On Ammonites silvestrei REYNES - a significant element of the Arietitinae in the Lower Jurassic (Lower Sinemurian, Bucklandi Zone, Rotiforme Subzone) of the NW European Province

摘要

A highly variable ammonite species of the lowermost Sinemurian, previously referred to under at least ten different names, is revised. The previous generic allocation to Coroniceras is discussed and rejected. Although there are features similar to Coroniceras, the most frequent genus of the Rotiforme Subzone, the prevailing morphological characters are of Epammonites SPATH, 1922: slow growth, lack of nodes, and different suture line. This genus is - revised on basis of its type species- characteristic of the latisulcatus group in the Conybeari Subzone. The morphological ambiguity of the species is consistent with its stratigraphic position - at the transition from the Conybeari Subzone to the Rotiforme Subzone as recognized in sections of the U.K. (best at East Quantoxhead, West Somerset), of France and of Germany. The previous allocation to the Rotiforme Subzone is here maintained. Hitherto the species was regarded as part of the Rotiforme Subzone and this use is maintained. The species seems appropriate for defining the base of the Rotiforme Subzone on the basis of the following: its co-occurrence with first specimens of Coroniceras deffneri (OPPEL) in the U.K., the transitional characters between Epammonites and Coroniceras, the special morphological features which render it easily recognizable, and the almost monotypic occurrence at its stratigraphic level. Stratigraphic relations to coeval ammonite faunas of the Alps are discussed.