6533b7d0fe1ef96bd125aa84

RESEARCH PRODUCT

The Sinemurian and Pliensbachian ammonites of the «Calcaires de Laffrey » (Isère, France). Description of a new taxon Pseudojamesonites nov. gen. mouterdei nov. sp. (Eoderoceratoidea)

subject

description

Some ammonites, formerly collected by René Mouterde within the Sinemurian and Pliensbachian beds of the “Calcaires de Laffrey” (Isère, France), are first described and illustrated in this work. The Sinemurian (Semicostatum chronozone) assemblage, chiefly constituted by Arnioceras gr. robustum (Quenstedt, 1884), is rather usual for the NW European faunas. Conversely, the Pliensbachian (Jamesoni chronozone, Brevispina subchronozone) assemblage is very peculiar. It is chiefly constituted by a very rare species Pseudojamesonites nov. gen. mouterdei nov. sp. (Eoderoceratoidea) which is here described for the first time. The presence, in significant proportion, of this new taxa in the Early Pliensbachian fossiliferous level of the “Calcaires of Laffrey”, designates this Alpine locality as a “singular fossiliferous locality” unparalleled in all the NW European and Mediterranean areas. The notion of “singular fossiliferous locality” is discussed. The role of palaeotectonics – which is significant during the Early Jurassic in the Laffrey area – is envisaged to explain the existence of the level with Pseudojamesonites nov. gen. mouterdei nov. sp.