Endocranial preservation of a Carboniferous actinopterygian from Lancashire, UK, and the interrelationships of primitive actinopterygians

The gross brain structure of an Upper Carboniferous (ca. 310 Myr ago) ray-finned fish (Actinopterygii) is described from exceptionally well-preserved fossil material from the Burnley region of Lancashire, UK. Previously identified as 'Rhadinichthys' planti, the species is reassigned to the genus Mesopoma. Morphological characters derived from these data are combined with reviews of cranial skeletal anatomy, enamel composition, oculomoter muscle insertion and paired fin morphology to test and reanalyse hypotheses of primitive actinopterygian interrelationships. Results indicate that ancestral chondrostean (sturgeon and paddlefish) and neopterygian (teleost, amiid and gar) lineages diverged earlier than current theories suggest. Palaeonisciformes, a taxonomic group widely used to include most Palaeozoic actinopterygians, include a significant number of primitive neopterygians, several of which may form a distinct monophyletic clade. Within this revised phylogenetic context, changes in gross brain morphology from primitive conditions, as revealed by fossil data, highlight likely specializations in extant non-teleostean actinopterygians.