The ornament on a small external cast in pink chert shows considerable similarity with that of various Middle Palaeozoic and Triassic fish genera. It comes from the Permian-Triassic Oruatemanu Formation of Arrow Rocks, Whangaroa area, eastern Northland. Conodont faunas from a few metres above and below the sample allow correlation with the Neospathodus pakistanensis zone of the Early Triassic, which is assigned to the late Dienerian (late Induan), with adjacent conodont zone faunas in their correct stratigraphic association. The cast is assumed to be that of a small fragment of fin spine, most likely from the junction area of the crown and root on the right-hand side of a dorsal fin spine, possibly anterior, of a marine ctenacanthoid shark, a basal shark order not previously recorded from New Zealand.