Rhyncholites and Rhynchoteuthis through time: unsuspected disparity and palaeoecological interpretations
Résumé
Rhyncholites, the arrow shaped calcite tips of the upper jaw of nautilus, are found abundantly in the fossil record from middle Triassic to nowadays, and are described as morphologically stable trough time. Another group of mineralized arrowed shaped fossils is found from Jurassic to Cretaceous: the rhynchoteuthis. Because they have never been found in situ, their affinity to nautiloids or ammonoids is still debated. With an overall morphology very similar to rhyncholites, they exhibit a greater shape diversity. Because of their feeding function, they should carry critical information related to their diet and environment. Yet, the morphological variability of these fossils has never been quantitatively investigated before. In this study, we thoroughly and quantitatively characterized the morphology of rhyncholites and rhynchoteuthis from Triassic to present-day. We digitized 262 specimens using an X-ray microtomograph (µCT) and performed a shape analysis using 3D geometrics morphometrics. The morphological continuum observed between rhyncholites and rhynchoteuthis along with the unfitting morphology of rhynchoteuthis with other contemporary cephalopods upper jaws, lead us to consider them as nautilids. Surprisingly, we observe broad variation in rhyncholites shape, ever since their apparition in Triassic. Specimens from that age are morphologically closer to modern ones, probably sharing their opportunistic carnivorous diet and able to feed on hard items. This ability might have given them the upper hand during the Marine Mesozoic Revolution. Through this period of intense arms race, the increasing competition might have favoured the emergence of new forms such as rhynchoteuthis, as well as exploration of new niches.