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Discussion on the calpionellid biozones and proposal of a homogeneous calpionellid scheme for the Tethysian Realm
Mohamed Benzaggagh

Building: Muséum d'Histoire Naturelle de Genève
Room: Amphithéâtre
Date: 2018-12-06 09:40 AM – 10:00 AM
Last modified: 2018-11-24

Abstract


Calpionellids constitute a group of planktonic microfossils usually occurring in large numbers in pelagic deposits from the late early Tithonian to Valanginian times, and showing a fairly uniform stratigraphic distribution all over the Tethysian Realm, from Argentina and Mexico to, at least, eastern Iran. Remane (1963, 1971) proposed a preliminary calpionellid scheme consisting of five zones and six subzones informally named: A (A1, A2, A3), B, C, D (D1, D2, D3), and E. This biozonal scheme based on specific associations, acme, and partial and/or total ranges, has represented so far a valid zonal framework towards a homogeneous and universal calpionellid scheme, only requiring changes at the level of subzones and lower subdivisions. Subsequently several works established new calpionellid frameworks that are often inconsistent and differing from one region to another. Such discrepancies noticeably hamper the purpose of calpionellids for biostratigraphy and long-distance correlations. These discrepancies between local schemes originated from the misguided choice of zonal indices: actually quite a number of subzones are grounded on either rare species with short ranges and sporadic appearances (e.g., Bermudezi, Andrusovi, Remanei, Praetintinnopsella,…), or rare and atypical forms (e.g., Catalanoi, Colomi, Doliphormis,…), or abundant long-ranged taxa (e.g., Longa, Remaniella, Cadischiana, Ferasini) in some stratigraphic levels that also yield species with more reliable stratigraphic value. Consequently, further consistent stratigraphical use of this fossil group requires a homogeneous framework with unambiguous definition of zones and subzones, preferably based on sound associations of two or more taxons (except for species with a typical form and a short range). The retained subzones should correspond to stratigraphic intervals wide enough to be identifiable in the widest possible geographical area. Their boundaries must correspond to major events or noteworthy changes in the composition of calpionellid assemblages. Short-duration events should be restricted to lower subdivisions. Consequently, it makes then sense to propose a coherent framework of 7 biozones, 15 subzones, and more than 5 intervals, all based upon the analysis of several dozens of sections from North Africa, France, and Iran, and the critical insights of former published works.