The ammonite fauna renewal at the boundary middle–late Toarcian: new data and consequences

Abstract

Abstract Several proposals have been made concerning the Toarcian informal substages. A three part division, here adopted, includes a Middle Toarcian encompassing the Bifrons and Variabilis ammonite zones, according to the definition well established by Monestier [1921]. Other authors, following Buckman [1888], admit a binary definition but with different meanings. The boundary is placed either below the Variabilis (Lower / Upper Toarcian limit), or below the Thouarsense zones (“Whitbian”–“Yeovilian” limit) [Howarth, 1992]. These differences are largely due to a strong sedimentary instability: condensations, gaps and faunal mixings were widespread during the Variabilis zone in NW Europe and eurocaucasian domain and its Tethyan equivalent, the Gradata zone. Contemporarily, a major renewal occurred in the ammonite faunas. This provoked a near general segregation that is the reason of the use of different zone standards. In NW Europe, the faunal renewal occurred during the Variabilis zone as it as been established in the stratotype localities of Poitou and Vendée (Poitou – Vendée) [Gabilly, 1973, 1976a]. This is also well known in the Causses [Guex, 1972, 1975]. These two areas give good reference sections for the ammonite succession at the top of the Middle Toarcian. These data have been supplemented by recent works concerning NW Europe as well as the Tethyan margins (Portugal, Betics, Morocco, Algeria, Apennines). New observations have also been realized in France (Poitou-Vendée, Causses, Lyon area). They allow to have a better knowledge of the development of the faunal renewal and of the appearance of new taxa (genus, sub and superfamilies). Special attention is given to the Phymatoceratids, Grammoceratids and Hammatoceratoids. In consequence, the main renewal did not happened at the end of the Middle Toarcian (end of the Variabilis zone) but sooner at the beginning of the Illustris subzone. New ammonite morphologies appeared that will dominate the assemblages at the beginning of the late Toarcian (Grammoceratinids) and that will be at the origin of the main Middle Jurassic assemblages (Hammatoceratids). Among the species from the Illustris subzone, some are good markers for the correlations between the faunal domains and provinces (“Pseudogrammoceras” aratum Buck., “Pseudogrammoceras” subregale Pinna, “Geczyceras” costatum (Gab.), Osperleioceras (Pseudopolyplectus) bicarinatum (Ziet.)). It appears that the difficulties to place the Middle–Upper Toarcian limit result closely of the sedimentary perturbations that happened in Europe from the west to the eurocaucasian domains and in the whole western Tethys. The substage problem is secondary as stated by Gabilly but it is convenient for long range comparisons. The main units to establish the correlations are the biostratigraphic zones (biozones). In such a matter, the Variabilis zone is of particular importance, according to the coeval sedimentary and palaeontologic events.