'This strange little palaeoniscid': a new early actinopterygian genus, and commentary on pectoral fin conditions and function

摘要

The early actinopterygian Mesopoma planti is reassigned to a new genus on the basis of data obtained from high-resolution computed tomography (CT) scans of an unusually well-preserved specimen from the Early Pennsylvanian of Lancashire, UK. The former M. planti is joined by two further Mesopoma species from the Late Mississippian of Scotland. CT scans of the key planti specimen bring to light new details of the dermal skull, pectoral girdle and fin. Among the cranial features, CT data reveal a specialised, anteriorly projecting preopercular bone, the location of the spiracular duct opening, presence of a so-called coronoid process on the lower jaw and the full three-dimensional shape of the snout. Of the pectoral girdle and fin, for the first time in a Palaeozoic actinopterygian it has been possible to complete a three-dimensional reconstruction of the entire endoskeleton in articulation. The fin presents new diversity within a conservative general pattern, revealing for the first time a double propterygium. Girdle shape shows that the fin orientation is derived: rotated with the leading edge dorsalmost. These details are used to identify unexploited character states for use in phylogenetic analyses, while functional implications of the fin and girdle suggest advanced locomotory control emerging among different groups of post-Devonian ray-finned fishes.