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Upper Frasnian ammonoids and gastropods from Boudouda (Benahmed region, Moroccan Meseta)

Auteur : Söte Till, Becker Ralph Thomas
Année de Publication : 2021
Type : Article
Thème : Sciences de la terre
Couverture : Maroc

Résumé/Sommaire :

The upper Frasnian of the Boudouda Formation of the Benahmed region yielded a diverse, originally pyritized (secondarily limonitized) ammonoid fauna that resembles assemblages from contemporaneous goniatite shales of the Rhenish Massif (Germany: Büdesheim Formation). There are nine species each of the Tornoceratidae and Gephuroceratoidea (Gephuroceratidae and Ponticeratidae). New endemic taxa are Crassotornoceras boudoudense n. sp., Costornoceras multiseptatum n. gen. n. sp., which both fall in the Crassotornoceratinae n. subfam., and Costamanticoceras hybris n. sp. Tornoceras aff. contractum and Trimanticoceras n. sp. (based on fragments) are left in open nomenclature. Eight more species are recorded for the first time from North Africa, Aulatornoceras auris and Lobotornoceras ausavense are illustrated for the first time from Morocco. Associated are three bactritid species, bivalves, brachiopods, and the small gastropod Goniphilus delicatulus n. gen. n. sp. Based on Aulatornoceras auris, Lobotornoceras ausavensis, “Ponticeras” prumiense, and Manticoceras carinatum, the ammonoid succession falls in the lower (pre-Kellwasser) part of the upper Frasnian (UD I-I and IJ). The local absence of Beloceratidae was controlled by the hypoxic facies. At species level, the fauna includes 2/3 common taxa with the distant Büdesheim region, while the similarity with the closer and contemporaneous Anti-Atlas faunas (Tafilalt, Maïder, and Dra Valley) is only 1/3. The insufficiently known upper Frasnian goniatite faunas of other Meseta regions are also different. This unusual pattern can be explained to a large extent by biofacies differences but the lack of Frasnian outcrops in the southern Meseta is also well-known. There may have been a palaeobiogeographic barrier caused by strong Eovariscan block faulting and uplift that started in the middle/upper Givetian. It was obviously biogeographically more relevant than the wide, structurally complex western Prototethys (Variscan Sea) between the Meseta and Rhenohercynian Zone.

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