Pseudomyona from the Cambrian of North Greenland (Laurentia) and the early evolution of bivalved molluscs

logical studies of present day bivalves that the Ordo­ vician–Recent members of the Class Bivalvia Linnaeus, 1758 form a monophyletic group, although some relation­ ships within the group remain unresolved (Schneider 2001; Giribet 2008; Plazzi et al. 2011; Sharma et al. 2012, 2013; Bieler et al. 2014; González et al. 2015; Lemer et al. 2019). However, the derivation of Bivalvia from ancestral univalved molluscs and its stem group evolution in the Cambrian remains the subject of speculation (Cope 1997, 2000; Carter et al. 2000, 2011; Fang & Sánchez 2012; Cope & Kříž 2013; Ponder et al. 2020). Two Cambrian bivalved groups are recognised as potential ancestors of Bivalvia but their morphological dissimilarity, together with the general absence of material from the late Cambrian (Furongian), obscures the evolutionary pathway. The Order Fordillida Pojeta, 1975 includes Fordilla Barrande, 1881 (Fig. 1L, N) and Pojetaia Jell, 1980 (Fig. 1M) and its oldest members appeared in the Terreneuvian. The Order Tuarangiida MacKinnon, 1982 contains Tuarangia MacKinnon, 1982 (Fig. 1K) and is described from the late Miaolingian Series (Guzhangian Stage), about 20 Ma later. Tuarangia and the youngest specimens of Pojetaia occur together in the Guzhangian of Denmark (Hinz­Schallreuter 2000). Carter et al. (2011) rationalised the dilemma con­ cerning origin(s) by recognising two major divisions of Class Bivalvia. A Grade Euprotobranchia Nevesskaja, 2009 embraced the two orders Fordillida and Tuarangiida, which were considered in some way likely to be an­ cestral to crown group bivalves, but together did not comprise a monophyletic entity. The grade recognises the dichotomy as stem group bivalves between Fordilla and Pojetaia, and Pseudomyona Runnegar 1983 and Tuarangia, discussed by Runnegar & Pojeta (1992), as a result of which Pseudomyona and Tuarangia were not considered to be Bivalvia. Carter et al. (2006) had considered Tuarangia, Pseudomyona, Watsonella Grabau, 1900 (Fig. 1J) and Anabarella Vostokova, 1962 to form a sister group to Pojetaia and Fordilla and post­Cambrian Bivalvia. A Clade Eubivalvia Carter in Carter et al. (2011) included the post­Cambrian groups traditionally regarded as Bivalvia, but Sharma et al. (2013), Bieler et al. (2014) and others considered Fordilla to be a crown group bivalve. Conversely, Ponder et al. (2020, p. 552) placed