title: A new Ordovician arthropod from the Winneshiek Lagerstätte of Iowa (USA) reveals the ground plan of eurypterids and chasmataspidids.
abstract: 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 Palaeozoic forms, the synziphosurines-may not represent a natural group. Basal euchelicerates are rare in the fossil record, however, particularly during the initial Ordovician radiation of the group. Here, we describe Winneshiekia youngae gen. et sp. nov., a euchelicerate from the Middle Ordovician (Darriwilian) Winneshiek Lagerstätte of Iowa, USA. Winneshiekia shares features with both xiphosurans (a large, semicircular carapace and ophthalmic ridges) and dekatriatan euchelicerates such as chasmataspidids and eurypterids (an opisthosoma of 13 tergites). Phylogenetic analysis resolves Winneshiekia at the base of Dekatriata, as sister taxon to a clade comprising chasmataspidids, eurypterids, arachnids, and Houia. Winneshiekia provides further support for the polyphyly of synziphosurines, traditionally considered the stem lineage to xiphosurid horseshoe crabs, and by extension the paraphyly of Xiphosura. The new taxon reveals the ground pattern of Dekatriata and provides evidence of character polarity in chasmataspidids and eurypterids. The Winneshiek Lagerstätte thus represents an important palaeontological window into early chelicerate evolution.
authors.affiliation: Department of Geology and Geophysics, Yale University, 210 Whitney Avenue, New Haven, CT, 06511, USA. james.lamsdell@yale.edu.
authors.forename: James C
authors.initials: JC
authors.lastname: Lamsdell
authors.affiliation: Department of Geology and Geophysics, Yale University, 210 Whitney Avenue, New Haven, CT, 06511, USA.|Yale Peabody Museum of Natural History, Yale University, New Haven, CT, 06511, USA.
authors.forename: Derek E G
authors.initials: DE
authors.lastname: Briggs
authors.affiliation: Iowa Geological Survey, IIHR-Hydroscience and Engineering, University of Iowa, 340 Trowbridge Hall, Iowa City, IA, 52242, USA.
authors.forename: Huaibao P
authors.initials: HP
authors.lastname: Liu
authors.affiliation: Department of Earth and Environmental Sciences, University of Iowa, 121 Trowbridge Hall, Iowa City, IA, 52242, USA.
authors.forename: Brian J
authors.initials: BJ
authors.lastname: Witzke
authors.affiliation: Iowa Geological Survey, IIHR-Hydroscience and Engineering, University of Iowa, 340 Trowbridge Hall, Iowa City, IA, 52242, USA.
authors.forename: Robert M
authors.initials: RM
authors.lastname: McKay
mesh_terms: D000818:Animals; D001181:Arthropods / Q000033:anatomy & histology* / Q000145:classification*; D005580:Fossils / Q000033:anatomy & histology*; D007484:Iowa; D010802:Phylogeny
keywords: Dekatriata; Ground pattern; Microtergite; Phylogeny; Synziphosurine; Tagmosis