The genera that never were: The impact of Janeia and Janacekia on phyletic and taxonomic relations within the Solemyidae (Bivalvia: Protobranchia)

Abstract

Proposed taxonomies of the Solemyoidea remain conflicted, particularly regarding the familiar genera, Solemya and Acharax, whose close phyletic kinship has been obscured by: 1) imperfect understanding of the chronological morphogenesis of diagnostically important ligament support structures, and 2) misconceptions triggered by two problematic Paleozoic genera, Janeia and Janacekia, including the prevalent mischaracterization that the primary ligament in Janeia was located in an internal chondrophore. Whereas Janacekia and certain Janeia are synonyms of Acharax, other alleged Janeia are shown to be clinopisthins such as Dystactella and Clinopistha. Morphological analysis supports DNA studies suggesting Acharax and Solemya are monophyletic, thereby challenging recent taxonomies placing them in separate families or superfamilies. The primary ligaments of Acharax and Solemya are basically similar (opisthodetic, parivincular, attached at nymphae), the main differences being placement: external in Acharax but slightly internal (submarginal) in Solemya. Character analyses suggest the external nymphae of Acharax are plesiomorphic, arising by the Early Devonian, whereas the submarginal placement of the nymphae in Solemya is shown to be a Mesozoic apomorphy involving depression and secondary enclosure of the ligament and nymphae by a shelly outer layer. Nymphal enclosure has had two concomitant effects: 1) dorsal occlusion of the posterior adductor muscle; 2) fusion of the nymphae to internal buttresses that plesiomorphically functioned as simple reinforcement for the anterior margin of the posterior adductor muscles whereas, by exaptation, they become apomorphically modified to serve as supporting braces for the submarginal nymphae by dorsally uniting with them. Jack Bowman Bailey. Department of Earth, Atmospheric and Geographic Information Sciences Western Illinois University, 1 University Circle, Macomb, IL 61455 USA. jb-bailey@wiu.edu