Scaphopodization in Palaeozoic molluscs

The relationship of a variety of problematic Early Palaeozoic fossils to the true scaphopods (Late Palaeozoic-Recent) is clarified by the recognition that a trend towards the development of a tubular shell is a recurrent theme within Palaeozoic benthic molluscs. This trend is here termed scaphopodization and can be recognized already in the Cambrian in helcionelloids such as Yochelcionella and Eotebenna. The problematic Early Palaeozoic tubular fossils Janospira, Jinonicella and Rhytiodentalium are interpreted as pseudo-scaphopod derivatives of the apparently exogastric ribeirioid rostroconchs. The direction of coiling of the protoconch indicates that they are not closely related to the true scaphopods, which were derived from endogastric concocardioidean rostroconchs in the Devonian or Carboniferous. Scaphopods show a comparable underlying morphological blueprint to these conocardioideans, but they represent a distinct stream-lining to the infaunal habitat.