Digging around the base of the Billingsellida: the evolution and phylogenetic relationships of early rhynchonelliform brachiopods

Brachiopods dominated the benthic Palaeozoic marine realm, both in terms of relative abundance and also species diversity. The Ordovician represented a pivotal period for brachiopods, with the phylum under taking a spectacular surge in biodiversity driven by an exponential increase in numbers at family, genus and species levels. However, brachiopods had a significant history prior to the Ordovician, with the majority of higher-level taxa already established during the Cambrian 'explosion'. Calcareous rhynchonelliform brachiopods tended to be a minor and relatively short-lived component of these Cambrian communities and there is a significant time lag between their appearance in the early Cambrian and the explosion of genera in the Ordovician. The relationship of many Cambrian rhynchonelliform brachiopods is poorly understood, with many genera displaying a combination of morphological features that are taxonomically confusing. Yet, the study of middle Cambrian -- early Tremadocian brachiopods is pivotal to fill the interval between the two major radiation phases in the early Palaeozoic. 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 the Billingsellida, a group considered by many to represent the ancestral stock of several Ordovician brachiopod lineages. The analyses portray the polytoechioids as derived billingselloids separate from the clitambonitoids that form a sister group. The enigmatic Roanella is interpreted as ancestral to the clitambonitoids within a new monogeneric family, Roanellidae nov. and the recently reappraised clitambonitoid Arctohedra is interpreted as a basal member of the entire order Billingsellida.