A preliminary study of Hettangian Bivalves from Triassic-Jurassic passage beds in northern Spain

MÁRQUEZ-ALIAGA, A.; DAMBORENEA, S.E.; GOY, A.

Hammamet, Túnez

Simposio; Fifth International Symposium of IGCP 506 on Marine and non-marine Jurassic; 2008

IGCP 506

The Triassic-Jurassic boundary beds from Asturias have been known for a long time, but have only recently been studied in detail from several points of view. The palynological contents of these beds is now very well known but the scarce invertebrate fauna is not enough studied, in part due to the poor preservation of the material involved. Some Hettangian bivalves from these beds were illustrated by Gómez et al., identified as Parallelodon hettangiensis (Terquem), Cuneigervillia rhombica (Cossmann), Eomiodon menkei (Dunker) and Pteromya tatei (Richardson y Tutcher). This material was found associated to ammonites referred to Caloceras pirondi Reynès and Psiloceras sp. All Hettangian shell beds examined have very low diversity, and in fact, most of them can be regarded as monotypic (Fabares 14-15, Pedrosa, Caravia and Bárcena in Asturias; Salinas de Pisuerga and Barrio San Pedro in Palencia). This in itself is a measure of the low complexity of the community structure, and a biological indication that they may belong to marginal marine environments, with high environmental stress levels. Even the more diverse association, the Pteromya-Cuneigervillia-Eomiodon association of Fabares and Colunga, was probably also salinity controlled, and beds with similar fauna were regarded by some authors as belonging to the brachyhaline regime (18-30%o). Other biological features of these beds include absence of stenohaline higher taxa, such as ammonoids, brachiopods, echinoderms and cnidaria; a conservative shell morphology typical of reduced salinity environments; and size selection. The fauna analyzed here clearly belongs to the same facies and environment as those described by Freneix and Cubaynes from Aquitaine. Apparently similar Rhaeto-Hettangian beds with Eotrapezium aff germari Dunker have also been mentioned from the Pont Tiout Formation, Algeria. This fauna is completely different from coeval bivalve associations from other European Hettangian localities, such as those from Bergamo and Lombardy.