Mass mortality of clam shrimp (Crustacea, Branchiopoda) from the Lower Devonian (Emsian) Fossil-Lagerstätte of Consthum, Luxembourg—paleoecologic and taxonomic implications

The hitherto oldest known mass mortality of clam shrimp is described from the Early Devonian (Emsian) of Luxembourg. This (almost) monospecific clam shrimp association allows for a much more comprehensive assessment and understanding of preservational and ontogenetic variation in a single taxon, Pseudestheria diensti (Gross, 1934). This suggests that other taxa originally described from the “classical” Willwerath locality, the type locality of P. diensti, are variants of the latter, and thus Pseudestheria subcircularis Raymond, 1946 and Palaeolimnadiopsis ? eifelensis Raymond, 1946 are synonymized here with P. diensti. A further clam shrimp taxon, for which we propose a new species, Palaeolimnadia stevenbeckeri n. sp., is found in the same stratum, but not in the mass mortality layer itself. The clam shrimp mass mortality is interpreted to reflect sudden destruction of the original habitat on a delta plain and subsequent transport and burial in a marginal marine low-energy setting. Non-technical Summary Clam shrimp are branchiopod crustaceans with a fossil record starting in the Early Devonian and consisting mainly of their bivalved carapaces. A new Fossil-Lagerstätte from Luxembourg yielded the oldest known mass mortality of clam shrimp, providing new insights into the extent of morphological/preservational variation within a single species. This allows us to show that some previously described “species” fall within this variation and must therefore be considered synonyms. Other finds are described as a new species. We assume that these clam shrimp populated a freshwater-dominated delta platform and were rapidly buried in a marginally marine environment following the destruction of their original habitat, possibly by a storm.