TESTING THE ‘MASS-MOULT-MATE' HYPOTHESIS OF EURYPTERID PALAEOECOLOGY

Eurypterids (Arthropoda: Chelicerata) are generally rare in the fossil record, but are sometimes found in great abundance and exceptionally preserved, for example, in the Late Silurian Bertie Group of New York State and Ontario. The mass-moult-mate hypothesis has been proposed to explain such occurrences, whereby eurypterids undertook mass migrations from their usual estuarine habitats into nearshore, tidal flat settings and lagoons to molt, mate and spawn—behavior analogous to that of living horseshoe crabs. This hypothesis was tested using measurements from over 600 Eurypterus specimens from three localities in the Bertie Group; Eurypterus remipes, from the Fiddlers Green Formation, and Eurypterus lacustris, from the overlying Williamsville Formation. Disarticulation patterns support previous evidence for molted assemblages, and the presence of larval, juvenile and adult instars at each locality indicates proximity to a breeding ground or nursery. A gender bias, similar to studies on modern Limulus populations, is noted in the significant predominance of the type “A” genital appendage at each locality. Therefore, using the traditional type “A”-as-female designation, a modified mass-mate-spawn-moult hypothesis has been proposed. An alternative hypothesis is also discussed in light of recent work that reverses the gender designations of the genital appendages. In this scenario, males competed for limited female mates, as in Limulus, allowing for an abundance of male exuviae. Further consideration is also given to the taphonomic conditions of the eurypterid-rich formations within the Bertie Group. Rather than representing long term, time-averaged assemblages, we suggest that the abundances of eurypterids in the Fiddlers Green and Williamsville Formations were the result of a taphonomic window that allowed for infrequent, but exceptional, preservation of exuviae due to burial during seasonal molting and mating events.