Abstract
The acanthodian Paucicanthus vanelsti gen. et sp. nov. is described from six body fossils from Lower Devonian (Lochkovian) rocks of the southern Mackenzie Mountains, Northwest Territories, Canada. This new species is unique among acanthodians in that it lacks both pectoral and pelvic fin-spines. In the absence of fin-spines, the leading edges of the pectoral and pelvic fins are reinforced by enlarged scales. The anatomy of the acanthodiform Traquairichthys pygmaeus is similar to P. vanelsti in that both lack pelvic fin-spines, although T. pygmaeus also lacks pelvic fins. Similarly, the acanthodian Yealepis douglasi lacks both paired and median fin-spines, and its anatomy resembles that of P. vanelsti based only on the loss of paired fin-spines. The lack of paired and (or) median fin-spines in these three taxa contrasts with the widely held view that acanthodian fins all were preceded by spines. The anatomy of P. vanelsti also is similar to that of the acanthodian Brochoadmones milesi in that both have a completely unossified endoskeleton, slightly elevated pectoral fins, and deep, compressed bodies. The median fin-spines of P. vanelsti have an anterior leading edge rib followed by a field of fine striations. This striated ornamentation coupled with few leading edge ribs also is seen on fin-spines of Cassidiceps vermiculatus and primitive acanthodiform acanthodians (e.g., Mesacanthus and Lodeacanthus species). I tentatively suggest that this fin-spine ornament indicates relationship between P. vanelsti, acanthodiform acanthodians, and C. vermiculatus. However, a cladistic analysis is required to test whether or not the characteristics such as fin-spine loss, unossified endoskeleton, elevated pectoral fins, deep compressed bodies, and (or) median fin-spine ornamentation are synapomorphies within the Acanthodii or evolved convergently within the class.
