Occurrence of early post‐hatching Jurassic Nautilida in Normandy, France: palaeobiologic, palaeoecologic and palaeobiogeographic implications

Abstract

Nautilida are present and sometimes abundant in various sedimentary environments of the Jurassic series (Sinemurian to Kimmeridgian) in Normandy, but in the majority of cases the individuals are mature or at a late stage of ontogenetic development. Comparative analysis of the palaeobiogeographic distribution of Nautilida according to their ontogenetic stages shows that early post-hatching specimens are present only in particular palaeoenvironments which are rarely visible and laterally limited in outcrop. The sedimentologic and palaeoecologic originality of the Carixian, Aalenian and Bajocian deposits bearing these peculiar specimens is related to their palaeogeographic position along the western border of the Anglo-Paris Basin, on the eastern margin of the Armorican Massif, where Jurassic deposits overlie Palaeozoic rocky reefs or shoals. The egg-laying places of the Jurassic Nautilida show palaeoenvironmental features similar to those of the recent Nautilus, having been controlled by the water temperature and the nature and morphology of the sea bottoms. A hard, anfractuous and stable substratum, low sedimentation rates and warm and well-oxygenated marine waters appear to have been required for egg-laying and for the embryonic development of these cephalopods. The discovery of early post-hatching fossil Nautilida is fundamental to a better understanding of their palaeobiogeographic distribution at local to global scales. The stenotopic nature of their egg-laying places explains the segregation between newly hatched individuals and the rest of the population and leads also to a distinction between the notions of ‘dispersal potential’ and ‘settlement potential’ for populations of Nautilida. Three palaeobiogeographic maps show that Nautilida were absent in the Jurassic Boreal realm. This exclusion could be closely dependent on the thermal requirements that govern their reproduction. By comparison, the contemporary occurrence of ammonoids in this realm appears to reflect differences in reproductive strategies.