Hips, tips and sweet sweptback rays: Looking beyond traditional cranial characters in Pachycormiformes

A focus on cranial characters for determining relatedness is a predominant trait in many datasets, however this overemphasis can lead to distortion from sampler bias. We report on revised analyses of pachycormids - a key group within Actinopterygii, as part of the Holostei-Teleostei transition, which display a phyletic trend towards reduced skeletal ossification with the increased adult size of a pachycormid taxon. This reduced preservation potential for the axial skeleton makes it difficult not to base phylogenetic assumptions primarily on the limited skull material present. However, pachycormids show a remarkable conservatism in their dermatocranial anatomy, the few differences being useful for showing the separation of genera, but of little utility in working out broader intrafamilial relationships. The combination of a paucity of postcranial characters in the Late Cretaceous pursuit predator Protosphyraena with a poor knowledge about the skulls of suspension-feeding pachycormids (SFPs) had led to the absence of Early Cretaceous predatory pachycormids being interpreted as indicating a ghost lineage between Protosphyraena and the European Upper Jurassic taxa Orthocormus and Hypsocormus over an almost 50-million-year gap. However, the inclusion of several features from the pectoral and pelvic fins, supplemented by splanchnocranial characters, produces a much clearer picture that questions the traditional perception of a single carnivore lineage: Protosphyraena emerges as secondarily carnivorous from the SFPs’ tribe, mirroring 130 years of misidentification of North American Bonnerichthys specimens as Protosphyraena. Confirmation of this will rely on the further recovery of data concerning the skull morphology of SFPs.