STEM GROUP BIVALVIA: WHERE TO BEGIN?

Ever since Laacaze-Duthiers (1857), scaphopods have been considered to be the living class of the Mollusca most closely related to the Bivalvia. If this were true, then rostroconchs are either stem group scaphopods or stem group diasomes (Runnegar & Pojeta, 1974; Salvini-Plawen, 1980: Steiner, 1992; Engeser & Riedel, 1996; Runnegar, 1996) and the Bivalvia stands alone as a monophyletic clade. Recently, Waller (1998) has overturned this traditional view by proposing that the Scaphopoda and Cephalopoda are sister groups within the subphylum Cyrtosoma. As a result, all rostroconchs and some helcionellacean univalves technically become stem group bivalves. This is because the total group is thought to have originated at the latest common ancestor of the Bivalvia and the Cyrtosoma: (Gastropoda (Cephalopoda, Scaphopoda)). There are four known fossil organisms that might represent early sister groups of the Scaphopoda: (1) the Ordovician ribeiriid rostroconch Pinnocaris (Pojeta & Runnegar, 1976); (2) the mid-Paleozoic rostroconch Conocardium (Morris, 1990; Engeser & Riedel, 1996); (3) the Ordovician Problematicum Janospira (Fortey & Whittaker, 1976; Runnegar, 1977; Paterson, 2001); and (4) the stem group cephalopod Plectronoceras (Waller, 1998). Choosing among these different scenarios requires an assessment of the few and often ambiguous characters that have been used in phylogenetic analysis.