Ordovician and Silurian ichnofossils from carbonate facies in Estonia: A collection-based review

Abstract

Trace fossils are common in the Ordovician and Silurian shallow marine carbonate succession of Estonia, with 45 ichnofossil genera and five bioclaustration structures identified, representing 31 categories of architectural designs and nine categories of ethological classification. Diverse soft sediment traces, bioerosional traces and bioclaustrations occur both in the Ordovician and Silurian. Diversity of trace fossils is similarly high in the Late Ordovician and Silurian, but markedly lower in the Middle Ordovician. This could be explained by the fact that during the Late Ordovician, Baltica drifted to the subtropical climatic zone where ichnofauna is usually more diverse than in temperate climatic settings. In addition, the Great Ordovician Biodiversification reached its peak in the Late Ordovician for many groups of organisms, which further contributed to the increase in ichnodiversity. Distribution of trace fossils is also controlled by the type of sedimentation, so that the mixed carbonate-siliciclastic systems prevailing in the Late Ordovician and Silurian have higher ichnodiversity than the pure carbonate settings of Middle Ordovician age. Feeding and locomotion traces are relatively rare in the Ordovician and Silurian of Estonia with the exception of the feeding structure Arachnostega, which is formed inside of protective shells and therefore has abundant occurrences. Bioerosional trace fossils may be extremely common in places, with a large number of different genera in the Upper Ordovician, supporting the idea of the Ordovician Bioerosion Revolution.