3D Reconstruction and Microarchitecture of a Reticular Belemnoid Cephalic Cartilage from the Late Triassic Polzberg Konservat-Lagerstätte

Abstract The internalmicroarchitecturecanbecrucial for the interpretation of fossil structures. High-resolution micro-computer tomography (micro-CT) and 3D reconstructions of 14 fossilcoleoid cephalic cartilages, embedded in dark, clayey limestones, provided insights into the internal structure of these unique Carnian (Upper Triassic) fossils. A well-developed branched, calcite-filled channel system was localized within the fossils. Measurements on these structures and how the channels are interlinked, along with actualistic comparisons, support the theory that these fossils were coleoidreticular cephalic cartilages, mineralized 234 million years ago. The findings provide new knowledge on the evolution of Triassic coleoids and in particular on the morphology and function of these fossilcephalopod cephalic cartilages. The present study combines classical morphometry from earlier work with modern 3D-visualization techniques of these unique fossils and discusses evolutionary relationships as well as the ecology of early coleoids. The structures are compared with today’s reticular cephalic cartilages of coleoids, which inhabit the pelagic realm and are mainly represented by the neutrally-buoyant order Oegopsida, which already provides clues to the lifestyle of basal coleoids.