Benthic marine palaeoecology and recovery from the end-Triassic mass extinction in the Hettangian and Sinemurian (Early Jurassic) of southern Germany

This paper provides a palaeoecological analysis and interpretation of benthic marine communities from the first two stages of the Jurassic in southern Germany, based on quantitative data that represent more than 1,000 individuals from six samples. Alpha-diversity of these samples varies from eight to 48 species, with a tendency towards higher richness in the stratigraphically younger samples. Dominance (Simpson D) was relatively low in all samples, ranging from 0.06 to 0.24. In most samples, bivalves are the richest and most abundant taxon. The presence of gastropods is variable, ranging from complete absence to dominance in one sample (Betakalkbank; Sinemurian). Accessory taxa include brachiopods, crinoids and echinoids. The number of guilds varies from two to eleven; the most common guilds are epibyssally attached suspension feeders, semi-infaunal suspension feeders, reclining suspension feeders and locally epifaunal grazers/detritivores. Local factors that controlled the faunal composition include substratum, water depth and energy, and possibly oxygen content. However, cluster analysis also revealed a strong stratigraphic signal, which outweighs lithological similarities. We interpret this stratigraphic signal as a superordinated macroevolutionary pattern, which reflects the evolutionary changes that were associated with the recovery from the end- Triassic mass extinction. Based on the data from this paper, recovery lasted at least to the Sinemurian, which is in accordance with bivalve diversity data from the UK. Globally, data on benthic recovery are still scarce, but it seems that there were notable differences among palaeogeographic regions. In comparison to the recovery from the end-Permian mass extinction, we found a less extreme change in richness and dominance, with neither a long lag phase nor an explosive diversification thereafter. We interpret this difference as a result of the lower extinction magnitude, which did not shatter the structure of marine benthos to the same extent than the end-Permian event did 50 Ma earlier. Keywords: Early Jurassic; benthos; bivalves; palaeoecology; end-Triassic mass extinction; recovery; Germany