Silurian tarphyceridDiscoceras(Cephalopoda, Nautiloidea): systematics, embryonic development and paleoecology

Abstract

AbstractTarphycerids were diverse and abundant in Ordovician marine faunas. Beginning at the Late Ordovician extinction, the diversity of tarphycerids declined throughout the Silurian, until their extinction in the latest Silurian. Two genera survived the Late Ordovician extinction:TrocholitesConrad, 1838 (from whichOphiocerasBarrande, 1865 probably diverged) andDiscocerasBarrande, 1867 (=GraftonocerasFoerste, 1925).Discoceras graftonense(Meek and Worthen, 1870), so far known from the US, China, and Australia, is recorded from the Silurian of Bohemia and Gotland.Discoceras stridsbergin. sp.,D.lindstroemin. sp., andD. sp. indet. from the Wenlock of Gotland andD.amissus(Barrande, 1865) from the Llandovery of Bohemia are all endemic species probably derived fromD.graftonense. The distribution ofD.graftonenseand the origin of four species ofDiscocerasin the latest Sheinwoodian and early Homerian represent the last diversification and dispersion of the Tarphycerida. No tarphycerid species originated after the mid-Homerian extinction (Mulde and Lundgreni events). SilurianDiscocerasretained the morphology and habitats of their Ordovician ancestors. The hatching time and autecology of juveniles has remained unclear. Evidence from the material studied suggests that juveniles were planktonic in habit, possessing a minute curved shell with few phragmocone chambers.Discoceras lindstroemin. sp. is exceptional owing to its heteromorphic planispiral shell with coiling that changed during ontogeny, resulting in a changing aperture orientation and decreased maneuverability.