The Middle Devonian acanthodian Orcadacanthus n. gen. from the Orcadian Basin of Scotland

Although first described more than 175 years ago and represented by many hundreds of specimens, the small (less than 60 mm long) mesacanthid acanthodiform originally named Acanthodes pusillus is the most poorly known acanthodian-grade vertebrate from the Middle Devonian of Scotland. The classification of the species received some attention in the late nineteenth century, but it is only in the last decade that more substantive interest has been shown in this little fish. Here we re-evaluate the type material for the species, and assess the validity of its putative junior synonym Mesacanthus peachi based on morphological and histological study of mesacanthid specimens from all levels of the Eifelian and Givetian from the Orcadian Basin. New information on the taxon revealed by these studies includes the shape of the jaw cartilages, ornamentation of the sclerotic plates, structure and morphology of the different fin spines, variation in scale shape and size over the body, and histological structure of the dermal elements. Our study supports synonymizing M. pusillus and M. peachi and erecting a new genus Orcadacanthus, type species O. pusillus. The depositional environment appears to have influenced the mode of preservation of the new taxon at different localities. Variability in the degree of mineralization, particularly in the head region, is linked to ontogenetic changes.