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The base of the Tithonian Stage – historical review and state of the art

Schweigert G., Atropos F.

Volumina Jurassica

2006

Vol. 4, no. 1

213-214

When introducing the Tithonian Stage in a preliminary way, Oppel in 1865 listed several localities, which, according to him, yielded typical faunas of this age. The original list of Oppel comprises 117 taxa, mostly ammonites. The main localities which proliferated these taxa are: Rogoenik, Radzichov (Polish Carpathians), Ignaziberg, Koniakow, Willamowice, TeĘin (Silesia), Stramberk (Moravia), Trento, Rovereto (Southern Alps), Ruhpolding (Eastern Alps), Solnhofen, Eichstatt, Neuburg a. d. Donau (Bavaria), Nusplingen (Swabia), Grenoble, Val d'Ormonds (Switzerland), Chambery (Savoie), Boulogne-sur-Mer (Normandy), and Portland (Dorset). However, no type locality was ever selected. One of the localities in Oppel's list, Nusplingen, is not a time equivalent of the Solnhofen Limestones in Franconia, as suggested by Oppel in disregard of the completely different ammonite faunas. In the original definition of the Tithonian no zonal subdivision was presented. The first ammonite zone of the Tithonian, the Hybonotum Zone, was informally proposed by Benecke (1866, "Schichten des Ammonites hybonotus and lithographicus"), who focussed on the correlation between the Tithonian in the Tethyan Late Jurassic - mainly the Ammonitico Rosso Superiore of the Southern Alps - and its supposed coeval deposits in SW Germany. This Hybonotum Zone was an assemblage range zone, which included the taxa from the Diphyakalk of the Southern Alps, intercalated between the beds containing Aspidoceras acanthicum and the Early Cretaceaous Biancone, thus being approximately synonymous with the Tithonian Stage of Oppel. Neumayr introduced the Beckeri Zone in 1873 by subdividing the Acanthicum Zone. An increase of provincialism in the Late Jurassic forced the establishment of several independent zonal schemes and stages. Today, the Tithonian Stage starting with the Hybonotum Zone is regarded as standard for chronostratigraphy in the Jurassic. More recently several attempts for a definition of the Kimmeridgian/Tithonian boundary, the base of the Hybonotum Zone, were made. Sections in SW Germany were studied in detail but it was not possible to find suitable sites for the proposal of a GSSP. The only published suggestion is from Contrada Fornazzo near Castellammare in W Sicily, but the flatter section is rather poor in respect of its ammonite faunas and hence of little correlation value, especially in contrast to sections in SE France (Canjuers, Mt. Crussol). The latter provide rich, well-preserved faunas with a succession of chronospecies of Hybonoticeras (in ascending order): Hybonoticeras kamicense (Schopen) - H. n. sp. - H. pseudohybonotum Vigh - H. n. sp. (=autharis sensu Berckhemer & Holder) - H. hybonotum (Oppel) - H. n. sp. They are accompanied by numerous perisphinctids, oppeliids and aspidoceratids. The final decision of a GSSP for the base of the Tithonian Stage is a formal convention, in which these valuable data should be taken into account, and we warn of a premature decision