An early chondrichthyan and the evolutionary assembly of a shark body plan

Although relationships among the major groups of living gnathostomes are well established, the relatedness of early jawed vertebrates to modern clades is intensely debated. Here, we provide a new description of Gladbachus, a Middle Devonian (Givetian ~385-million-year-old) stem chondrichthyan from Germany, and one of the very few early chondrichthyans in which substantial portions of the endoskeleton are preserved. Tomographic and histological techniques reveal new details of the gill skeleton, hyoid arch and jaws, neurocranium, cartilage, scales and teeth. Despite many features resembling placoderm or osteichthyan conditions, phylogenetic analysis confirms Gladbachus as a stem chondrichthyan and corroborates hypotheses that all acanthodians are stem chondrichthyans. The unfamiliar character combination displayed by Gladbachus, alongside conditions observed in acanthodians, implies that pre-Devonian stem-chondrichthyans are severely under-sampled and strongly supports indications from i...