Ontogeny, morphology and pedicle attachment of stenothecoids from the middle Cambrian of North Greenland (Laurentia)

fossils in the Cambrian, but the adaptation to enclose the soft parts within two articulating shells is widespread in the Cambrian biota. Amongst arthropods, bradoriids and phosphatocopines are also diverse and widely distributed (Hinz-Schallreuter 1993, Williams et al. 2007, Zhang 2007, Maas et al. 2003, McMenamin 2020), but a variety of other bivalved or pseudo-bivalved arthropods is known, mainly through occurrences in Lagerstätten such as the Burgess Shale and Chengjiang biotas (Briggs et al. 1994, Hou et al. 2017). Members of the molluscan Class Bivalvia are few in the Cambrian, but one of these, Pojetaia Jell, 1980, is widely distributed in the early and middle Cambrian (Elicki & Gürsu 2009, Vendrasco et al. 2011a). Shell symmetry in the problematic inequivalved bivalves Apistoconcha Conway Morris in Bengtson et al., 1990 and Aroonia Bengtson in Bengtson et al., 1990, originally described from the early Cambrian of Australia (Bengtson et al. 1990), is similar to that of brachiopods. Parkhaev (1998) placed Apistoconcha within a new class, Siphonoconcha, of uncertain position. Both genera may be stem group brachiopods (Bengtson 2004, Li et al. 2014). The calcareous shells of most members of the Class Stenothecoida Yochelson, 1968 (= Phylum Stenothecata Rozov, 1984) differ from brachiopods (usually inequi v alve but equilateral) and Bivalvia (usually equivalve but inequilateral) in being neither equivalve nor equilateral (Yochelson 1969, Rozov 1984; Fig. 1), but their systematic position is equivocal. Laurentian stenothecoids are generally referred just to Stenothecoides Resser, 1938 (Rasetti 1954, 1957; Robison 1964; Yochelson 1969; Peel 1988; Johnston et al. 2017; Johnston 2019; Fig. 1A, H), but a number of other genera and numerous species have been described from the Cambrian of Siberia and eastern Asia (Horný 1957; Sytchev 1960; Aksarina 1968; Yochelson 1969; Koneva 1976, 1979a, b; Pelman 1976, 1985; Aksarina & Pelman 1978; Voronin et al. 1982; Rozov 1984; Esakova & Zhegallo 1996; Yu 1996). Stenothecoids are distributed through Cambrian Series 2 and the Miaolingian (Rozanov & Zhuravlev 1992). Stenothecoides groenlandica Peel, 1988, from the late Miaolingian Series,