation, in Solemya it apomorphically serves as a supporting brace for the submarginal nymph by dorsally uniting with it.

Presumptions of past authors that the submarginal PL of Solemya is a primitive trait are unsupported. The external PL is symplesiomorphic, occurring widely in Paleozoic solemyids (Psiloconcha, Dystactella, Clinopistha, Mazonomya) and in Devonian Acharax, whereas the submarginal PL condition of Solemya is apomorphic, apparently derived from Acharax ancestry in the Jurassic. Acharax and Solemya (along with Pennsylvanian Mazonomya) form a single clade. Current paraphyletic taxonomies placing them in separate families or subfamilies are rejected in favor of monophyletically uniting them within the subfamily, Solemyinae. Genetic disparities separating modern representatives of the genera are explainable by the mid-Mesozoic split.