Organophosphatic stem group brachiopods: implications for the phylogeny of the subphylum Linguliformea

he recognition of potential organophosphatic-shelled stem group brachiopods has important implications for the understanding of brachiopod phylogeny. These groups fall outside the two currently recognized classes of the subphylum Linguliformea -the Lingulata and Paterinata. However, their organophosphatic shell structure and evidence of penetrative setae demonstrate that they are linked phylogenetically with the linguliforms. The proposed Early Cambrian stem-group brachiopods include the problematic, possibly vermiform, organophosphatic sclerite-bearing tannuolinids and the more brachiopod-like Mickwitzia and Heliomedusa. A columnar shell fabric, which was considered previously as a derived feature of acrotretids, is now known also from the tannuolinid Micrina as well as from Mickwitzia, thus indicating that this type of shell structure may be a plesiomorphic character; it was retained in acrotretids and some lingulids, like the Lingulellotretidae, which is here shown to include a wide variety of columnar fabrics. A columnar shell structure (sensu lato) is also identified here from the enigmatic South American Ordovician linguliform brachiopod Bistramia. The shells of Mickwitzia and Heliomedusa also have various types of thicker cylindrical columns ('tubes'), some of which were clearly open to the exterior surface and can be inferred to have contained setal structures penetrating the shell. Identical perforations are present in Micrina, and a similar function can be inferred for both the columnar fabric of some recently discovered paterinids described here and possibly also of some siphonotretids, indicating that these groups may also be closer to the stem of the brachiopods