Eurypterid palaeoecology: palaeobiological, ichnological and comparative evidence for a ‘mass–moult–mate’ hypothesis

Eurypterid (Eurypterida, Chelicerata) palaeoecology is reviewed, in light of a statistical analysis of 74 Silurian and Early Devonian eurypterid assemblages. Kjellesvig-Waeringʹs three eurypterid ‘biofacies’ model is rejected as this analysis indicates considerable mixing of eurypterids from each supposed biofacies. A new model is proposed, which regards each eurypterid genus as having a broad palaeoecological range, determined by several environmental and palaeophysiological criteria, including an ‘ontogenetic gradient’, whereby juveniles preferentially adopted marginal and near shore habits (here interpreted as representing a true population distribution and not a taphonomic or collecting bias). A ‘mass–moult–mate’ hypothesis is proposed, which depicts eurypterids as having migrated en masse into nearshore and marginal environments (e.g. lagoons) to moult and mate. This hypothesis is evidenced by: (i) the occurrence of abundant accumulations of eurypterid remains (e.g. the Bertie and Saarema assemblages), which are interpreted as concentrations of exuviae in the absence of any evidence that these remains represent mass mortalities. (ii) Eurypterid respiratory and reproductory palaeobiology (i.e. eurypterids possessed accessory aerial respiratory tissues, enabling amphibious excursions and they were capable of spermatophore exchange and storage, allowing the delay of fertilisation of their eggs). (iii) Occurrences of abundant, variously sized, sub-parallel, eurypterid trackways, indicating migrations en masse, in a similar direction, across marginal environments. Comparisons with modern analogues (semi-terrestrial crabs and xiphosurans) are made, which employ a similar behaviour.