Early Ordovician sea scorpions from Morocco suggest Cambrian origins and main diversification of Eurypterida

Eurypterida were a diverse clade of aquatic euchelicerates that occupied environments ranging from freshwater to fully marine and included several of the largest euarthropods on record. Although a Middle Ordovician megalograptid hitherto represented the oldest evidence of this clade, its phylogenetic position suggested an earlier history for the origin and main diversification within Eurypterida. Here, we report unequivocal eurypterid fragments from the Early Ordovician Fezouata Biota of Morocco, pre-dating the previously oldest record of this group by 12-15 million years. We describe ?<i>Carcinosoma aurorae</i> n. sp. based on several distinctively spinose isolated appendages diagnostic of the eurypterine clade Carcinosomatidae. This discovery demonstrates that the major morphological and ecological diversifications within Eurypterida between swimming Eurypterina and benthic crawling Stylonurina had taken place by the Early Ordovician. Furthermore, the derived phylogenetic position of carcinosomatids implies that most eurypterine clades had already diversified by that time. A cuticle patch with dense scales, reminiscent of pterygotids, likely belongs to a second eurypterid species. The remarkable diversity of euchelicerates in the Fezouata Biota indicates undocumented Cambrian origins and provides further evidence for an early eurypterid radiation centred off Gondwana. Significantly, the sister-group relationship between Eurypterida and Arachnida entails equally early arachnid origins.