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Ameghiniana

versión On-line ISSN 1851-8044

Ameghiniana vol.47 no.2 Buenos Aires jun. 2010

 

ARTÍCULOS ORIGINALES

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

 

Ana Márquez-Aliaga1, Susana Damborenea2 , Juan José Gómez3 and Antonio Goy4

1Departamento de Geología e Instituto Cavanilles de Biodiversidad y Biología Evolutiva (ICBiBE-UVEG). Universidad de Valencia. 46100 Burjassot, Valencia, España. Ana.Marquez@uv.es
2Consejo Nacional de Investigaciones Científicas y Técnicas, División Paleontología Invertebrados, Museo de Ciencias Naturales La Plata, 1900 La Plata, Argentina. sdambore@fcnym.unlp.edu.ar
3Departamento de Estratigrafía, Facultad de Ciencias Geológicas. Universidad Complutense de Madrid e Instituto de Geología Económica (CSIC-UCM). 28040 Madrid, España. jgomez@geo.ucm.es
4Departamento de Paleontología, Facultad de Ciencias Geológicas. Universidad Complutense de Madrid e Instituto de Geología Económica (CSIC-UCM). 28040 Madrid, España. angoy@geo.ucm.es

 


Abstract. Bivalve mollusks 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.

Resumen. Bivalvos del tránsito Triásico-Jurásico en el norte de España (Asturias y Cuenca Vasco-Cantábrica occidental). 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-Cuneigervillia- Eomiodon), 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.

Key words. Triassic-Jurassic boundary; Rhaetian; Hettangian; Bivalvia; Asturias; Palencia; Spain.

Palabras clave. Límite Triásico-Jurásico; Retiense; Hettangiense; Bivalvia; Asturias; Palencia; España.


 

Introduction

The palaeontology and stratigraphy of the Triassic-Jurassic (T-J) transition in westernmost Europe are inadequately known due to the scarcity of fossils, their poor preservation, and the fact that in many places the deposits show a wide range of facies variability. For this reason any contribution to this subject is welcome, and this paper is an attempt to improve its knowledge.
The Rhaetian-Hettangian transition beds from Asturias in northern Spain (figure 1) have been known for a long time (Dubar et al., 1963; Suárez-Vega et al., 1971; Suárez-Vega, 1974; Mouterde et al., 1977) 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., 2002, 2006a, 2006b) but the scarce invertebrate fauna was not, in part due to the poor preservation of the material involved (Márquez-Aliaga et al., 2008a, 2008b). The precise location of the T-J boundary is difficult to established in this region due to the scarcity or lack of ammonites at most sections. The only reports of ammonites from this area include an Arcestidae? from the late Triassic, Psiloceras sp. from the Hettangian of the Colunga section (Gómez et al., 2005) and ex-situ occurrences of Caloceras pirondi (Reynès) near Avilés (Dubar et al., 1963; Suárez-Vega, 1974).


Figure 1. Location map of the fossil localities mentioned in text. A, location of the studied areas in northern Spain. B, map showing the outcrops of the Jurassic deposits in Asturias and in the western Basque-Cantabrian Basin. Abbreviations of the fossil localities: Cv-Corvera, Fa-Fabares, Ba-Barzana, Co-Colunga, Ca-Caravia, Bs-Barrio de San Pedro, Sa-Salinas de Pisuerga, Re-Reinosilla / mapas mostrando la situación de las localidades fosilíferas mencionadas en el texto. A, situación de las áreas estudiadas en el norte de España. B, mapa mostrando los afloramientos de los depósitos del Jurásico en Asturias y en la parte oeste de la Cuenca Vasco-Cantábrica. Abreviaturas de las localidades donde se han recogido los fósiles: Cv-Corvera, Fa-Fabares, Ba-Barzana, Co-Colunga, Ca-Caravia, Bs-Barrio de San Pedro, Sa-Salinas de Pisuerga, Re-Reinosilla.

The Rhaetian bivalve assemblage is dominated by Isocyprina concentrica (Moore) and Bakevellia (Bakevelloides) praecursor (Quenstedt). Other bivalves are Isocyprina cf. ewaldi (Bornemann), Pteromya cf. crowcombeia (Moore), Pseudoplacunopsis alpina (Winkler), Modiolus? sp. (cf. minimus J. Sowerby). These bivalves belong to an assemblage similar to that found in the Westbury and Lilstock formations (Penarth Group) in the late Rhaetian of southern England. Some Hettangian bivalves from Asturias were illustrated by Gómez et al. (2005), identified as Parallelodon hettangiensis (Terquem), Cuneigervillia rhombica (Cossmann), Eomiodon menkei (Dunker) and Pteromya tatei (Richardson and Tutcher). These bivalves were found associated with ammonites referred to Caloceras pirondi (Reynès) and Psiloceras sp.
All the available bivalves from the boundary beds in Asturias and the western Basque-Cantabrian Basin (Palencia province, figure 1) are here systematically revised and illustrated, and an environmental interpretation is proposed. This paper also provides new evidence for the palaeogeographical reconstructions of the Rhaetian-Hettangian times in western Europe. The palaeontological content of the T-J transition beds is summarized in table 1. Bivalves are by far the most abundant and best preserved organisms, while ammonites and gastropod specimens are few and very badly preserved.
The fossils described here were collected at the following localities in Asturias (from West to East): Corvera, Fabares, Barzana, Colunga and Caravia; and in Palencia: Salinas de Pisuerga, Barrio de San Pedro and Reinosilla (figure 1).

Table 1. Locality distribution of the fauna from the Triassic-Jurassic passage beds studied in this paper / distribución por localidades de los bivalvos del tránsito Triásico-Jurásico descritos en este trabajo

Geological setting

The T-J transition beds in Asturias are located within the well bedded limestones, dolomites and mudstones known as the Solís Member of the Gijón Formation (figure 2), which contain all the bivalves studied here. The Solís Member overlies the mudstones and evaporites known as "Transition Beds of the Caravia Beach" (Suárez-Vega, 1974) or the "Mudstones and evaporites Unit" (Barrón et al., 2006b), and are covered either by the carbonates, evaporites and marls of the Barzana Member or the equivalent collapse breccias of the Fabares Member of the Gijón Formation. The T-J boundary was originally placed within the "Transition Beds of the Caravia Beach" (Suárez-Vega, 1974), but these beds are now regarded as fully Rhaetian in age (Martínez- García et al., 1998; Barrón et al., 2002). A Rhaetian age for the lower part of the Solís Member is supported by palynology studies (see Gómez et al., 2007) and is also suggested by the presence of Arcestidae? ammonoids (Gómez et al., 2005). The discovery (Llopis, 1961; Dubar et al., 1963) near Corvera (southeast of Avilés, figure 2) of an ex situ limestone boulder containing a well preserved specimen of Caloceras pirondi (Reynès) together with bivalves (figure 4), and the report of Psiloceras sp. from Colunga, indicate that at least the upper part of the Solís Member should be Hettangian in age, also supported by the palynological studies of these beds (Barrón et al., 2006b; Gómez et al., 2007).


Figure 2. Sections of the Triassic-Jurassic transition in Asturias. A, geological sketch of northern Asturias showing the location of the fossil localities: Cv-Corvera, Fa-Fabares, Ba-Barzana, Co-Colunga, Ca-Caravia. B, Colunga and Fabares stratigraphical sections showing the distribution of the identified mollusks / secciones de la transición Triásico-Jurásico en Asturias. A, esquema geológico del norte de Asturias mostrando la situación de las localidades donde se han recogido los fósiles: Cv-Corvera, Fa-Fabares, Ba-Barzana, Co-Colunga, Ca-Caravia. B, secciones estratigráficas de Colunga y Fabares, mostrando la distribución de los moluscos identificados.


Figure 3. Sections of the Triassic-Jurassic transition in the western Basque-Cantabrian Basin. A, geological sketch of the western Basque- Cantabrian Basin showing the location of the fossil localities: Bs-Barrio de San Pedro, Sa-Salinas de Pisuerga, Re-Reinosilla. B, Barrio de San Pedro, Salinas de Pisuerga and Reinosilla stratigraphical sections showing the distribution of the identified mollusks / secciones de la transición Triásico-Jurásico en la parte oeste de la Cuenca Vasco-Cantábrica. A, esquema geológico de la parte oeste de la Cuenca Vasco-Cantábrica mostrando la situación de las localidades donde se han recogido los fósiles: Bs-Barrio de San Pedro, Sa-Salinas de Pisuerga, Re-Reinosilla. B, secciones estratigráficas de Barrio de San Pedro, Salinas de Pisuerga y Reinosilla mostrando la distribución de los moluscos identificados.


Figure 4. 1, Caloceras pirondi (Reynès) and 2, associated bivalves, most of them referable to Isocyprina sp., some to Pteromya sp. and one specimen probably to Cuneigervillia sp. (white arrow); Corvera, southeast of Avilés, RM-00941, material found ex situ mentioned by Dubar et al., 1963; Gómez et al., 2005, 2007, and figured by Suárez-Vega, 1974, pl. 1, fig. 1; natural size / 1, Caloceras pirondi (Reynès) y 2, bivalvos asociados, la mayoría referibles a Isocyprina sp. y Pteromya sp., uno de ellos probablemente a Cuneigervillia sp. (flecha blanca); Corvera, sudeste de Avilés, RM-00941, material encontrado ex situ mencionado por Dubar et al., 1963; Gómez et al., 2005, 2007, e ilustrado por Suárez-Vega, 1974, lám. 1, fig. 1; tamaño natural

The Solís Member comprises limestones and dolostones with intercalated marls, and commonly contains thin layers of bivalve-rich tempestitic lime packstone (figure 2.B, see detailed log of the Barzana section in Barrón et al., 2006a, 2006b and Gómez et al., 2007). The carbonates of the Solís Member are organized in shallowing-upward sequences composed of a lower unit of subtidal carbonates (locally bioturbated) that occasionally contain bivalve-rich shell layers, a middle part composed of intertidal to supratidal carbonates, and an upper part interpreted as distal fan-delta facies (Gómez et al., 2007). The middle and upper parts of the Solís Member show a d13Corg negative excursion which starts at the T-J transition beds and extends up to the top of the overlying Barzana Member as studied in a borehole located south of Gijón (Gómez et al., 2007).
The stratigraphy of the T-J transition at the western part of the Basque-Cantabrian Basin is poorly known. Well-log analysis of the oil wells drilled in the region reveals, above the salt (halite) regionally attributed to the Upper Triassic, Keuper facies, a thick succession (more than 275 m thick in the Sal-1 well, see Gómez et al., 2007) of carbonates and evaporites with interbedded lutites which are located below the Lower Jurassic marine carbonates. These carbonate units and lutites (most evaporites seem to be dissolved in surface) are cropping out in some areas and have been grouped under the name Puerto de la Palombera Formation (Quesada et al., 2005). These authors interpreted a Rhaetian-Hettangian age for these deposits and a depositional setting corresponding to a broad supratidal flat and sabkha environments.
The three sections here studied are located in the southwestern part of the Basque-Cantabrian Basin (figures 1, 3), corresponding to some of the mentioned carbonates and lutites containing the T-J transition, which are cropping out in the Palencia province.

Material and methods

All bivalve fossil specimens described and illustrated here are deposited in the palaeontology collections of the University of Valencia Geology Museum (MGUV), with the following collection numbers: MGUV 3705 to 3752 and 4359 to 4500. The stratigraphical location of the field sample numbers mentioned in the Material section of each species is shown in the sections depicted in figures 2 and 3. The specimen of Caloceras pirondi (Reynès) was originally deposited in the Geological Laboratory of the Université Catholique of Lyon (France) and later moved to the Department of Paleontology of the Complutense University of Madrid, collection number: RM-00941.
In the systematic description that follows, synonymy lists have been limited to the original reference and material from northern Spain and neighbouring areas (Portugal, France). They have been critically compiled following the indications in Matthews (1973).

Systematic palaeontology

Superfamily PTERIOIDEA Gray, 1847
Family BAKEVELLIDAE King, 1850

Genus Bakevellia King, 1848

Subgenus Bakevelloides Tokuyama, 1959

Type species. Gervillia hekiensis Kobayashi and Ichikawa, 1952,

Upper Triassic of Japan, by original designation. See Hayami (1975, p. 159) for comments and diagnosis.

Bakevellia (Bakevelloides) praecursor (Quenstedt, 1856)
Figures 5.1-2


Figure 5. 1-2, Bakevellia (Bakevelloides) praecursor (Quenstedt), 1, MGUV 3748, left valve with part of the shell showing growth lines visible on the posterior wing / valva izquierda con parte de la concha mostrando las líneas de crecimiento muy visibles en la aurícula posterior. 2, MGUV 3749, ventral view of an articulated specimen with part of the shell / vista ventral de un ejemplar articulado. 3, Isocyprina cf. ewaldi (Bornemann), MGUV 3737, left valve of an internal mould / molde interno de la valva izquierda. 4, Modiolus? sp. (cf. minimus J. Sowerby), MGUV 3747, left valve of an internal mould / molde interno de la valva izquierda. 5, Isocyprina concentrica (Moore), MGUV 3740, slab with several internal and external moulds / placa con moldes internos y externos de varios ejemplares. 6, Pteromya cf. crowcombeia (Moore), MGUV 3743, slab with several internal moulds of articulated specimens / placa con moldes internos de varios ejemplares articulados. 7, Pseudoplacunopsis alpina (Winkler), MGUV 3744, left (upper) valve with part of shell showing the strong concentric wrinkles / valva izquierda (superior) con parte de la concha mostrando fuertes pliegues. 8, Isocyprina concentrica (Moore), MGUV 3738, mould of right valve showing concentric shell ornamentation / molde de la valva derecha mostrando la ornamentación concentrica. 9-10, Pteromya cf. crowcombeia (Moore), 9, MGUV 3742, internal mould of an articulated specimen showing part of the left valve / molde interno de un ejemplar articulado mostrando parte de la valva izquierda. 10, MGUV 3747, internal mould of a left? valve / molde interno de una valva izquierda? All specimens from Colunga (Asturias) / todos los especimenes son de Colunga (Asturias). Scale bar = 5 mm. / escala = 5 mm.

* 1856. Gervillia praecursor Quenstedt, p. 29, pl. 1, figs. 8-11.
v 2005. Bakevellia praecursor (Quensted); Gómez et al., p. 183-184, figs. 2-3.

Material. Three right valves, one left valve and an articulated specimen from the Solís Member (Gijón Formation) (Barrón et al., 2006b) Colunga, Asturias, samples Co-3/2, Co-3/8, Co-13/1, Co- 13/2 (MGUV 3738, 3745, 3746, 3748, 3749).

Description. Shell small (all specimens less than 20 mm long), pterioid rhomboidal in outline but not greatly elongated. Umbo nearly terminal. Equivalve and slightly inflated to flat valves. An obtuse ridge crosses each valve obliquely forming a 30º angle with the dorsal margin. Large posterior wing obtuse and clearly separated from body of shell. Anterior auricle not differentiated. Dorsal line long but shorter than maximum shell length. Anterior margin convex with no visible indentation. Postero-ventral margin convex. Surface ornamented with prominent growth lines and commarginal undulations. Hinge characters unknown.
Comparison and comments. Despite the lack of hinge characters, the specimens here described can be referred to Bakevellia, a genus well represented in Middle and Upper Triassic beds of Europe, and which extends to the late Cretaceous worldwide. The Asturian specimens are very similar in size and form to those figured and described by Ivimey-Cook et al. (1999, pl. 12, figs. 4-5) as Gervillella praecursor (Quenstedt), which is particularly common towards the top of the Westbury Formation and in the base of the Cotham Member (Lilstock Formation) (Hallam, 1990b; Mander et al., 2008), from the Rhaetian Penarth Group of England and Wales. The Asturian specimens are very close to those figured as Gervillia praecursor by Troedsson (1951, pl. 3, figs. 12-13) from the Rhaetian beds of the Höganäs Series (Sweden). Schmidt (1928, p. 151, fig. 317) figured a specimen from Rhaetian beds of Württemberg (South Germany) which is very similar to the Asturian specimens, but which are different from G. praecursor as figured by Alberti (1864, pl. 1, fig. 6) from the Kössen beds of Tübingen. The latter look similar to B. costata (Schlotheim) and to G. praecursor figured by Stoppani (1860, pl. 34, fig. 13) from the Avicula contorta beds of Dijon (France), which is very similar to a Modiolus. From the Rhaetian of the Bakony Mountains (Hungary), Vörös (1981) referred some specimens to B. faberi (Winkler, 1859) which show radial ornamentation, he remarks that the relation between this species and B. praecursor is somewhat confused and mentions transitional forms between both species. A study of more material is needed to elucidate this problem.

Genus Cuneigervillia Cox, 1954

Type species. Gervillia hagenowii Dunker, 1846, Lower Jurassic of Germany, by original designation.

This genus was regarded as a Bakevellidae by Cox et al. (1969) but as an Isognomonidae by Muster (1995, p.12) on account of the lack of teeth in adult shells, a character rarely preserved. It is here maintained in the Bakevellidae because the presence, characteristics, and taxonomic significance of dentition in these groups of bivalves are still to be assessed.

Cuneigervillia rhombica (Cossmann, 1904)
Figures 6.1-4


Figure 6. 1-4, Cuneigervillia rhombica (Cossmann), MGUV 4364, Fabares, 1-3, left valves / valvas izquierdas. 4, right valve / valva derecha. 5-8, Sphaeriola? sp., Fabares. 5-7, MGUV 4375, internal view of isolated valves / vista interna de valvas aisladas. 5, left valve? with traces of hinge teeth / valva izquierda? con restos de dientes. 6, two left valves?, white arrows on the specimen at the bottom point to strong conical teeth / dos valvas izquierdas?, las flechas blancas en el ejemplar inferior apuntan a los dientes cónicos fuertes. 7, right valve? / valva derecha? 8, MGUV 4396, external view of right? valve / vista externa de valva derecha? 9, Eomiodon? sp., MGUV 4400, left valve, Colunga / valva izquierda, Colunga (specimen illustrated in Gómez et al., 2005, fig. 2.6 as Eomiodon menkei). 10-14, Pteromya cf. tatei (Richardson and Tutcher), Fabares. 10, MGUV 4365, left valve / valva izquierda. 11, MGUV 4367, fragment of shell showing detail of ornamentation / fragmento de conchilla mostrando detalle de la ornamentación. 12, MGUV 4364, two internal moulds (specimens ilustrated by Gómez et al., 2005, fig. 2.5, as Parallelodon hettangiensis) / dos moldes internos (ejemplares ilustrados por Gómez et al., 2005, fig. 2.5, como Parallelodon hettangiensis). 13, MGUV 4373, left valve / valva izquierda. 14, MGUV 4365, right valve / valva derecha. Scale bar = 1 mm / escala = 1 mm.

*1904. Gervilleia rhombica Cossmann, p. 507-509, pl. 16, figs. 19-21.
1984. Cuneigervillia rhombica (Cossmann); Freneix and Cubaynes, p. 9-10, pl. 1, figs. 1-3.
v 2005. Cuneigervillia rhombica (Cossmann); Gómez et al., p. 183- 184, fig. 2.8.

Material. Six left valves and one right valve preserved as internal moulds from Fabares, Asturias, samples Fa-1, Fa-2, Fa-5 (MGUV 4364, 4365, 4369). Doubtfully assigned here is a specimen present in the same slab as Caloceras pirondi (figure 4-2), RM-00941.

Description. Shell very small (most specimens less than 5 mm long), pterioid trapezoidal in outline, inflated, almost equivalve, with umbo nearly terminal, and flat posterior wing. Shells are slightly longer than high, prosocline. Large posterior wing not clearly separated from body of shell. Anterior auricle small but distinct. Left valve slightly more inflated than right valve, with wider and higher umbones. Dorsal line long but shorter than maximum shell length, postero-ventral margin evenly convex, in some specimens with a faint concavity just below the dorsal line. Postero-dorsal angle obtuse. Antero-ventral margin nearly straight, without sinus or traces of byssal gape. Both valves exteriorly smooth with only very faint commarginal growth lines. Hinge characters unknown.
Comparison and comments. The specimens described can be referred to Cuneigervillia although they do not show the key hinge characters. A relatively large specimen from Corvera shows traces of the multivincular ligament and may belong to Cuneigervillia also (figure 4-2), but it is difficult to assign specifically. Cuneigervillia is very well represented in earliest Jurassic beds of Europe. Gómez et al. (2005) figured one specimen from Fabares and listed more specimens from Corvera, associated to Caloceras pirondi (Reynès). Most Asturian specimens are smaller than those from the Hettangian of Vendée described by Cossmann (1904) and those from Aquitaine described by Freneix and Cubaynes (1984), but are otherwise quite similar in shape. This species differs from C. hagenowii (Dunker, 1846), widely distributed in Hettangian and Sinemurian beds of western Europe (Philippi, 1897, pl. 16, fig. 3; Boehm, 1901, pl. 9, fig. 6; Freneix and Cubaynes, 1984, pl. 1, figs. 9-10, text-fig. 3), and from C. infraliasica (Quenstedt in Terquem and Piette, 1865, pl. 12, figs. 13-14; Dumortier, 1867, pl. 12, figs. 3-4), because its shell is more prosocline, the umbones are not terminal and it has better defined anterior auricles (see also Boehm, 1901, pl. 9, fig. 6; 1903, pl. 2, fig. 6; Cossmann, 1904, pl. 16, figs. 25-27; Troedsson, 1951, pl. 4, figs. 15-16). The Hettangian species C. angelini (Lundgren) (Troedsson, 1951, pl. 4, figs.12-14), from Sweden, and C. coimbrica (Choffat) (Boehm, 1901, pl. 9, figs. 3, 12, text-figs. 14-16; 1903, pl. 2, figs. 3, 12a), from Portugal, have terminal umbones and flat ventral regions, and are thus different from the material here described. Bakevellia (Bakevelloides) praecursor (Quenstedt), described here from Rhaetian beds of northern Spain (figures 5.1-2), is larger and less inflated with more terminal umbones.

Superfamily MYTILOIDEA Rafinesque, 1815
Family MYTILIDAE Rafinesque, 1815

Genus Modiolus Lamarck, 1799

Type species. Mytilus modiolus Linne, 1758, living species, Europe.

A number of species referred to this genus are recorded from the Rhaetian of the Penarth Group (England). This cosmopolitan and long ranging genus is mentioned in numerous publications about Rhaetian faunas in Europe.

Modiolus? sp. (cf. minimus J. Sowerby, 1818)
Figure 5.4

cf. *1818. Modiolus minimus, Sowerby, p. 110, pl. 130, fig. 6.
v 2005. Modiolus minimus (J. Sowerby); Gómez et al., p. 184.

Material. Two left valves and several small valves, from two levels of the Solís Member of the Gijón Formation (Barrón et al., 2006b), Colunga, Asturias, samples Co-3/3, Co-13/2 (MGUV 3740, 3747).

Description. Small internal moulds, up to 6 mm long, elliptical and convex. Mytiloid outline, dorsal margin straight, anterior margin relatively pointed. No other shell details are visible.
Comparison and comments. Modiolus minimus is known from the Hettangian Blue Lias of England and Wales, and the lectotype, designated by Hodges (2000, p. 52), is a specimen from the Lower Lias, probably Pre-planorbis Beds or planorbis Biozone. The Asturian material compared to this species is very scarce and badly preserved, they are very small valves, similar in form to those figured and described by Ivimey-Cook et al. (1999, pl. 16, figs. 1-2) and Hodges (2000, pl. 5, figs. 1-17) as Modiolus minimus (see range in Mander et al., 2008). This species exhibits considerable variation both in degree of elongation and shell inflation. Another species close to this group is Modiolus minutus (Goldfuss), figured by Troedsson (1951, p. 140, pl. 3, figs. 10-11) on the basis of well preserved specimens from the Rhaetian of Sweden and mentioned from several other European countries. A revision of this Triassic-Jurassic group is necessary; some specimens attributed to different Modiolus species may even be juveniles of other taxa. In the Westbury Formation the species is scarce, and although frequently mentioned, M. minimus is also rare in other European Rhaetian localities.

Superfamily PLICATULOIDEA Watson, 1930
Family PLICATULIDAE Watson 1930

Genus Pseudoplacunopsis Bittner, 1895

Type species. Plicatula (Pseudoplacunopsis) affixa Bittner, 1895, Upper Triassic (Carnian) of St. Cassiano, North-East Alps (Italy), by original designation.

Comments. Hautmann (2001) emended the diagnosis of this genus based on his revision of the type specimens from the Cassian Formation housed in the Naturhistorisches Museum (Wien). He also discussed the genus Placunopsis Morris and Lycett and proposed a stratigraphical range from Carnian (P. affixa) to Kimmeridgian (Plicatula ogerieni Loriol). In our opinion, Placunopsis is a Jurassic genus (Todd and Palmer, 2002) and Pseudoplacunopsis should be regarded as a Triassic genus (Middle and Upper Triassic) with a cosmopolitan, mainly tethysian, palaeogeographical distribution (Ros, 2009).

Pseudoplacunopsis alpina (Winkler, 1859)
Figure 5.7

*1859. Anomia alpina Winkler, p. 5, pl. 1, figs. 1 a-c.
v 2005. Placunopsis cf. alpina (Winkler); Gómez et al., p. 184, pl. 2, fig. 2.

Material. Two left valves, from two levels of the Solís Member of the Gijón Formation (Barrón et al., 2006b), Colunga, Asturias, samples Co-3/7, Co-13/1 (MGUV 3744, 3746).

Description. Small to medium-sized shell, up to 16 mm long, with an irregular ovoid, higher than long valve outline; hinge margin short, curved and without auricles.
Shell surface almost flat, only the anterior part is gently convex during early ontogeny of the shell, becoming flat or concave later due to xenomorphic growth. The umbo is not prominent and lies in the middle of the very short hinge line. The growth rugae are very irregular in strength and distribution but heavier near the ventral area, forming strong concentric wrinkles. Radial ornamentation consists or irregular, wavy, rather coarse costellae showing allomorphic sculpture. No clear xenomorphic surface is detectable in the umbonal-middle region; thus these specimens are regarded as left valves. Internal characters are not visible.
Comparison and comments. The Asturian material of this cemented bivalve is very scarce, the specimens have only part of the shell preserved, and are similar in size and form to those figured and described by Ivimey-Cook et al. (1999, pl. 14, figs. 4-6) as Placunopsis alpina. This species is not very common but is usually recorded in the Westbury Formation and in the basal beds of the Cotham Member of the Lilstock Formation in England and Wales (Mander et al., 2008). The species is also widely distributed in the Rhaetian Kössen Formation of the northern Calcareous Alps (Austria, Germany) and the Zu Limestone in Lombardia (Italy) (Tomasovych, 2006). Vörös (1981) described P. alpina from the Rhaetian of the Bakony Mountains (Hungary).
Several taxa are mentioned in this group, Placunopsis plana Giebel (1856, p. 64, pl. 2, fig. 6), P. gracilis Giebel (1856, p. 64, pl. 6, fig. 2) and P. oblique Giebel (1856, p. 64, pl. 6, fig. 3) from the "Muschelkalk" of Lieskau, Middle Triassic of Germany. Subsequently, Würm (1911, p.107, figs. 13a-b) described P. teruelensis from the "Muschelkalk" of Aragón (Iberian Range) and this form was recorded as very abundant in several outcrops of the Spanish Ladinian (Márquez-Aliaga, 1985 and Márquez-Aliaga et al., 2005). From the lower part of the Avicula contorta beds, Stoppani (1860-1865) described some new "Anomia" taxa from the "Infralias" of the Lombardy Alps, that he regarded very close to a Plicatula, as Anomia mortilleti, A. talegii and A. favri. He emphasized the resemblance of his forms to A. schafhaeutli (Winkler, 1859) and A. alpina (Winkler), in all cases the description is based only on external morphological features.
The very common Anomia fissistriata Winkler (1861, p. 467, pl. 10 figs. 10 a-c), described from the "Ober-Keuper" Rhaetian of Bavaria, has the upper and lower valves bearing the typical thin radial ornamentation, but hinge details are unknown. This species was recorded from the Carnian of Bavaria (Wöhrmann, 1889), from the Ladinian of the Jordan Valley (Cox, 1924), Israel (Lerman, 1960) and Nevada (USA) (Waller and Stanley, 2005) and from the Anisian of the Dolomites (Posenato, 2008).
In summary, several Triassic species of Pseudoplacunopsis have been described during the 19th century but many of them must be considered synonyms, because broad intraspecific variability surely occurred in these bivalves (Posenato, 2008). A revision of this species is being prepared by one of us (A.M-A.), based on many Spanish Ladinian specimens, and some of them show hinge morphological characters, such as the crura and the pit for reception of primary ligament. We have data on the microstructural features of those specimens and the bimineralic composition of the shell (De Renzi and Márquez-Aliaga, 1980) with a foliate calcite outer layer and an inner aragonitic crossed-lamellar layer (Márquez-Aliaga and Márquez, 2000) and their epizoic mode of life has been studied (Márquez-Aliaga and Martínez, 1994). We propose that these Triassic plicatulids should be separated into two morphological groups with different ornamentation pattern, the first one with very thin radial ornamentation (close to Placunopsis plana Giebel) that includes all the species mentioned above. The second group has thick-folded shells which are similar in form to the "false-oysters type" such as A. matercula Quenstedt (1851, pl. 40, fig. 6). The Spanish P. flabellum Schmidt (1935, p. 64, pl. 5, figs. 4-7) from the Ladinian of Siles (Jaén) may be regarded as a synonym of A. matercula and not related to any species of Umbostrea (= Enantiostreon), a true Triassic oyster (Márquez-Aliaga et al., 2005). All Triassic Pseudoplacunopsis species show great variability, as is usual in cemented bivalves; the majority of the proposed names are not justified because they are based on the shell outline form alone. At the moment, until the systematic revision is finished, we regard the Asturian specimens are close to the Rhaetian P. alpina as a form of the P. plana (Giebel) group.

Superfamily LUCINOIDEA Fleming, 1828
Family FIMBRIIDAE Nicol, 1950

Genus Sphaeriola? Stoliczka, 1871

Type species. Cardium madridi d´Archiac, 1843, from the Bathonian of Europe, by original designation.

Sphaeriola? sp.
Figures 6.5-8

Material. A few specimens from Fabares can be doubtfully referred to this genus, from samples Fa-1, Fa-7, Fa-9 to 20, ex situ), Fa-13 (MGUV 4374, 4375, 4380, 4383, 4385, 4386, 4387?, 4389?, 4391, 4393, 4396).

Description. Medium-sized, thick, ovoid, globose shells, with nearly orthogyrous umbones situated at mid length. Slightly longer than high. Without lunule or escutcheon. Anterior shell margin widely curved from umbo, posterior margin also curved but with a nearly straight dorsal portion. Inner margin of shell smooth, some specimens show a well-defined pallial line. Details of hinge unknown; on left valves there are two massive conical cardinal teeth (figure 6.6). Surface with commarginal lines only.
Comparison and comments. The specimens described here do not show the internal characters in enough detail and are referred very doubtfully to Sphaeriola on the basis of shell shape and appearance of hinge teeth, although the shell margin is smooth. The material could also be referred to Schafhaeutlia, which was widely distributed around the world during late Triassic times. About the Triassic-Jurassic boundary there are several species which are difficult to refer to one of these two genera on shell morphology alone, and while late Triassic species of this group are referred to Schafhaeutlia, Hettangian species are usually placed into Sphaeriola.
Some comparable species are Sphaeriola leedae Marwick (1953, pl. 10, figs. 12, 15, 16) from the Aratauran (Hettangian) of New Zealand, S. nipponica Hayami, 1959 (Hayami, 1975, pl. 6, fig. 2) from the Sinemurian of Japan, and S. subglobosa (Tate) (Palmer, 1966, pl. 2, figs. 5-6) from the Pliensbachian of Great Britain.
Small-sized Schafhaeutlia species from the latest Triassic which have a shape similar to the specimens here described are: Schafhaeutlia astartiformis (Münster), from Carnian-Rhaetian deposits (see for instance Bittner, 1895, pl. 3, figs. 1-4; Scalia, 1912, pl. 3, fig. 67; Allasinaz, 1966, pl. 54, figs. 6-7; Zhang et al., 1979, pl. 61, figs. 27-29, 31; ) and S. mellingi (Hauer) (in Wöhrmann, 1889, pl.10, figs. 4-6; Broili, 1904, pl. 27, fig. 32; Toula, 1910, pl.12, figs. 14-15; Ichikawa, 1950, pl. 5, fig. 8; Kiparisova et al., 1966, pl. 23, figs. 3-4), both are higher than long and have a nearly circular shape. Schafhaeutlia nakazawi Tokuyama (1960, pl. 13, figs. 12-14) from the Late Triassic of Japan, has the umbones placed posterior to mid-length and a more rounded anterior margin. Schafhaeutlia schafhaeutli (Salomon, 1895, pl. 5, figs. 44-45) has a larger and more rounded shell.

Superfamily ARCTICOIDEA Newton, 1891
Family ARCTICIDAE Newton, 1891

The material here analyzed is poorly preserved and generally lacks details of hinge and pallial line, but one silicified young specimen with well preserved right valve hinge allows reference to Isocyprina (Eotrapezium). The rest of the material can only be identified on the basis of general shape, presence of strong anterior and posterior lateral teeth, position and characteristics of the umbonal region and external features.

Genus Isocyprina Röder, 1882

Type species. Cardium cyreniforme Buvignier, 1852, from the Oxfordian of Europe, subsequent designation by Cossmann, 1920.

Isocyprina concentrica (Moore, 1861)
Figures 5.5, 5.8

*1861. Axinus concentricus Moore, p. 503, pl. 17, figs. 19-21.
v 2005. Isocyprina (Eotrapezium) concentricum (Moore); Gómez et al., p. 183-187, fig. 1.

Material. One right and two left? valves of the Solís Member of the Gijón Formation (Barrón et al., 2006b), Colunga, Asturias, samples Co-3/2, Co-3/3, Co-13/3 (MGUV 3738, 3740, 3751).

Description. Shell small, up to 12 mm long, triangular- rhomboidal to very elongate in outline, not very compressed. Anterior margin short, rounded; posterior margin extended and straight, bounded by a strong carina from umbo to postero ventral margin. Umbones somewhat raised, convex and anterior of mid-line. Surface covered by commarginal growth lines which cross the posterior area to meet the postero- dorsal margin at about 90º. Hinge characters unknown.
Comparison and comments. The Asturian specimens are very similar in size and form to those figured and described by Ivimey-Cook et al. (1999, pl. 17, fig. 5) as Isocyprina (Eotrapezium) concentricum (Moore), which is particularly abundant in the Westbury Formation and basal Cotham Member (Lilstock Formation) from the Rhaetian Penarth Group of England and Wales (Mander et al., 2008). This species has been frequently referred to Schizodus ewaldi (Bornemann) (Richardson, 1911), another Isocyprina species common in Rhaetian beds.

Isocyprina cf. ewaldi (Bornemann, 1854)
Figure 5.3

*1854. Taeniodon ewaldi Bornemann, p. 66.
v 2005. Isocyprina ewaldi (Bornemann); Gómez et al., p. 183-187.

Material. Two left and four right valves of the Solís Member of the Gijón Formation (Barrón et al., 2006b), Colunga, Asturias, samples Co-3/1, Co-3/3, Co-3/8 (MGUV 3737, 3740, 3741, 3745a, b, c).

Description. Shell very small, up to 6 mm long, rather triangular, smooth, with very fine commarginal growth lines, not compressed. Anterior margin rounded, posterior margin slightly extended with a depressed ridge extending from umbo to posteroventral margin but not very visible. Umbones raised, convex and anterior of mid-line. Hinge characters unknown.
Comparison and comments. The Asturian specimens are very small, and are not well preserved, but are very similar in size and form to those figured and described by Ivimey-Cook et al. (1999, pl. 17, figs. 3- 4) as Isocyprina (Eotrapezium) ewaldi (Bornemann). This species is very common in the Westbury and Lilstock Formations of the Rhaetian Penarth Group of England and Wales (Mander et al., 2008) and in the Langport Member of Devon, where it is always less abundant than I. concentrica (see Hallam, 1990b). Two specimens, MGUV 3737 and 3745a, have a very similar outline to the figures of I. germari (Dunker, 1844) in Ivimey-Cook et al. (1999, pl. 17, figs. 3- 4) but the umbonal region is not well preserved in either of them. A more elongated form could be interpreted as normal specific variation, and thus we prefer to leave this specimens in open nomenclature, but close to I. ewaldi, noting that this is a very common species in this type of fossil association. Some specimens are close to I. ewaldi described by Schmidt (1928, p. 201, fig. 484) from Rhaetian beds of North Germany. This species has been mentioned by Alberti (1864, p. 119) based on Winkler (1861, pl. 7, figs. 6a-d) from the "Oberkeuper" of the Bavarian Alps, but those figured specimen should be referred to the Myophoridae.

Subgenus Eotrapezium Douvillé, 1913

Type species. Mesodesma germari Dunker, 1846, from the Lower Jurassic of Germany, by original designation.

Eotrapezium was regarded as a synonym of Isocyprina by Arkell (1934, p. 261, 264) on account of their very similar hinge characters, but Cox (1947) pointed out that shell shape is different in the two groups, and then Eotrapezium was included as a subgenus of Isocyprina by Cox et al. (1969).

Isocyprina (Eotrapezium) germari (Dunker, 1846)
Figures 7.1-13, 8.1-5


Figure 7. 1-13, Isocyprina (Eotrapezium) germari (Dunker). Scale bar = 1 mm / escala = 1 mm. All specimens from Barrio San Pedro / todos los ejemplares proceden de Barrio de San Pedro. 1-2, 6, 9, general view of slabs containing numerous complete isolated valves, sometimes nested-in / vista general de lajas conteniendo numerosas valvas completas aisladas, ocasionalmente encajadas. 1, MGUV 4438, 2, 6, MGUV 4439. 9, MGUV 4440. 3, two right valves in nearly dorsal view, the upper one showing posterior tooth (arrow) / dos valvas derechas en vista casi dorsal, la superior mostrando el diente posterior (flecha), MGUV 4438. 4, right valve internal view / vista interna de valva derecha, MGUV 4439. 5, 13, dorsal view of a specimen with closed valves showing escutcheon / vista dorsal de un ejemplar con valvas cerradas mostrando el escudete, MGUV 4442. 7-12, internal views of isolated valves, lateral teeth indicated by white arrows / vista interna de valvas aisladas, dientes laterales indicados por flechas. 7, 12, MGUV 4442; 8, 10, MGUV 4439. 9, 11, MGUV 4440.

*1846. Mesodesma Germari Dunker, p. 40, pl. 6, figs. 20-22.
1901. Isocyprina Germari Dunker; Boehm, p. 242-244; pl. 10, figs. 5- 8; text-figs. 22-28.
1971. Isocyprina (Eotrapezium) aff. germari (Dunker); Fischer and Palain, p. 116-118; text-fig. 4; pl. 1, figs. 4-6.

Material. This is the most abundant species at all localities. A small silicified right valve and several fragments from Fabares, sample Fa-3 (MGUV 4368), many specimens, mostly preserved as internal moulds, from several levels at Fabares, samples Fa-2, Fa-14-15, Fa-14-1, Fa-15b, Fa-15-2 (MGUV 4360, 4362, 4363, 4365, 4380-4382, 4384, 4387, 4389, 4393). Additional relatively well-preserved and abundant material was found at Barrio de San Pedro and Salinas de Pisuerga, Palencia; samples Bs-1, Bs-2, Sa-I, Sa-II, Sa-10.1+0.3, Sa-10.1+2, Sa-10.2+0.91-1.31, Sa-10.2+2, Sa-10.2+2.5, Sa-10.2+4, Sa-10.2+5.5 (MGUV 3705-3720, 4438-4500); and Reinosilla, samples Re-A, Re-B, Re-C, Re-D, Re-E (MGUV 3721-3736). Some poorly preserved specimens from other localities (Caravia, Barzana) can doubtfully be included here as well; samples Ca- 15/1.1, Ba-A, Ba-B, Ba-C, Ba-3+9, Ba-14, Ba-15 (MGUV 4411-4413, 4425-4437). Several fragmentary specimens found ex situ near Corvera, in a slab with Caloceras pirondi, could also belong to this species (figures 4.1-2), RM-00941.

Description. Small, compressed equivalve shells, ovate-subtrapezoidal in outline, with prosogyrous umbo situated at about one third of total shell length. Shell consistently longer than high. Some specimens show a faint umbonal carina. Escutcheon well defined in small specimens, narrow and long (figure 7.5). Anterior and ventral margins evenly curved, posterior margin shorter than anterior and slightly truncate (figure 8.5).


Figure 8. 1-4, Isocyprina (Eotrapezium) germari (Dunker) hinge, fragment of juvenile right valve superficially silicified / charnela, fragmento de una valva derecha juvenil superficialmente silicificada, MGVU 4368. 1, SEM view of hinge area / vista del área charnelar con MEB. 2-3, view of specimen before coating and a sketch with indication of teeth nomination / vista del ejemplar antes de ser cubierto y un diagrama con indicación de la nomenclatura dentaria. 4, SEM detail view of tooth 3b / vista de detalle del diente 3b tomada con MEB. 5, camera lucida drawings of shell shape variation in specimens from Salinas de Pisuerga / dibujo a cámara clara de la variación de forma de la conchilla en ejemplares de Salinas de Pisuerga.

Young right valve hinge (figure 8.1-4) with two anterior lateral teeth (AI and AIII), strong bifid cardinal 3b, colaminar cardinal 3a at the posterior end of AIII; colaminar cardinal 1 only incipient at posterior end of AI, in front of 3a and well anterior to the beak. Hinge of left valve unknown, but right valve with a deep socket for reception of strong 2b. Both valves also with at least one strong posterior lateral tooth (figure 7.10). Ligament external on nymphs. Valve margins smooth.
Shell surface with very faint growth concentric lines only. Muscle scars and pallial line not seen.
Comparison and comments. The hinge characters of this material can be described as early cyprinoid in the sense of Gardner (2005). They mostly agree with the hinge of the type species, I. (E.) germari (Dunker) as figured by Philippi (1897, pl. 16, figs. 4a-c); Boehm (1901, figs. 22-25); Cox (1947, figs. 1a-b) and Cox et al. (1969, fig. E128.1). The specimens from Asturias can be referred to I. (E.) germari (Dunker) since, apart from the hinge characters already mentioned, they have the same shell shape, inflation, presence of a well-defined escutcheon and low carina. This species is widely distributed in uppermost Triassic and lowermost Jurassic deposits in Europe from Sweden to Portugal (Dunker, 1846, pl. 6, figs. 20-22; Philippi, 1897, pl. 16, figs. 4a-c; Boehm, 1901, pl. 10, figs. 5-8; text-figs. 22-28; Douvillé, 1913, text-figs. 38-39; Troedsson, 1951; Ivimey-Cook et al., 1999, pl. 17, fig. 10).
Shell shape and variation are very similar to those of the material from the Hettangian of Portugal described by Fischer and Palain (1971, p. 116, fig. 4, pl. 1, figs. 4-6) as I. (E.) aff. germari (Dunker), but Spanish specimens are somewhat smaller and have a less sharp posterior carina. This species was also listed by Meddah et al. (2007) from the Saharian Atlas in Algeria. On the basis of the well preserved specimens, material from Barzana and Caravia with a similar external shell shape was included in this species, although in some of them preservation was indeed poor. Despite this, the abundance of material allows the description of shell shape variation (figure 8.5).

Family NEOMIODONTIDAE Casey, 1955

Again the material was referred to this family on account of its external shell shape, as it is very poorly preserved.

Genus Eomiodon? Cox, 1935

Type species. Eomiodon indicus Cox, 1935, Middle Jurassic, India, by original designation.

Eomiodon? sp.
Figure 6.9

?1984. Eomiodon aff. menkei (Dunker); Freneix and Cubaynes, p. 14- 15; pl. 1, figs. 9, 12, 13; text-fig. 4.
v 2005. Eomiodon menkei (Dunker); Gómez et al., p. 184-185, fig. 2.6.

Material. This species was recognized at Colunga, sample Co-15 (MGUV 4400). The specimens are all preserved as subinternal moulds. Gómez et al. (2005) mention this species also from Fabares and Caravia.

Description. The shell is subtrapezoidal in outline, with wide prosogyrous umbones situated at about 1/4 of the total shell length from the anterior end. All shell margins are evenly rounded, with a slightly extended and narrower posterior portion. Shell inflated with a low umbonal carina. Shell surface appears to be smooth with only faint growth lines. They show no details of hinge or muscle scars.
Comparison and comments. Specimens were referred doubtfully to Eomiodon on the basis of their shell shape, inflation and position of umbones, and presence of a broad umbonal carina. According to Gómez et al. (2005) this species is associated with Psiloceras sp. at Colunga, Fabares and Caravia.
Shell shape is similar to Eomiodon lunulatus (Yokoyama) and E. vulgaris Hayami from the Hettangian of Japan (see illustrations in Kondo et al., 2006), but the lack of hinge characters in the Asturian specimens makes comparison very difficult.

Superfamily PHOLADOMYOIDEA Gray, 1847
Family CERATOMYIDAE Arkell, 1934

Genus Pteromya Moore, 1861

Type species. Pteromya crowcombeia Moore, 1861, from the Rhaetian of England, subsequent designation by Stoliczka (1871, p. xv).

Comments. Cox (1963) made a revision of the type and of all known species of the Pteromya and their hinge characters, concluding that the genus should be referred to the Ceratomyidae, and this has been maintained in Cox et al. (1969) and by Runnegar (1974).
The geographical distribution of this genus was originally thought to be restricted to northern Europe (mainly Great Britain), i.e. Boreal domains (Liu, 1995), but it is also present in several localities in France (Freneix and Cubaynes, 1984), and the specimens here described extend its latitudinal distribution to northern Spain. Doubtful records from North America and New Zealand (MacFarlan, 1998) are not confirmed by illustrations.
The material here analysed is not well preserved, and many characters cannot be observed. Nevertheless, it is assigned to this genus on account of the general shape, inflation of the valves and ornamentation with close commarginal undulations.

Pteromya cf. crowcombeia (Moore, 1861)
Figures 5.6, 5.9-10

*1861. Schizodus crowcombeia Moore, p. 506, pl. 15, figs. 22-23.
v 2005. Pteromya aff. crowcombeia (Moore); Gómez et al., p. 184.
v 2005. Pteromya langoportensis (Richardson and Tutcher); Gómez et al., p. 184, pl. 2, fig. 4.

Material. One articulated internal mould and more than 40 specimens preserved as internal moulds of single valves, from three levels of the Solís Member of the Gijón Formation (Barrón et al., 2006b), Colunga, Asturias, samples Co-3/5, Co-3/6, Co-13/1,Co- 13/2, (MGUV 3742, 3743, 3752, 3747).

Description. Shell small, up to 15 mm long. Elongated-ovoid in outline. Umbones broad and fairly prominent placed anterior to mid-line. Inequilateral and slightly inequivalve, the right valve is a little more convex than the left one. Dorsal and posterior margins meet at well marked obtuse angle. Some specimens have a very faint posterior ridge. Anterior and ventral margins slightly curved. None of the specimens has the shell preserved so the typical concentric folds are not visible. Internal characters unknown.
Comparison and comments. The Asturias specimens are very small and are badly preserved, but they are similar in size and form to those figured and described by Cox (1963, pl.79, figs. 1-7) and Ivimey- Cook et al. (1999, pl. 18, figs. 3-5) as Pteromya crowcombeia (Moore). The first author reexamined Moore's syntypes from the Lower Rhaetian of Bere Crowcombe, near Somerset (England), and considered P. simplex Moore 1861 a synonym of this taxon. On the other hand, Cox (1963) regarded that the Garden Cliff (Yorkshire) species, referred to as Myacites musculoides Schlotheim by Tate (1876), belongs to P. crowcombeia instead. Some of the Asturian specimens, from the upper part of the Colunga section, look close to P. langportensis (Richardson and Tutcher) but the poor preservation of the isolated valves does not allow a determination, and we prefer, following Cox (1963), to regard this slight outline differences in elongation as species variability, until we have more material to study.

Pteromya cf. tatei (Richardson and Tutcher, 1916)
Figures 6.10-14

cf. 1916. Pleuromya tatei Richardson and Tutcher; p. 52, pl. 8, figs. 3a-c, 6.
v 2005. Pteromya tatei (Richardson and Tutcher); Gómez et al., p. 183, 184, 185, fig. 2.7.
v 2005. Pteromya cf. vaurensis (Freneix and Cubaynes); Gómez et al., p. 184.
v 2005. Pteromya sp.; Gómez et al., p. 185, 184.
v 2005. Parallelodon hettangiensis (Terquem); Gómez et al., fig. 2.5.

Material. Twelve specimens and fragmemnts from five different levels at Fabares, samples Fa-1, Fa-2, Fa-3, Fa-6, Fa-7, Fa-9 (MGUV 4364-4367, 4370, 4371, 4373) and from Colunga, sample Co-15 (MGUV 4399). Gómez et al. (2005) figured one specimen from Colunga, and listed material referred to Pteromya from Corvera (RM-00941), Fabares, Caravia and Barzana, occasionally associated with Hettangian ammonites. The available specimens are very poorly preserved, merely as moulds of isolated valves, showing the external shape only and indication of growth lines and clear commarginal ornamentation.

Description. Shell sub-ovate, well inflated, with ventral and posterior margins meeting at an obtuse angle. Anterior and ventral margins evenly curved. A very wide umbonal carina is present in some specimens. Umbones broad, placed slightly anterior to mid-length. Surface with regular commarginal undulations covering the whole shell (figures 6.10-11). The hinge region and muscle scars are not preserved.
Comparison and comments. The material is difficult to assign to a species, nevertheless, the best preserved specimens can be closely compared with P. tatei, a widely distributed species from the Lower Hettangian of Great Britain (Richardson and Tutcher, 1916, pl. 8, figs. 3, 4, 6; Arkell, 1933, pl. 29, fig. 8; Cox, 1963, pl. 79, figs. 9-19; pl. 80, figs. 1-2; Hodges and Simms, 2004) with which they share the general shell shape and ornamentation pattern. The specimens from Asturias are more sub-triangular in outline, with a wider and more protruding umbo, and a more defined umbonal carina, but all these features are very variable in P. tatei. Other Hettangian comparable nominal species (probably synonyms) from France are P. vaurensis Freneix and Cubaynes (1984, pl. 1, figs. 5-8, text-fig. 2A) and P. coxi Freneix and Cubaynes (1984, text-fig. 2B), both are more elongated and lack commarginal ornamentation.
The stratigraphical distribution of Pteromya tatei extends across the Triassic-Jurassic boundary in localities with a good continuous record (Hodges and Simms, 2004; Simms and Jeram, 2007; Mander et al., 2008).

Preservation and taphonomy

Bivalve preservation is poor in all localities. Specimens appear mostly as internal in moulds with few inner details preserved in micritic limestones. The material which shows inner hinge details comes from the Barrio de San Pedro and Salinas de Pisuerga sections (Palencia), preserved in micritic limestones.
At some levels in Fabares (Fa-2 and 9) the bivalves appear as external and sub-internal moulds and shell shape variability is increased by deformation. The fossils comprise at least three bivalve species, referred here to Pteromya cf. tatei, Isocyprina (E.) germari and Cuneigervillia rhombica, and one gastropod species. Other levels from the same locality (Fa-13-15) only bear very poorly preserved bivalves which seem to belong to Isocyprina (Eotrapezium) germari and a different species, referable with doubts to Sphaeriola? sp. Some samples bear numerous ostracod remains and several poorly preserved microgastropod species. One sample has abundant fragments of tubelike calcareous structures probably belonging to calcareous algae. In Colunga there are abundant but poorly preserved bivalves referable to Pteromya? sp. and Eomiodon? sp., whilst in Caravia and Barzana limestones only bear bivalve ghosts.
At the western Basque-Cantabrian Basin, the TJ transition beds contain bivalves but they are nearly always monospecific. At Salinas de Pisuerga the abundant bivalves are referred to Isocyprina (E.) germari and appear in micritic limestones in dense concentrations on bedding planes (pavements). In some samples the shells are articulated and closed, but most samples are almost coquinas with isolated but complete concordant valves, oriented mostly with their convex side up. There seems to be some kind of size sorting, since very small specimens are extremely scarce and most valves are of very similar size, around or slightly less than 1 cm long.
At Barrio de San Pedro the situation is similar both in lithology and in fossil contents, Isocyprina (Eotrapezium) germari is abundant in sample Bs-1, forming dense pavements of mostly concordant isolated valves, sometimes nested-in (figures 7.1, 7.6, 7.9). At other levels of this locality shells are scarce and badly preserved (Bs-2).

Palaeoecology

The only available data on the environmental conditions of these deposits were provided by the coeval palynomorph assemblages from the Asturias region (Barron et al., 2006b; Gómez et al., 2007). These studies indicate that the diverse Late Triassic xerophilous community (indicating arid conditions) was followed by an impoverished but hygrophilous flora just above the T-J transition, and then a recovery of arid conditions later in the Hettangian.
Most of the Rhaetian-Hettangian bivalve species described here are shallow burrowers, Pteromya is regarded as a medium-depth burrower by Runnegar (1974), and Cuneigervillia and Bakevellia are the only epifaunal byssate bivalves.
Around the Triassic-Jurassic mass extinction event most known biotic assemblages were of very low diversity. All shell beds examined have very low species richness, and in fact, most of them can be regarded as oligotypic and even monotypic (Caravia and Barzana in Asturias; Salinas de Pisuerga and Barrio de San Pedro in the western Basque-Cantabrian Basin). 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, most probably related to low salinity. The consistent small size of all specimens adds to this argument. Coeval faunas with similar composition were regarded as deposited in a marginal marine environment in the Aquitaine Basin (brachyhaline open lagoon according to Freneix and Cubaynes, 1984; Cubaynes et al., 1984) and Pyrenean Basin (hypersaline conditions according to Fauré, 2002). The Penarth Group in Great Britain contains a variety of facies, but its fauna is far less diverse than in the Alps and was interpreted by Hallam and El- Shaarawy (1982) as deposited in a shallow epicontinental sea with a salinity level well below that of the Tethyan ocean (see also Gallois, 2007). Compared to the Penarth fauna (Mander et al., 2008), the end- Triassic Asturian bivalves are far less diverse, and marine stenohaline elements, such as palaeotaxodonts, are absent.
Even the more diverse assemblages, the Hettangian Pteromya-Cuneigervillia-Eomiodon assemblage of Fabares and Colunga, and the Rhaetian Pteromya-Bakevellia-Isocyprina assemblage of Colunga, were probably also salinity controlled, and beds with similar fauna were interpreted by Hallam (1976), Fürsich (1993, p. 342) and Kondo et al. (2006) as belonging to the brachyhaline regime (18-30‰). The assemblages with Eomiodon were referred to the lower part of this range (16-23 ‰) by Hallam (1976). Other biological features of these beds are: the absence of stenohaline higher taxa, such as brachiopods, echinoderms and cnidaria; the extreme paucity of ammonoids; a conservative bivalve shell morphology typical of reduced salinity environments; and size selection. All bivalve taxa have a very low maximun size, well below half the size attained by the same species at other localities.

Palaeogeography

The opening of seaways at the beginning of the breaking of Pangea during the Rhaetian started deposition of marine sediments in many places of Western Europe. Earliest Jurassic sediments in Western Europe were deposited in shallow epicontinental seas, and around the T-J boundary an extensive shallowing event was proposed, including emergence in some areas, followed by rapid deepening associated to anoxic-dysoxic facies (see discussion in Hallam, 1997; 2001). Marginal marine settings around the boundary are thus widespread in western Europe and northern Africa (Golonka, 2007, and figure 9 here). Gómez et al. (2007) indicate that shallow carbonate platform environments were present in the Asturian area around the T-J boundary, without major changes in sea-level.


Figure 9. Regional palaeogeographic map for the western Tethys, future central Atlantic and adjacent areas during earliest Jurassic times (modified after Golonka, 2007), with the location of the areas discussed in the text / mapa paleogeográfico regional del Tethys occidental, futuro Atlántico central y áreas adyacentes durante el Jurásico Inferior (modificado de Golonka, 2007), con la localización de las áreas mencionadas en el texto.

The bivalves analyzed here clearly belong to the same biofacies and environments as those described by Dalbiez et al. (1959), Cubaynes et al. (1984) and Freneix and Cubaynes (1984) from Aquitaine and by Fauré (2002) from the Pyrenees. In fact, the bivalves from Asturias seem to be an impoverished version of those assemblages. The general environmental setting seems to have been similar to those of northwestern Europe, for instance the Höganäs Formation in Sweden (Troedsson, 1951; Lindström and Erlströn, 2006) and the already mentioned Penarth Group in Britain (Hallam and El Shaarawy, 1982), where a variety of marginal marine sediments interbedded with terrestrial sediments occurs. A recent and very detailed study of the Triassic/Jurassic transition beds in SW Britain (Mander et al., 2008) records the ranges of several species in common with northern Spain, though the British assemblages (both Rhaetian and Hettangian) are far more diverse.
Apparently similar Rhaetian-Hettangian beds with Isocyprina (Eotrapezium) aff. germari have also been recorded at the Pont Tiout Formation, Saharian Atlas, Algeria (Meddah et al., 2007, p. 28), and coeval beds with salinity-controlled bivalve assemblages have been reported from eastern Morocco (Olsen et al., 2006).
This assemblage is different from bivalve assemblages reported in other European Rhaetian- Hettangian open-marine northern regions such as northern Ireland (Simms and Jeram, 2007) and Tethyan settings, such as those from northern Italy (Berini, 1957; Bertuletti, 1962; Gaetani, 1970; Allasinaz, 1992; McRoberts, 1994; McRoberts et al., 1995), southern Italy [Sirna, 1968; Senowbari-Daryan and Zamparelli, 2003, Spain (Goy and Márquez- Aliaga, 1998)] and Austria (Hallam, 1990a). It shares some elements with faunas from somewhat restricted marginal marine habitats interpreted as having reduced salinity, such as those from Portugal (Fischer and Palain, 1971), England and northwestern Europe (see discussion in Hallam and El-Shaarawy, 1982 and Wignall and Bond, 2008), but again these are more diverse than the Asturian faunas.

Conclusions

Both the Rhaetian and Hettangian bivalve assemblages here described from northern Spain have very low systematic diversity, with about half a dozen species each, belonging to seven families. The bivalves which form the trophic nucleus of each assemblage have a comparable systematic composition. Each assemblage is dominated by different species of the arcticid Isocyprina (I. concentrica and I. cf. ewaldi in the Rhaetian and I. germari in the Hettangian), each has a species of a small bakevellid (Bakevellia praecursor in the Rhaetian and Cuneigervillia rhombica in the Hettangian) and distinct species of the ceratomyid Pteromya (P. cf. crowcombeia and P. cf. tatei respectively). Isolated specimens of two other bivalve taxa were identified in the Rhaetian assemblage, referred to Modiolus? cf. minimus and Pseudoplacunopsis alpina. Likewise, Hettangian beds also contain a few specimens of Sphaeriola? sp. and Eomiodon? sp.
The T-J transition beds Asturias and the western Basque-Cantabrian Basin were deposited in a marginal marine environment. Benthic fauna was dominated by shallow burrowing, suspensivorous bivalves, with subordinate participation of epibyssate suspension feeders. The Rhaetian bivalve assemblage at Colunga is relatively more diverse than the Hettangian one, but the comparable systematic composition suggests little change in environmental conditions across the T-J boundary in this region. The Hettangian bivalves are T-J transition as part of a community which lived in highly stressed environments probably with reduced salinity.
No relationship between the Rhaetian-Hettangian bivalves found in Asturias and in the western Basque-Cantabrian Basin can be established with the open-marine faunas described from the Tethys, but they are similar to those of northwestern Europe and northern Africa instead.
The scarcity of ammonites prevents for the moment the precise location of the T-J boundary within the studied beds, and bivalves themselves are not helpful. As happens in many other localities of western Europe, this hinders the analysis of the Triassic- Jurassic extinction event in this region until more data are available.

Acknowledgements

We are deeply grateful to the "Patronat Sud-Nord" of Valencia University, and in particular its chairwoman Eva Barreno, for the appointment of one of us (SD) to the Cátedra UNESCO during the fall term 2008, allowing the revision of the bivalves. This research was financed by projects CGL 2008-03112 of the Spanish Ministerio de Educación y Ciencia, CCG07-UCM/AMB-2478 and CCG07- UCM/AMB-2132. This is a contribution to IGCP Project 506 "Marine and non-marine Jurassic". The paper benefited from editorial revisions by C. McRoberts and M.J. Comas.

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Recibido: 2 de junio de 2009.
Aceptado: 18 de octubre de 2009.

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