Abstract
Mid- to Late Palaeozoic sharks and holocephalans display a wide range of armour, with bodies that range from sleek, pelagic forms to slow-swimming, chimaeroids or ray-like bottom dwellers. Despite this Late Palaeozoic diversity, there still is an expectation that early chondrichthyans will be anatomically like later species. Recent discoveries from eastern Canada (Doliodus problematicus), and several heavily spined fishes from the MOTH locality in the Northwest Territories, including Kathemacanthus and Seretolepis, described here, challenge this expectation. These fishes show scale and endoskeletal features thought to be characteristic of chondrichthyans, yet they have paired fin spines, anal fin spines, and in some cases rows of prepectoral and prepelvic spines as would be expected from primitive acanthodians. Kathemacanthus and Seretolepis do not fit neatly within the cur- rent taxonomy, demonstrating that previous distinctions between acanthodians and chondrichthyans, including scale-based criteria, fail to account for the diversity being discovered in the fossil record.
