A revision of the anatomy of the Early Devonian jawed vertebrate Ptomacanthus anglicus Miles

The spine-bearing jawed vertebrates (gnathostomes) assigned to the assemblage 'Acanthodii' play a key role in understanding the early evolution of osteichthyans and chondrichthyans. Amongst 'acanthodians', the genus Ptomacanthus has played a prominent role owing to its shark-like tooth files. Recently described braincase material from this taxon contrasts strongly with the osteichthyan-like braincase of Acanthodes, the only other 'acanthodian' for which this anatomy is well known. This seriously challenges acanthodian monophyly or at least the prevailing interpretation of Acanthodes. This study presents a redescription and updated comparison of the anatomy of Ptomacanthus based on unfigured material from the type and referred specimens, as well as adding new data on spine and scale histology. Ptomacanthus is remarkably heterosquamous compared to some other 'acanthodians'. Further to its resemblances to Climatius, Ptomacanthus shares some features of the external morphology of its scales with the enigmatic genus Obtusacanthus. The scale crowns of Ptomacanthus exhibit both superpositional and areal growth. The bases, however, grow separately from the crown. As in Acanthodes, scales are here inferred to be added to the body from posterior to anterior. In spite of its chondrichthyan-like form and characters, Ptomacanthus remains difficult to place on either the chondrichthyan, osteichthyan or gnathostome stem. Different placements each require invoking considerable character incongruence. Nevertheless, these new data from Ptomacanthus provide broader anatomical context for hard tissue characters commonly described on the basis of isolated remains.