The pharynx of the iconic stem-group chondrichthyan Acanthodes Agassiz, 1833 revisited with micro-computed tomography

Acanthodes has long been the primary source of information on the pharyngeal skeleton of ‘acanthodians’. Because of this its anatomy has played a disproportionate role in attempts to understand the evolution of the jawed vertebrate pharynx and the clade as a whole. However, the anatomy of the pharynx of Acanthodes, now understood to be a stem-group chondrichthyan, remains poorly characterized and subject to several competing interpretations. We used computed tomography to image the articulated pharyngeal skeletons of three specimens of Acanthodes confusus from Lebach, Germany. Acanthodes has a mélange of osteichthyan-like and chondrichthyan-like morphologies in its pharyngeal skeleton. Like many other chondrichthyans, Acanthodes lacked hypohyals, and had four pairs of posteriorly oriented pharyngobranchials. Like osteichthyans, Acanthodes possessed an interhyal, but lacked the separate infra- and supra-pharyngobranchial elements present in osteichthyans and the crown-chondrichthyan Ozarcus. Using these new data we built and animated a digital 3D model of the pharyngeal endoskeleton in Acanthodes, showing that the jaws could have swung outwards during the opening cycle, increasing the anteriorly facing area of the gape for suspension feeding. These new data provide a more definitive picture of the anatomy of a taxon that has long been of great significance in early vertebrate palaeontology.