The Lyckholm acme of cephalopods – Review of the late Katian (Vormsi–Pirgu regional stages) Ordovician cephalopods of Estonia

A revision of more than 660 specimens of fossil cephalopods from the Vormsi–Pirgu regional stages (late Katian Stage), Ordovician Period, of Estonia, available from the Estonian palaeontological collections, reveals the extraordinarily high cephalopod-richness of this interval, termed “Lyckholm acme of cephalopods” herein. Ninety species can be distinguished, belonging to 35 genera, and 17 families. The high turnover between assemblages from the Vormsi and Pirgu stage strata is remarkable. Only 16 species range through both stages, where the former is dominated in abundance by tarphycerids and orthocerids, and the latter by multiceratoids, probably reflecting pronounced palaeo-environmental changes. Species abundance distributions from the main collection localities show differences in sampling practice and give evidence for potential problems in species delineation. Difficulties in the species delineation of, e.g., Discoceras exist because critical morphological details are commonly not preserved. A cluster analysis and comparison of the Estonian assemblage with assemblages of other regions reveals a high similarity with that of the Laurentia-palaeocontinent, a fact which suggests a possible connection of the Lyckholm-acme with the “Richmondian Invasion” of Laurentia. The revision, presented herein, is a first step toward an in-depth global-scale analysis of the spatiotemporal patterns of Katian cephalopod occurrences, in which this hypothesis could be tested.