Bivalves from the Triassic-Jurassic transition in northern Spain (Asturias and western Basque-Cantabrian Basin)

Abstract

Bivalve molluscs from the Triassic-Jurassic transition collected in eight localities in Asturias and the western Basque-Cantabrian Basin (Palencia province) are systematically revised. Preservation is poor at all localities. The dominant Rhaetian bivalves are Isocyprina concentrica (Moore) and Bakevellia (Bakevelloides) praecursor (Quenstedt). These species, together with Isocyprina cf. ewaldi (Bornemann), Pteromya cf. crowcombeia (Moore), Pseudoplacunopsis alpina (Winkler), and Modiolus? sp. (cf. minimus J. Sowerby), with a specimen of Arcestidae (?), belong to an assemblage similar to that found in the Westbury and Lilstock formations (Penarth Group) in the late Rhaetian of southern England. The most abundant Hettangian species is Isocyprina (Eotrapezium) germari (Dunker). Others are referred to Cuneigervillia rhombica (Cossmann), Sphaeriola? sp., Eomiodon? sp. and Pteromya cf. tatei (Richardson and Tutcher). All Hettangian shell beds examined are monotypic or have very low diversity, a biological indication that they may belong to a restricted marine environment, with high environmental stress levels. Even the more diverse assemblage (Pteromya-Cuneigervillia-Eomiodon) was probably also salinity controlled. The fauna analyzed here clearly belongs to the same facies and environment as those described from Aquitaine (France) and the Pyrenees and is different from coeval bivalve assemblages from other European Hettangian localities. The Triassic/Jurassic boundary cannot be precisely located at the studied sections on the basis of the bivalve faunas alone, but these indicate that the transition beds in Asturias were deposited in a marginal marine environment and the benthic fauna was dominated by shallow burrowing, suspensivorous bivalves.Se revisan sistemáticamente los taxones de moluscos bivalvos procedentes de sedimentos del tránsito Triásico-Jurásico en ocho localidades en Asturias y la parte occidental de la Cuenca Vasco-Cantábrica (provincia de Palencia). La preservación del material es muy pobre en todas las localidades. Las especies dominantes en el Retiense son Isocyprina concentrica (Moore) y Bakevellia (Bakevelloides) praecursor (Quenstedt). Estas especies, junto con Isocyprina cf. ewaldi (Bornemann), Pteromya cf. crowcombeia (Moore), Pseudoplacunopsis cf. alpina (Winkler), Modiolus? sp. (cf. minimus J. Sowerby), y un ejemplar de Arcestidae (?), representan asociaciones similares a las de las formaciones Westbury y Lilstock (Grupo Penarth) en el Retiense del sur de Inglaterra. La especie hettangiense más abundante es Isocyprina (Eotrapezium) germari (Dunker), y otras son referidas a Cuneigervillia rhombica (Cossmann), Sphaeriola? sp., Eomiodon? sp. y Pteromya cf. tatei (Richardson and Tutcher). Todos los niveles fosilíferos hettangienses son monotípicos o poseen muy baja diversidad, indicando que pueden pertenecer a ambientes marinos marginales con alto estrés ambiental. Es probable que aún la asociación más diversa (Pteromya-CuneigervilliaEomiodon), se encontrara controlada por la baja salinidad. La fauna analizada pertenece a las mismas facies y ambientes que las descritas de Aquitania (Francia) y de los Pirineos, y es diferente a la de otras localidades del Hettangiense europeo. El límite Triásico-Jurásico no puede ser situado con precisión teniendo en cuenta las asociaciones de bivalvos, pero éstos parecen indicar que las capas de tránsito en Asturias se depositaron en un ambiente marino marginal con una asociación dominada por bivalvos suspensívoros excavadores superficiales.