Convergent evolution of giant size in eurypterids

Eurypterids, Paleozoic marine and freshwater arthropods commonly known as sea scorpions, repeatedly evolved to remarkable sizes (over 0.5 m in length) and repeatedly colonized continental aquatic habitats. We compiled data on the majority of eurypterid species and explored several previously proposed explanations for the evolution of giant size in the group including the potential role of habitat, sea surface temperature and dissolved sea surface oxygen levels using a phylogenetic comparative approach with a new tip-dated tree. Overall, there is no compelling evidence that the evolution of giant size was driven by temperature or oxygen levels, nor that it was coupled with the invasion of continental aquatic environments, latitude, or local faunal diversity. Eurypterid body size evolution is best characterized by rapid bursts of change that occurred independently of habitat or environmental conditions. Intrinsic factors likely played a larger role than previously recognized in determining the convergent origin of gigantism in eurypterids.