Abstract

Tarphycerids 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: Trocholites Conrad, 1838 (from which Ophioceras Barrande, 1865 probably diverged) and Discoceras Barrande, 1867 (= Graftonoceras Foerste, 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 stridsbergi n. sp., D. lindstroemi n. sp., and D. sp. indet. from the Wenlock of Gotland and D. amissus (Barrande, 1865) from the Llandovery of Bohemia are all endemic species probably derived from D. graftonense. The distribution of D. graftonense and the origin of four species of Discoceras in 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). Silurian Discoceras retained 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 lindstroemi n. sp. is exceptional owing to its heteromorphic planispiral shell with coiling that changed during ontogeny, resulting in a changing aperture orientation and decreased maneuverability.

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