Jaws of Triassic ammonoids from New Zealand

Abstract

Cephalopod jaws have been recovered from the Etalian (Middle Triassic = Anisian-Ladinian) of New Zealand. The jaws are found as isolated elements in the Tilson Siltstone near Kaka Point, southeast Otago. The structures are interpreted as jaws of ceratitid andphylloceratid ammonoids. Most of the jaws are small, <10 mm in length, but two specimens are much larger: c. 35 mm in length. The lower jaws conform to the published descriptions of anaptychus type jaws. Each jaw is convex on the ventral side and terminates in a projected apex. The surface is covered with regularly spaced ridges that parallel the posterior margin. The inner lamella is preserved near the anterior margin and in areas where the outer lamella has eroded away. The upper jaws are rarer, with their inner lamella ending in a bilobate posterior margin. The lower jaws are similar to those of Paleozoic ammonoids and contrast with those of the Ancyloceratina and most of the Ammonitina, which are characterised by a midline flange with a central groove and a pair of calcareous plates (aptychi), and the Lytoceratina, which bear calcareous deposits at the apex.