A new Ordovician arthropod from the Winneshiek Lagerstätte of Iowa (USA) reveals the ground plan of eurypterids and chasmataspidids

摘要: Euchelicerates were a major component of Palaeozoic faunas, but their basal relationships are uncertain: it has been suggested that Xiphosura—xiphosurids (horseshoe crabs) and similar forms, the synziphosurines—may not represent natural group. Basal euchelicerates rare in fossil record, however, particularly during initial Ordovician radiation Here, we describe Winneshiekia youngae gen. et sp. nov., euchelicerate from Middle (Darriwilian) Winneshiek Lagerstatte Iowa, USA. shares features with both xiphosurans (a large, semicircular carapace ophthalmic ridges) dekatriatan such as chasmataspidids eurypterids (an opisthosoma 13 tergites). Phylogenetic analysis resolves at base Dekatriata, sister taxon to clade comprising chasmataspidids, eurypterids, arachnids, Houia. provides further support for polyphyly synziphosurines, traditionally considered stem lineage xiphosurid horseshoe crabs, by extension paraphyly Xiphosura. The new reveals ground pattern Dekatriata evidence character polarity eurypterids. thus represents an important palaeontological window into early chelicerate evolution.