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-f inned fish (Actinopterygii) is described from exceptionally well-preserved fossil material from the Burnley region of Lancashire, UK. Previously ident ified as 'Rhadinichthys' planti, the species is reassigned to the genus Mes opoma. Morphological characters derived from these data are combined with r eviews of cranial skeletal anatomy enamel composition, oculomoter muscle in sertion and paired fin morphology to test and reanalyse hypotheses of primi tive actinopterygian interrelationships. Results indicate that ancestral ch ondrostean (sturgeon and paddlefish) and neopterygian (teleost, amiid and g ar) lineages diverged earlier than current theories suggest. Palaeoniscifor mes, a taxonomic group widely used to include most Palaeozoic actinopterygi ans, include a significant number of primitive neopterygians, several of wh ich may form a distinct monophyletic clade. Within this revised phylogeneti c context, changes in gross brain morphology from primitive conditions, as revealed by fossil data, highlight likely specializations in extant non-tel eostean actinopterygians.