Publication Date:
2011-11-01
Description:
The Liassic is generally and rightly considered to be a time period in the history of the Earth when reefal activity was restricted in comparison to the intervals that preceded and followed it [Kiessling et al., 1999]. Extinctions at the end of the Rhaetian and at the beginning of the Toarcian are largely responsible for this [Kiessling et al., 2007; Lathuilière and Marchal, 2005; 2009]. The Toarcian stage, in particular, is a time when coral reef-building was extremely limited and the little that we know is clearly insufficient [Beauvais, 1986]. The reefs described in the Moroccan Middle Atlas by Elmi et al. [2002] open a window into the Toarcian coral fauna, still poorly known throughout the entire planet. They were described in the Upper Toarcian of the Middle Atlas of Morocco. They are located in the Awragh-Afennourir syncline (western Middle Atlas, just at south of the tabular Middle Atlas, fig. 1). They consist of bioherms, 1 to 12 m thick, placed on top of tilted blocks (fig. 2). These reefs lie on a formation consisting of yellow bioclastic limestones, dated as Levisoni Zone Gemma Subzone, and are covered by a terebratulid bed, the bottom of which is assigned to the Upper part of the Meneghinii Zone [Elmi et al. 2002]. These reefs occurred at a paleolatitude of 18°N [Besse and Courtillot, 1991].A set of 18 coral samples collected by S. Elmi is identified and analyzed in this paper. The material was sampled from several reefs. In most cases the specimens are free of debris. I did not have the opportunity to access the outcrops myself; the samples were entrusted to me by S. Elmi. His approach was to sample taxa representative of the diversity of these reefs. Consequently, I had at my disposal only a few specimens per taxon and therefore open nomenclature is often preferred, in order to avoid the proliferation of specific taxa before having a clearer understanding of intraspecific variability. The material is deposited in the collections of the University H. Poincaré in Nancy (UHP).The taxa belong to the genera Cladophyllia, Diplocoenia?, Montlivaltia (two species), Thecosmilia (two species), Isastrea?, Latomeandra, Periseris, Microphyllia (three species) and Proleptophyllia (two species).Comparisons are performed with the only general monograph on Liassic corals from Morocco [Beauvais, 1986]. Among the 82 taxa described by this author, only 19 taxa are identified in the “Upper Liasssic” from South Rifian ridges or from the High Atlas. The fauna described here is derived completely from the Middle Atlas. Only three of 15 genera are in common between the two studies : Montlivaltia, Isastrea and Microphyllia. Some differences may be explained by particular ecologies (e.g. Ellipsoidastraea, Cardiastraea), but these differences must not be overinterpreted because the number of specimens is low in both studies.The samples consist of 12 colonies and 7 solitary forms. They include massive and meandroid forms having a high level of colonial integration and providing a strong argument for the presence of symbiotic zooxanthellae [Coates and Jackson, on 1987; Coates and Oliver, on 1973]. Accordingly, these observations indicate the location of these constructions in the photic zone. Subject to a sedimentological field analysis, mud-supported matrices associated with corals suggest environments not exposed to fair weather waves. Bioerosion is moderate (Gastrochaenolites, Entobia and worm-like traces of possible sipunculids).A sample including branching corals shows microbial crusts, with abundant sponge spicules. We cannot exclude a temporal shift between coral building and microbial settlement [Olivier et al. 2006]. Nevertheless, one observation worthy of generalization, involves the frequent development of pennular morphologies in corals, which suggests a rather mesotrophic reefal ecosystem [Dupraz and Strasser, 2002; Gill et al., 2004; Olivier et al., 2004].Based on a recent compilation of the stratigraphic distributions of genera [Lathuilière and Marchal, 2009], the present study substantially modifies our knowledge of the distribution for some genera. The first appearances of the genera Cladophyllia, Latomeandra and Periseris are now older, perhaps also for Diplocoenia. Conversely, the last appearance of the genus Proleptophyllia, previously known as Pliensbachian, is younger.In this initial examination, the coral reef fauna from the Toarcian of Middle Atlas seems rather diverse (9 genera identified for 18 specimens). These reefs with zooxanthellate corals lived in optimal conditions with regard to the latitude and the depth. This fauna has more features in common with the faunae of Dogger than with that of Pliensbachian. A more complete sampling is obviously needed to improve the taxonomic precision, describe the structure of the populations better and define more precisely the paleoenvironment of these reefs, which remain an exception as much temporal as geographical.
Print ISSN:
0037-9409
Electronic ISSN:
0037-9409
Topics:
Geosciences
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