Graptolite biostratigraphy and biodiversity dynamics in the Silurian System of the Prague Synform (Barrandian area, Czech Republic)

Forty six graptolite biozones and seven subzones are recognized in the first compilation of high-resolution Silurian graptolite biostratigraphy in central Bohemia. The durations of these graptolite biozones, deduced from correlation with the Geological Time Scale 2020 age model and the global standard graptolite biozonation, range between ca 0.1 and 1.74 Myr. Each biozone is defined as an interval biozone named after the biozone index species, characterized by a typical biozone assemblage and delineated by bounding horizons with the stratigraphically lowest and/or highest occurrences of the respective index taxa. The Petalolithus folium Biozone is introduced as a replacement of the nearly equivalent Pribylograptus leptotheca Biozone, because the latter is a less common and less easily identified index taxon. Detailed range charts of 385 species of planktic graptolites based on in situ records from 88 localities and section logs in the offshore Silurian succession of the Prague Synform provide a solid data source for the proposed biozonal scheme and subsequent study of regional graptolite faunal dynamics, traced by means of species richness per biozone, mean standing diversity, time-normalized Van Valen's metrics, and FADs/LADs score per biozone. The moderate graptolite diversity of the lower Rhuddanian biozones rose to the mid-Aeronian maximum succeeded by stepwise decline forced by five globally recognized extinction events (mid-Aeronian sedgwickii Event, early Sheinwoodian murchisoni Event, mid-Homerian lundgreni Event, early Ludfordian leintwardinensis Event, and mid- Ludfordian kozlowskii Event). Sixth, early Telychian utilis Event is not observed in the Prague Synform as a results of a graptolite barren interval that separates linnaei and turriculatus biozones. Each mass extinction, although succeeded by recovery and adaptive radiation, resulted in a progressive step-wise reduction of graptolite diversity.

Automated summary

This article presents a high-resolution Silurian graptolite biostratigraphy study in central Bohemia, identifying 46 graptolite biozones and 7 subzones. The durations of these biozones range between approximately 0.1 and 1.74 million years. Each biozone is defined by a specific assemblage of species and bounding horizons. Detailed range charts of 385 planktic graptolite species provide valuable data for studying regional graptolite faunal dynamics. The article highlights the step-wise decline of graptolite diversity due to five globally recognized extinction events. The sixth event is not observed in the study area.