Introduction and geological setting
The Cantabrian Zone is the most external area of the Iberian Massif. Cropping out throughout the area is a nearly complete succession of Carboniferous strata. Differences in the stratigraphical succession and facies of Palaeozoic rocks distinguish several units in the Cantabrian Zone (Fig. 1). The reader is referred to Sánchez de Posada et al. (1990) and Fernández et al. (2004) for summaries of the Carboniferous stratigraphy of the region.
Fig. 1. Sketch map of the Iberian Massif showing the position of the Cantabrian Zone (right) and position of the Entrago Section within the Cantabrian Zone (left).
A rather peculiar ostracod was found in late Mississippian limestones of the Cantabrian Zone, at the locality of Entrago (Teverga, Asturias, northern Spain). This ostracod is mainly represented by isolated valves (some of them broken) along with a few carapaces. The ornamental pattern of a posterior dorsoventral ridge ending in dorsal and ventral spines, and a pitted area in front of the ridge is strikingly similar to that of Criboconcha Cooper, 1941. However, the differences in the junction of the valves indicate that the ostracod studied in this paper belongs in the Bairdiocypridoidea Shaver, 1961.
The fossils were obtained by processing silicified limestones with acetic and formic acids. The sample was collected at the section of Entrago (Menéndez-Álvarez, 1978, 1991), exposed in an abandoned quarry 2 km northeast of the village of Entrago (Teverga, Asturias, Spain; Fig. 1), on the southeast side of the road As 228 (today the route of the road at this point has been slightly changed) linking the village of Trubia to Puerto Ventana (Ventana Pass).
The rocks exposed in the quarry range in age from Famennian (Late Devonian) to Serpukhovian (Namurian in terms of regional western European Carboniferous stratigraphy), comprising the Baleas, Alba and Barcaliente formations with a total thickness of some 33 m, although a more extensive sequence of Carboniferous rocks can be recognised in the neighbouring area. Cephalopods (Delépine, 1943; Kullmann, 1962; Wagner-Gentis, 1963), conodonts (Higgins in Higgins et al., 1964; Budinger & Kullmann, 1964; Pello, 1972; Menéndez-Álvarez, 1978, 1991; Higgins & Wagner-Gentis, 1982; Sanz-López et al., 2004 ), tintinids (Cuvillier & Barreyre, 1964), trilobites (Gandl, 1977) and ostracods (Bate, 1968; Sánchez de Posada, 1987) have been recorded from this section. Menéndez-Álvarez (1978, 1991) investigated the conodonts of the Alba Formation in detail, and the strata below and above it, and discovered a complete succession of conodont zones ranging from Tournaisian to lower Namurian (Serpukhovian) (Fig. 2).
Fig. 2. Upper part of the Alba Formation and base of the Barcaliente Formation at the Entrago Section. First occurrences of some important conodont species are depicted in the figure.
The ostracod here described comes from grey limestones (level J-2 of Menéndez-Álvarez, 1978, 1991) belonging to the San Adrián Member of the Alba Formation, although previous authors included it in the overlying Barcaliente Formation (Fig. 2). Conodonts found correspond to Gnathodus bilineatus bilineatus, G. bilineatus cf. bollandensis, Lochriea nodosa, L. mononodosa, Pseudognathodus homopunctatus, and L. costata (Menéndez-Álvarez, 1991, and new data). The presence of small elements of G. bilineatus cf. bollandensis in lower Serpukhovian beds of the Cantabrian Mountains was mentioned by Sanz-López et al. (2007). Two new mature elements associated with those small ones from sample J-2 show a shorter caudal parapet than that present in Gnathodus bilineatus bilineatus. Lower Namurian ammonoids (E1 Zone) coming from an unknown red nodular limestone bed below J-2 were reported by Wagner-Gentis (1963).