ENCRUSTING COMMUNITIES IN THE LIGHT OF THE FRASNIAN-FAMENNIAN (LATE DEVONIAN) MASS EXTINCTION

Comprehensive data on encrusting organisms at the community level through the Late Devonian Frasnian-Famennian mass extinction is lacking. This is especially a result of the scarcity of suitable facies containing hard substrates. The Central Devonian Field in the European part of Russia is an excellent area to study this interval because the upper Frasnian and lower Famennian deposits contain abundant and well-preserved brachiopods that acted as a secondary hard substrates for various encrusters.

During the preliminary field reconnaissance in Russia, late Frasnian (Late rhenana Chron) and early Famennian (crepida Chron) brachiopods were collected and investigated with respect to encrustation patterns. The pre-extinction late Frasnian brachiopods are encrusted by tubeworms (cornulitids and microconchids), foraminifers, tabulate and rugose corals, trepostome bryozoans and problematic Ascodictyon. In the assemblages, cornulitids dominated, followed by microconchids and foraminifers. In the post-extinction recovery interval during the early Famennian crepida Chron, on the other hand, the assemblages were dominated by microconchids, while cornulitids and other taxa were much less abundant. Corals and foraminifers were absent.

The crepida Chron in the Central Devonian Field was a time of a major transgression following the regression that occurred at the Frasnian-Famennian transition. This is why the lowermost Famennian deposits of the triangularis Zone are missing in the area. During the transgression, the brachiopods, dominated by Ripidiorhynchus and Cyrtospirifer, rebounded and the new encrusting assemblages started to occupy the hard substrates available. Microconchids flourished in large numbers as a single species, dominating the whole community. It is worth noting that the encrusting community structure during the post-extinction recovery interval (crepida Chron) is similar to the later stages of recovery (Spathian) from the end-Permian mass extinction when microconchids still dominated while the associated encrusters were minor.