Gruendelella, a new Carboniferous ostracod genus from the Namurian of the Cantabrian Mountains (N. Spain)

Etching limestones of Arnsbergian (Namurian A) age from the northern slope of the Cantabrian Mountains (N. Spain) has yielded a diverse ostracod fauna. One of the forms found is especially interesting as it represents a new genus ressembling in some aspects the family Rectonariidae. The new genus Gruendelella is described.


INTRODUCTION
Recent data on Namurian ostracods from Western Europe are scarce. Since the pioneer studies of Jones and Jones & Kirkby in the second half of the nineteenth century, most of the work on marine Carboniferous ostracods has been on Dinantian forms and Westphalian faunas described mainly by Bless, Jordan & Michel (1969) and Bless (1974) collected in several localities in the Aegir Marine Band and its equivalents (cf. SBnchez de Posada, 1977 andSohn &Jones, 1984 for a review on this subject). Recently, Robinson (1978) gave a summary of selected British ostracod species ranging in age from Tournaisian to Chokerian.
Becker & Bless (in Becker, Bless & Kullmann, 1975) and Becker (1976) studied several ostracod faunas composed of the entomozoid genus Truyolsina coming from shales of Namurian A age from the Cantabrian Mountains and Sanchez de Posada (1974) found an interesting fauna of Namurian B age at the locality of MerC some 100 km east of Entrago, in the Nappes Province of the Cantabrian Mountains, which will be published shortly. Perhaps the most striking fact about the latter fauna is the presence, in this association (as it occurs at Entrago), of strongly spinose species, with an "old aspect" which leads the author (Shnchez de Posada, 1974 to compare it with ostracod faunas described by German authors (especially Grundel and Blumenstengel) from nodular limestones of Devonian and Dinantian ages.
Processing of samples for conodont study by J. R. MenCndez-Alvarez yielded a reasonably well preserved ostracod fauna of Namurian A age (Arnsbergian) including some twenty different species, which are currently being studied. One of them is especially interesting as it represents a new genus with some characteristics of the carapace (particularly the great length of the posterior spines) which gives it some external resemblance to a peculiar group of ostracods, the rectonarids.
This new genus, Gruendelella is considered to be a metacopid.

GEOLOGICAL SETTING
The Cantabrian Zone ( Fig. 1) is the most external area of the Iberian Massif. Cropping out throughout the area is a succession of Carboniferous rocks deposited in several sedimentary environments. The reader is referred to Truyols &Sanchez de Posada (1981) andMartinez Diaz (1983) for a summary of the Carboniferous stratigraphy from the Cantabrian Mountains.
The Early Carboniferous (Tournaisian to Arnsbergian) is represented by a rather uniform condensed sequence some 40m thick (Fig. 2). The lowermost Carboniferous rocks (Candamo or Baleas Formation) are light grey bioclastic limestones which have yielded conodonts of Upper Famenian age (costatus Zone) in the lower part (Pello, 1972;Budinger & Kullmann, 1964;Higgins et al., 1964;Rio & MenCndez-Alvarez, 1978) and of Upper Tournaisian age near the top. At some localities, the Candamo Formation is in part replaced by black shales (Vegamihn Formation) with conodonts, cephalopods and ostracods of Upper Tournaisian age near the boundary with the overlying formation. The Baleas or VegamiBn Formations are overlain by the "Caliza Griotte", a very distinctive horizon of red, pink or grey, nodular "wavy bedded" limestones with interbedded shales, usually with a remarkable siliceous member and containing cephalopods and conodonts of the Upper Tournaisian, Visean and Lower Namurian zone ages. The "Caliza de MontaAa" overlies the Alba Formation in most units of the Cantabrian Zone; only in the most internal structural units (those located towards the convex part of the Asturian Arc), are these limestones replaced by a turbiditc sequence. The lower part of the "Caliza de Montaiia" (Barcaliente Formation) is made up of dark laminated foetid limestones with a very poor fossil  content. Only conodonts indicating a Namurian B age for its upper beds and conodonts and cephalopods of Namurian A age (Eumorphoceras Zone), coming from transitional beds between Alba and Barcaliente Formations at several localities, are recorded in the literature.

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The ostracods described in this paper were obtained from a silicified limestone exposed in a quarry, 2 km N.E. of the village of Entrago (Teverga, Asturias), just on the S.E. side of the road linking Trubia (Oviedo) and Puente Orugo (Babia baja, Province of Leon) in the area called after Julivert (1967) "Region de Pliegues y Mantos" (Fold and Nappe Province) (Fig.  1B). The rocks exposed in this quarry range in age from Famenian to Arnsbergian (although a more extensive sequence of Carboniferous rocks can be recognised in the neighbouring area) comprising the Baleas, Alba and Barcaliente Formations with a total thickness of some 33m (Fig. 3).
The ostracod genus here described comes from grey limestones (level 5-2 of Menendez-Alvhrez) belonging to the zone of Gnathodus bilineatus bollandensis (bearing Gnathodus bilineatus bilineatus, G. bilineatus bollandensis, Paragnathodus cruciformis, P. commutatus, P. mononodosus and P. nodosus) which was considered by that author to be the base of the Barcaliente Formation. Remarks. The great length of the posterior spines in the only known species of the genus, the different size of the posterodorsal and posteroventral spines and, to some extent, the lateral outline of the carapaces are characteristics giving Gruendelella some resemblance to members of the family Rectonariidae Gruendel, 1962. In fact, some species belonging to this family are characterised by the unusual size of the spines and by the different development of the dorsal and ventral spines (cf. Blumenstengel, 1965;Gruendel, 1962). However, Gruendelella seems to have differences from the Rectonariidae both in internal and external characters. Rectoplacera Blumenstengel and Triplacera Grundel are the only rectonarid genera with their main spines in a posteroventral position. However, the type species of Rectoplacera (R. elongata Blumenstengel) and Triplacera (T. triquetra Grundel), as well as other species assigned to both genera, possess only one spine on one of their valves and not two as in the case of the genus here discussed. The dorsal spines of the known species of Rectoplacera are directed posterodorsally and the lateral outline of Gruendelella, Rectoplacera and Triplacera are different. In Rectoplacera the dorsal margin is arched or, if straight, it runs parallel to the ventral border and in Triplacera the dorsal and ventral margins converge strongly to the anterior end producing a roughly "subtriangular" outline. In the only known species of Gruendelella the dorsal border is straight and converges slightly with the ventral margin towards the posterior end. The limited knowledge of the internal characteristics of members of Rectonariidae (and as a consequence its controversial systematic position) hampers their comparison with the new genus here described.

SYSTEMATIC DESCRIPTION
The presence of a contact groove (presumably excavated on the outer lamella) in the larger valve of Gruendelella is considered indicative of metacopid affinities (see Adamczak, 1976 on the constitution of free margin in podocopids and metacopids). As far as I know, such a structure has not been described in rectonarids, which had been considered as belonging to the Podocopida by Grundel (1962) (on the basis of the close relationships between Rectoplacera and Triplacera and the existence of a calcareous inner lamella and a vestibulum in this genus), to the Metacopida by Blumenstengel (1965) (although he did not give additional details on the inner morphology), and as a probable heterogeneous group, but not metacopids, by Becker (1981) (although according to this author some species of Orthonaria have a clear metacopid aspect). The hinge of Gruendelella is quite different from the hinge of Rectonariidae described by Grundel (op. cit.) as being simple, without clear furrows or teeth. On the contrary, it is quite similar to that described and figured by Adamczak (1976)      Lateral surface smooth. Near the dorsal and ventral extremities of the posterior margin and at some distance from it, there are in each valve two spines of unequal size and directed backwards, slightly ventrally; the dorsal spine is always much shorter than the ventral. In specimens with unbroken spines the dorsal spine does not exceed 15% of the maximum length of the valve and the ventral one reaches 40 -60% of the maximum length. Along the anterior and posterior margins and passing along the dorsal and ventral surfaces there is a faint rim, which seems to be due to a faint thickening of the valve. This rim is more obvious at the anteroventral border.
In dorsal and ventral views the sides of the carapace are slightly convex, with two small concavities near the anterior and posterior ends. The outline of the carapace is rhomboidal, very asymmetrical with regard to the minor (= dorsoventral) akis; the posterior sides being much shorter than the anterior ones. The spines lie at the dorsal and ventral vertex of the posterior side. Maximum width posterior, near the area of insertion of spines. Hinge slightly depressed.
In spite of the masking of these structures by the silification, the hinge can be studied in some specimens. In the left valve it consists of two terminal sockets extending towards the central part of the hinge so that the central bar is much more reduced or even absent. In the right valves the hinge is not as well preserved. Apparently, it consists of a hinge groove and cardinal lists. The left valves are provided with a contact groove which seems to be interrupted ventrally.
Left valve larger than the right and overlapping it. In the only complete carapace found the overlap is maximum at the ventral margin and very reduced along the remainder of the outline.

REMARKS
At the locality of Entrago, Gruendelella was found at the base of the Barcaliente Formation, which comprises dark, bedded, micritic, foetid limestones, very poorly fossiliferous and possibly deposited in a quiet and poorly oxygenated environment. The new genus is a part of a peculiar ostracod fauna mainly made of Triplacera, Healdiopsis? , healdiids, primitive bythocytherids, tricorninids, bairdiids, quasillitids and cladocopines, associated to conodonts and primitive and little diverse foraminifers. Some of these forms are peculiar ostracods mainly known from nodular limestones of Devonian and Dinantian strata from the German Democratic Republic and had not been described from the usual faunas of Carboniferous age of North America and Europe.
The position and direction of the spines of Gruendelella resembles that of healdiids, but the constitution of the hinge prevents a close relationship being assumed between the new genus and those organisms.
The great size of the posterior spines of Gruendelella may be related to the great development of these attributes of the carapace in some associations of ostracods (Thuringian associations, "Thuringer Okotype" of Becker, 1975, in Band1 & Becker). In spite of the lack of knowledge of the inner morphology of rectonarids (one of the typical members of Thuringian ostracod association provided long posterior or dorsoposterior spines), Gruendelella seems to have a morphology sufficiently different to be considered as a member of a different family.
Progress in the research of Carboniferous ostracods from facies like that of the locality of Entrago or somewhat similar (sediments deposited in quiet waters containing at least a moderate amount of pelagic fossils) will disclose the extent of the geographical distribution of Gruendelella, its facial range and stratigraphic usefulness. At the moment, Gruendelella remains one of the few ostracod genera known from limestones with silicified faunas in Western Europe.