Quantitative analysis of the late Famennian and early Tournaisian ammonoid stratigraphy

The occurrences of 65 late Famennian (Late Devonian) ammonoid species from twelve sections and 52 early Tournaisian (Early Carboniferous) ammonoid species from seven sections in the Rhenish Mountains are analysed for their succession. Three quantitative stratigraphical methods are used, (1) Unitary Associations (UA), (2) Ranking and Scaling (RASC) and (3) Constrained Optimization (CONOP) to test the quality of the existing ammonoid zonation and to evaluate which of the three methods is best suited for such a test. Principally, the UA, RASC and CONOP methods lead to similar outcomes with respect to the succession of occurrence events. On the basis of the results of the three analyses, the existing modern ammonoid zonation can be confirmed for the late Famennian and the early Tournaisian. The most suitable method for biostratigraphical analyses depends on the data available and the purpose of the investigation. The UA method is considered to be the most suitable for the analysed data because the result perfectly mirrors the empirical data from the single outcrops. Using the obtained robust biozonation, a diversity analysis across the study interval reveals four clearly separable units with respect to species richness as well as extinction, origination and thus turnover rates: A low diversity, lower part (traditional Clymenia Stufe) with high turnover rates is followed by a symmetrical diversity rise and fall with a maximum of 27 species in the traditional Wocklumeria Stufe paralleled by low turnover rates, the interruption by the mass extinction at the Hangenberg Event with slow recovery in the aftermath, and the Gattendorfia Stufe with rapid re-installation of the pre-extinction diversity and a stable high species richness and low turnover rates.