The mid-Homerian (Silurian) biotic crisis in offshore settings of the Prague Synform, Czech Republic: Integration of the graptolite fossil record with conodonts, shelly fauna and carbon isotope data

Abstract

The middle Homerian biotic crisis resulted in the almost complete eradication of graptolites. The shale-dominated Kosov Quarry section, central Bohemia, preserves the most complete graptolite record across the crisis in peri-Gondwanan Europe. The pre-extinction graptolite assemblage of the upper lundgreni Biozone, composed of ten species vanished in three extinction phases recognized in an interval 1.6 m thick. The crisis commenced with the increasing dominance of generalist taxa and subsequent extinction of several abundant species including Cyrtograptus lundgreni. The second phase coincided with the extinctions of genera Cyrtograptus and Testograptus, whereas long-ranging, generalist monograptids prevailed before they became extinct as the crisis culminated in its third phase in the flemingii Biozone. The lower part of the overlying parvus Biozone contains only Pristiograptus parvus, which became abundant in the upper part of the biozone, together with incoming Gothograptus nassa. The recovery interval is marked by a moderate diversification of monograptids and retiolitids. The extinction did not affect the diversity of pelagic cephalopods although their abundance was reduced. A bivalve-dominated benthic fauna disappeared throughout the extinction interval and re-appeared not earlier than in the latest Homerian. It was temporarily replaced by a time-specific fauna of anachronistic trilobites and brachiopods. The extinction interval coincided with sea-level fall, indicated by limestone slump-beds in a generally shaly succession. The post-extinction interval corresponds with a lowstand systems tract with deposition of condensed shelly limestone and burrowed shale. A positive carbon isotope excursion started in the flemingii Biozone and δ13C values increased up to the lower parvus Biozone above which the plateau of the first peak started. The beginning of the graptolite extinction predated the early phase of the late Homerian carbon isotope excursion. The terminal phase of the extinction, nevertheless coincided with the onset of the carbon isotope excursion and change in benthic fauna.