Enigmaconidae Mackinnon, 1985

Kouchinsky, Artem, Bengtson, Stefan, Clausen, Sébastien & Vendrasco, Michael J., 2015, An early Cambrian fauna of skeletal fossils from the Emyaksin Formation, northern Siberia, Acta Palaeontologica Polonica 60 (2), pp. 421-512 : 436

Family Enigmaconidae Mackinnon, 1985 Genus Enigmaconus Mackinnon, 1985

Type species: Enigmaconus parvus Mackinnon, 1985 ; Ptychagnostus cassis Zone, Boomerangian Stage ; Cobb Valley, Nelson Province, South Island, New Zealand.

Remarks.— Enigmaconus Mackinnon, 1985 is represented by a similar pegma-bearing, but slightly cyrtoconic whith a more extended apex species E. parvus Mackinnon, 1985 . Pegma-like structures in the sub-apical part of the shell are also known from several forms of molluscs attributed to different genera. The pegma was first described in ribeirioid rostroconchs ( Pojeta and Runnegar 1976). It represents a hard internal plate that connected the left and right sides of the ribeirioid rostroconch shell and is visible as a groove on internal moulds. In some lower–middle Cambrian helcionellids, a similar structure is produced by a sub-apical invagination or internal thickening of the wall. When Enigmaconus and Eurekapegma were described from the Middle Cambian of New Zealand ( Mackinnon 1985), the pegma was known only from the cosmopolitan lower Cambrian mollusc Watsonella

= Heraultipegma ). At that time, Watsonella was considered a rostroconch ( Pojeta and Runnegar 1976), and Mackinnon

1985) speculated that Enigmaconus may have been ancestral to rostroconchs, based on the shared characteristic of the pegma. Since that time, other pegma-bearing species have been reported from the lower Cambrian of France ( Kerber 1988) and Mongolia ( Esakova and Zhegallo 1996), middle Cambrian of Greenland ( Peel 1994) and Australia ( Vendrasco et al. 2010). Runnegar (1996) suggested that because of the widespread nature of pegma-like structures in Cambrian molluscs, they likely evolved convergently in different lineages.