The many faces of ‘ Mesonichthys ’: new anatomical and taxonomic diversity among Carboniferous ray-finned fishes (Actinopterygii) from the U.K.

The Carboniferous represents a period where ray-finned fishes (actinopterygians), which today account for half of all living vertebrates, appear to increase exponentially in abundance as well as in taxonomic and anatomical diversity. However, complicated historical taxonomic and descriptive practices present an obstacle to understanding the magnitude and drivers of this radiation. The increased level of anatomical information offered by the recent application of computed tomography (CT) scanning represents an opportunity to resolve outstanding questions of taxonomic identity and anatomical diversity, ultimately feeding into better understanding of relationships and evolutionary patterns. Here we focus on ‘Mesonichthys,’ from the Pennsylvanian of the U.K., to which almost 50 specimens from disparate parts of England and Scotland have been referred over the last century and a half. Computed tomography-based revision of key specimens contained within this taxon reveals the presence of at least three distinct morphotypes differentiated by features of the cheek, jaws and cranial endoskeleton. We revise and stabilize the taxonomy of each, erecting three new taxa—Confoundichthys aitkeni, Mesonichthys culmensis, and Unionichthys friedmani—to contain them, and leave several specimens in open nomenclature pending further study. Unionichthys bears features unusual in a Carboniferous fish, including pronounced processes on the hyomandibula, separately ossified anterior and posterior ceratohyals, and two interhyoid elements arranged in dorso-ventral series. This unexpected morphology suggests affinity with Triassic and younger taxa, and the hitherto unappreciated taxonomic diversity previously contained within a single species underlies the importance of targeted revision of exceptionally preserved but historically overlooked Carboniferous actinopterygian specimens.