Tremadocian microbrachiopods from the southern Montagne Noire (France) and their paleobiogeographical significance
1 Departamento de Ciencias de la Tierra (Paleontología). Universidad de Zaragoza. 50009 Zaragoza. Spain. E–mail: firstname.lastname@example.org
Key words: Microbrachiopods. Tremadocian. Paleobiogeography. Montagne Noire. France.
Phosphate-shelled brachiopods from the Tremadocian of the northwestern Gondwana margin are well known in Bohemia (Barrandian Basin), with nearly 30 described species (Havlícek, 1982; Mergl, 1994, 2002) and also from the Anglo–Welsh Basin with about one dozen described species (Cocks, 1978; Sutton et al., 1999, 2000). Outside these areas, the only european regions originally placed at the northwestern Gondwana margin where Lower Ordovician phosphatic brachiopods have been described from are Germany (Sdzuy, 1955) and the Montagne Noire (Havlícek, 1980) with only one described species from the latter region.
This paper presents the study of new phosphatic microbrachiopods found in rare carbonate lenses and beds dispersed in lower–middle Tremadocian successions of the southern Montagne Noire, which allow to widen the biogeographical distribution of this group on the Mediterranean Province stretching from present-day Turkey to NW Argentina (Spjeldnaes, 1961). The results presented here should be considered as preliminary because of difficulties in taxonomical identification; this is due to the strongly distorted character of the etched material, and the scarcity and incompleteness of some shells.
Geological and stratigraphical setting
The stratigraphical sections that have yielded the new material are placed in the Pardailhan nappe (southern Montagne Noire, southern France; Figure 1A). The studied faunas occur (i) in the ‘Combes de Barroubio’ area, in limestone levels of the Mounio Formation (up to 30 m thick), which consists of alternating reddish and purple, shales and limestones, the latter changing laterally into dolomites; and (ii) in the vicinities of Saint–Martial, in isolated limestone nodules of the Saint Chinian Formation (Figure 1B). The presence of the trilobite Shumardia pusilla in these levels (Courtessole and Pillet, 1978; Vizcaïno et al., 2001) allows correlation with the mid Tremadocian of England and Norway (Brögger, 1882; Lake, 1907).
Palaeontological study and palaeobiogeographical remarks
The acid etching of the limestone levels from both sections has yielded a microbrachiopod assemblage composed of the taxa Acrotreta aff. dissimilis, Broeggeria sp., Elkania? sp., Eurytreta sp., Obolidae gen. et sp. indet., Orbithele sp., Schizambon sp. and Siphonotretella filipi. The most abundant is Eurytreta (68%), followed by Obolidae gen. et sp. indet. (25%) while the five resting taxa only represent the 7%.
Figure 1. A. Geological sketch of the Southern Montagne Noire with location of Combes de Barroubio and Saint–Martial sections (after Guérangé–Lozes and Burg, 1990). B. Synthetic columnar section of lower-middle Tremadocian succession in Saint-Martial outcrop with location of the fossiliferous level.
Cambro–Ordovician linguliformean brachiopods are generally widely dispersed through equatorial and tropical latitudes, so that they are usually considered of small interest for palaeobiogeographical analyses. Even more, they may be more substrate and depth related than climate related (Mergl, 2002). Nevertheless, little is known about those that thrived in high-latitude regions, as the northwestern Gondwana margin, during early Ordovician times. Despite their high-latitude setting, the occurrence of carbonate lenses and beds in the Lower Ordovician of the Montagne Noire would suggest temperate waters in this particular region of North Gondwana. Probably, this has permitted the survival in this area of taxa typical of low-latitude regions, such as Acrotreta, Broeggeria, Elkania, Orbithele and Siphonotretella. Nevertheless, the closest affinities of the studied fauna are with Bohemia, as could be expected, since all the known genera from Montagne Noire are also known from this region, except Eurytreta (although this is possibly a problem of sampling). One identified species, Siphonotretella filipi, is known in Bohemia evidencing the connections between this area and the Montagne Noire. Outside the northwestern Gondwana margin, the main biogeographical affinity is with Baltica as can be deduced from the common occurrence to both areas of almost all the genera and the species Acrotreta aff. dissimilis. The former palaeobiogeographical conclusions agree with those proposed by Havlícek et al. (1994), Havlícek (1999) and Sdzuy et al. (2001) for the Mediterranean Province. These authors described Tremadocian endemic faunas (trilobites and calcite- and phosphate-shelled brachiopods) but also with significant Baltic influences. Baltica should have been placed opposite to the Barrandian Basin and this, in turn, placed at the periphery of Gondwana.
This is a contribution to the project PB98–1625. C. González–Gómez receives a predoctoral research grant from the Ministerio de Ciencia y Tecnología.
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Received: February 15, 2003
Accepted: June 15, 2003