The diplacanthid fishes (Acanthodii, Diplacanthiformes, Diplacanthidae) from the Middle Devonian of Scotland

The Diplacanthiformes are a clade of acanthodian fishes which were widespread during the Middle and early Late Devonian. They are best represented in the Middle Devonian, by articulated fossils, fin spines, and abundant scales, the latter particularly from northern Europe. Three species of diplacanthid diplacanthi forms, Diplacanthus crassisimus, Diplacanthus tenuistriatus, and Rhadinacanthus longispinus, are found in Middle Devonian (Eifelian–Givetian) assemblages of articulated fish in northern Scotland. Our detailed study of the dermal structures and endoskeletal shoulder girdles in these fish supports the validity of Rhadinacanthus as a separate genus from Diplacanthus, with the two being differentiated by spine morphology, scale morphology, and histology, and shape and form of the pectoral girdle. In Orkney and Caithness, D. crassisimus first occurs in the Thursius macrolepidotus vertebrate biozone and disappears by the Millerosteus minor +Thursius pholidotus vertebrate biozone. Diplacanthus tenuistriatus and R. longispinus range from the Coccosteus cuspidatus biozone to the end of the Millerosteus minor + Thursius pholidotus biozone. Through comparing the results of our detailed work on the morphology and histology of fin spines and scales from the articulated fish with diplacanthid taxa based on isolated scales and fin spines from the Baltic region, Belarus, and Severnaya Zemlya, we recognize many of the latter taxa from contemporary deposits as junior synonyms of the Scottish species. Phylogenetic analysis of selected gnathostome genera shows the diplacanthiforms Diplacanthus, Rhadinacanthus, Uraniacanthus, and Culmacanthus form a well-supported clade within a larger clade comprising all acanthodian taxa plus a monophyletic Chondrichthyes.