Shell ornament, systematic position and hatching size of Epicymatoceras vaelsense (Nautilida): New insights based on specimens in mould preservation from the Upper Cretaceous of Poland

Abstract

The fossil record of Nautilida in carbonate facies is skewed towards moulds (steinkerns) in various states of preservation, which complicates assessment of the original shell ornament of many taxa. As a remarkable exception, moulds of Epicymatoceras vaelsense from upper Campanian and lower Maastrichtian (Upper Cretaceous) siliceous limestones (opoka) in Poland retain details of the original shell ornament, allowing conclusions on the original structure of the shell wall, systematic position and hatching size of this nautilid to be drawn. Both external and internal moulds are present in the material studied, which are referred to as taphomorph 1 and 2, respectively. Based on morphology and inferred taphonomy, it is proposed that the external ribbing of E. vaelsense was originally composed of overlapping, tile-shaped lamellae of the outer prismatic layer, a character recently suggested to be a synapomorphy for the cymatoceratid clade. The diameter of the embryonic conch of E. vaelsense is estimated to have been around 30 mm, which is near the maximum range of hatching size recorded for Cretaceous and younger nautilids. On the basis of the inferred shell structure of E. vaelsense, it is speculated that living individuals of this nautilid were covered by a dense periostracal cover similar to that of Recent Allonautilus scrobiculatus. This study demonstrates that nautilid moulds may provide a better source of palaeontological data than conventionally accepted.