Testing the ‘mass-moult-mate’ hypothesis of eurypterid palaeoecology

The eurypterids (Arthropoda: Chelicerata), some of the earliest arthropods to undertake amphibious excursions onto land, are generally rare in the fossil record, but are sometimes found in great abundance, for example in the Late Silurian Bertie Group of New York State. The mass-moult-mate hypothesis has been proposed to explain such occurrences, whereby eurypterids undertook mass migrations into near shore settings and lagoons to moult, mate and spawn, similar to the behaviour of living horseshoe crabs. This hypothesis is tested using measurements from over 600 Eurypterus specimens from three localities in the Bertie Group; Eurypterus remipes, from the Fiddlers Green Formation, and the slightly larger Eurypterus lacustris, from the overlying Williamsville Formation. Disarticulation patterns support previous evidence for moulted assemblages. A significant predominance of female exuviae is noted at each locality, unlike studies on modern Limulus populations. Therefore, a modified mass-mate-spawn-moult hypothesis is proposed here: males returned to deeper waters after mating, whereas females, having mated, remained at the breeding sites to deposit their eggs before moulting. After hatching, eurypterid larvae and juveniles remained in these spawning grounds until they matured and could move to deeper water, in comparison with Limulus. This hypothesis is also discussed in light of recent work on the gender determination of the type “A” and type “B” genital appendages.