Devonian ancestors ofNautilus

A Famennian nautilid is described that partially fills the morphologic and stratigraphie gap between Givetian and Tournaisian records of the order. A new generic nameDasbergoceras gen. nov. is proposed for it. Its loosely coiled, slender juvenile shell indicates close relationships to earlier Lechritrochoceratidae, rather than the Oncoceratida. The shell ornamentation, with oblique ribs and longitudinal striation, appears to be the primitive feature of the order. Tight coiling of the adult shell progressively developed during the Early Carboniferous while tightly coiled embryonic shells seem to be a post-Permian feature. The septal morphology, which is the most important feature distinguishing RecentNautilus from all the Tertiary nautilids, may be inherited from Cretaceous ancestors of theCimomia Aturia lineage. The nautilids probably originated from the kionoceratid Orthoceratida in late Early Ordovician.