An unusual conchorhynch from the upper Maastrichtian of the southeast Netherlands and the distinction between nautiloid and ammonoid conchorhynchs (Mollusca, Cephalopoda)

Abstract

A single, atypical conchorhynch (calcitic tip of a cephalopod lower jaw), recovered from the uppermost Meerssen Member (Maastricht Formation, upper Maastrichtian) at the former ENCI-HeidelbergCement Group quarry, south of Maastricht, is described as a new parataxon, Conchorhynchus illustris sp. nov. The specimen can be differentiated from all previous conchorhynch records on account of its large size, elongated shape and, in particular, of the structure of its apical part which is smooth and forwardly elongated. During the Late Cretaceous, conchorhynchs formed part of the jaw apparatus of nautilids and of two ammonoid suborders, Phylloceratina and Lytoceratina. Since conchorhynchs are most often found separated from jaws, establishing to which group of cephalopods their bearer belonged can be complicated. Here, for the first time, we propose a set of morphological criteria to differentiate clearly between nautiloid and ammonoid conchorhynchs. Although Conchorhynchus illustris sp. nov. is distinct from all currently known nautilid conchorhynchs, the sum of its morphological features is indicative of assignment to that cephalopod group. The upper portion of the Maastricht Formation in the Maastricht area (Nekum and Meerssen members) has yielded internal and external moulds of shells of the nautilid Eutrephoceras and the hercoglossid Cimomia. The new conchorhynch type described herein most likely belonged to one of these shell-based taxa. Judging from its unusual shape, the feeding strategy of its bearer must have differed from that of modern nautilids, in that it held and pierced prey rather than crushed sturdy shells.