Éléments de comparaison entre les faunes liasiques d'ammonites du nord-ouest européen, de la Téthys occidentale et de l'ouest de l'Amérique du Nord

Abstract

The first ammonites known in the Western Americas belong to the ubiquist genus Psiloceras and they are very close from the species from NW Europe and from the Alps (Western Tethys). Then the communities sustain a strong differentiation; the genus and even the subfamilies differ (Guex 1980). In this general context there are some striking similarities between some western american species from the Nevada (Taylor collection, Portland University, Oregon, USA) and the Tethyan fauna as it is known from the Austro-Alpine Domain. They are both different from the NW European fauna. However, a few species or genus are represented in the French Subalpine Basin (Ardèche), in the Austro-Alpine Domain as well as in the Nevada: Waehneroceras and Sunrisites (= Franziceras sensu Elmi & Mouterde 1965). In consequence, seaway connections must have been open between the Tethys and the East Pacific, at least episodically. Their actual position is debated as the connection through the “Hispanic Corridor” is not evidenced. A relative homogeneization of the faunas occurs during the Sinemurian, probably as a consequence of the Tethyan transgressions, but the main affinities are between Nevada and Alpine communities ( Paracaloceras, Pseudotropites). The informations on the Lower Pliensbachian (Carixian) ammonites are poor, but Metaderoceras, Reynesoceras and Dubariceras occur in Nevada (Taylor coll.) and they evidence that the connection remained possible with the Tethys. During the Late Pliensbachian (Domerian), the similarities remained at least at generic level ( Protogrammoceras, Arieticeras) but heterochrony and vicariance can have occurred; these mechanisms are bably known owing to strong nomenclatural problems. Aalenian fauna from the Oregon are striking by the lack of Ludwigia and Graphoceras. A strong Tethyan character is indicated by Vacekia and Asthenoceras as well as very similar Hammatoceratids.