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Jurassic ammonites from Central Saudi Arabia (Jebel Tuwaiq and adjacent areas)

Enay, Raymond; Mangold, Charles

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Revue de Paléobiologie
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The mapping of Phanerozoic rocks at 1:250000 scale by the Saudi Arabian Deputy Ministry for Mineral Resources and the French Bureau de Recherches Géologiques et Minières, since 1980, covered most of the Jurassic outcrops in central Saudi Arabia. Stratigraphic, sedimentological and palaeogeographic studies provided a precise framework for the ammonite faunas which are the main purpose of the present monograph. These faunas have already been applied by the authors to build a biochronological scale specific to the Arabian province correlated with that of Western Europe, very different from the former scales of Arkell and Imlay. A first result of the palaeontological study is an update of the Arab biochronological scale (based on the faunas observable in the field) and correlations with Western Europe. In this paper 17 zones or horizons are recognized which, for the most part, are based on taxa specific to the Arabian Province, with two exceptions for which the low diversity forced to choose a Western Europe species (Stephani Zone and Athleta Horizon). The ammonites studied are more than 2143 specimens, of which 2013 could be determined on at least a generic level, and whose ages range from the Early Toarcian up to the Early Kimmeridgian. About 104 species are described (28 in open nomenclature), of which 10 are new. These are distributed among 45 genera including 3 new (Eoermoceras, Saudisphinctes and Alienisphinctes), within 14 families. These 14 families are represented, very unequally in relation to each other, but also for each of them in terms of the present and/or dominant genera. The five best represented families are the Hildoceratidae of the Toarcian (7 species), the Sonniniidae in the Bajocian (12 species, including 2 new ones), the Stephanoceratidae in the Bajocian (20 species including 15 for the Ermoceratinae, a new genus with 2 new species, and 2 other possibly new species), the Bajocian-Bathonian Oppeliidae (18 species, dominated by the Arabian genera Micromphalites, Thamboceras and Thambites, and Rebouliceras), and the Callovian Pachyceratidae (20 species) with two dominant Arabian genera, Pachyerymnoceras and Kurnubiella. The other families, Graphoceratidae, Hammatoceratidae, Strigoceratidae, Sphaeroceratidae, Spiroceratidae, Perisphinctidae, Tulitidae, Aspidoceratidae, and Ataxioceratidae (with a new genus and species), although less represented, include several forms known from Western Europe. These ammonites have a determinant role in correlations between the Arabic and European zonal scales. The palaeoenvironmental and palaeobiogeographical setting of the Arabian ammonite faunas is the purpose of the third part of the monograph. The geographic distribution and composition of the successive faunas compared with the palaeoenvironmental conditions are dealt with, as well as the monospecific or monogeneric composition opposite to the more or less diversified faunas. The endemic character of the Arabian types, and their migration outside the Arabian Province which, together with the arrival in the Arabic domain of NW and/or Indian-Madagascan taxa, played a major role in the correlation of the biostratigraphical scales.

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