Rhaetian and Hettangian Bivalves from the Triassic-Jurassic boundary beds in Asturias and Palencia

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

Colunga, España

Jornada; XXIV Jornadas de la Sociedad Española de Paleontología; 2008

Sociedad Española de Paleontología

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 (see review in Gómez et al., 2007). The palynological contents of these beds is now very well known (Barrón et al., 2006) but the scarce invertebrate fauna is not, in part due to the poor preservation of the material involved. All the available bivalve material from the boundary beds in Asturias and Palencia was systematically revised and an environmental interpretation is attempted. The material described was collected at the following localities in Asturias (from west to east): Corvera, Fabares, Bárzana, Colunga, Caravia y Pedrosa; and in Palencia: Barrio San Pedro and Salinas de Pisuerga. Preservation is poor at all localities, specimens appear as moulds or abraded recrystalized shells, with few details preserved. The dominant Rhaetian bivalves are Isocyprina concentrica (Moore) and Bakevellia praecursor (Quenstedt). These species, together with Isocyprina cf. ewaldi (Bonnermann) , “Pteromorphus”  cf. elongatus (Moore), Pteromya  cf. longportensis (Richarson & Tucher), “Placunopsis” cf. alpina (Winkler), Modiolus cf.  minimus (Sowerby) and Paleocardita cf. austriaca (von Hauer), with a specimen of Arcestidae (?), represent a similar assemblage to that found in the Westbury and Lilstok formations (Penarth Group) in the late Rhaetian of southern England (Ivimey-Cook et al., 1999).  The most abundant Hettangian species can be referred to the genus Eotrapezium. Some Hettangian bivalves from these beds were illustrated by Gómez et al. (2005), identified as 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. 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 (1984) from Aquitaine and by Fauré (2002) from the Pyrenees. 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 (Allasinaz, 1992).