Ancestral billingsellides and the evolution and phylogenetic relationships of early rhynchonelliform brachiopods

The relationship of many Cambrian rhynchonelliform brachiopods is poorly understood, with many genera displaying a combination of morphological features that are taxonomically confusing. The study of middle Cambrian-early Tremadocian brachiopods is critical because this interval is sandwiched directly between the two largest radiation phases in the early Palaeozoic and provides raw data for deciphering the events leading up to the explosion of brachiopod genera in the Ordovician. Here we present a parsimony analysis of a wide selection of Cambrian and Ordovician brachiopod genera with a particular focus on the evolution and phylogeny of Billingsellida. The billingselloids were widespread by the late Cambrian and the group was originally thought to represent the ancestral stock of many Ordovician brachiopod lineages. The phylogenetic analyses portray the polytoechioids as derived billingselloids separate from the clitambonitoids that form a sister group. The Gondwanan brachiopod Roanella is interpreted as ancestral to the clitambonitoids within the Billingsellida and is reassigned to Clitambonitoidea within a new monogeneric family, Roanellidae nov. Antigonambonites displays no obvious relationship with the clitambonitoids and should be formally transferred to the polytoechioids. The monogeneric family Chaniellidae exhibits characters reminiscent of members of the polytoechioids and is transferred to the superfamily Polytoechioidea. The recently reappraised clitambonitoid Arctohedra is interpreted as a basal member of the entire order Billingsellida.