Sci Nat (2015) 102: 63 DOI 10.1007/s00114-015-1312-5 ORIGINAL PAPER A new Ordovician arthropod from the Winneshiek Lagerstätte of Iowa (USA) reveals the ground plan of eurypterids and chasmataspidids James C. Lamsdell1 & Derek E. G. Briggs 1,2 & Huaibao P. Liu3 & Brian J. Witzke4 & Robert M. McKay3 Received: 23 June 2015 /Revised: 1 September 2015 /Accepted: 4 September 2015 /Published online: 21 September 2015 # Springer-Verlag Berlin Heidelberg 2015 Abstract Euchelicerates were a major component of xiphosurid horseshoe crabs, and by extension the paraphyly of Palaeozoic faunas, but their basal relationships are uncertain: Xiphosura. The new taxon reveals the ground pattern of it has been suggested that Xiphosura—xiphosurids (horseshoe Dekatriata and provides evidence of character polarity in crabs) and similar Palaeozoic forms, the synziphosurines— chasmataspidids and eurypterids. The Winneshiek may not represent a natural group. Basal euchelicerates are Lagerstätte thus represents an important palaeontological win- rare in the fossil record, however, particularly during the initial dow into early chelicerate evolution. Ordovician radiation of the group. Here, we describe Winneshiekia youngae gen. et sp. nov., a euchelicerate from Keywords Dekatriata . Ground pattern . Microtergite . the Middle Ordovician (Darriwilian) Winneshiek Lagerstätte Phylogeny . Synziphosurine . Tagmosis of Iowa, USA. Winneshiekia shares features with both xiphosurans (a large, semicircular carapace and ophthalmic ridges) and dekatriatan euchelicerates such as Introduction chasmataspidids and eurypterids (an opisthosoma of 13 ter- gites). Phylogenetic analysis resolves Winneshiekia at the base Euchelicerates, represented today by xiphosurids (horseshoe of Dekatriata, as sister taxon to a clade comprising crabs) and arachnids (scorpions, spiders, ticks, and their rela- chasmataspidids, eurypterids, arachnids, and Houia. tives), comprise chelicerates (arthropods with cheliform Winneshiekia provides further support for the polyphyly of deutocerebral appendages) with a cephalic tagma of six ap- synziphosurines, traditionally considered the stem lineage to pendages united beneath a dorsal carapace (Weygoldt and Paulus 1979). During the Palaeozoic, four major lineages of Communicated by: Sven Thatje euchelicerate were present: xiphosurids, eurypterids, arach- nids, and the rare chasmataspidids (Dunlop 2010). During this Electronic supplementary material The online version of this article (doi:10.1007/s00114-015-1312-5) contains supplementary material, time, euchelicerates were major components of both aquatic which is available to authorized users. and terrestrial faunas and some groups occupied the role of apex predators (Lamsdell and Braddy 2010). The earliest con- * James C. Lamsdell firmed euchelicerates are xiphosurids from the Lower [email protected] Ordovician (Tremadocian) of Morocco (Van Roy et al. 2010) while potential chasmataspidid specimens from the late 1 Department of Geology and Geophysics, Yale University, 210 Cambrian (Dresbachian) of Texas await description (Dunlop Whitney Avenue, New Haven, CT 06511, USA et al. 2004). There is a large gap in the fossil record between 2 Yale Peabody Museum of Natural History, Yale University, New the Tremadocian occurrences and the major radiation of the Haven, CT 06511, USA four euchelicerate clades in the Silurian, occupied by fewer 3 Iowa Geological Survey, IIHR-Hydroscience and Engineering, than ten eurypterid species (Lamsdell et al. 2013a), a University of Iowa, 340 Trowbridge Hall, Iowa City, IA 52242, USA xiphosurid species from the Airport Cove locality (Katian) 4 Department of Earth and Environmental Sciences, University of of Manitoba, Canada (Rudkin et al. 2008), and a Iowa, 121 Trowbridge Hall, Iowa City, IA 52242, USA chasmataspidid species from Sevier County (Middle 63 Page 2 of 8 Sci Nat (2015) 102: 63 Ordovician) of Tennessee, USA (Dunlop et al. 2004). locality on the Upper Iowa River near Decorah, but only the Furthermore, until recently, there were no convincing stem upper 4 m has been systematically collected due to the limited lineage taxa for any of these clades; the affinities of the sup- thickness accessible. The Winneshiek Shale is confined to the posed proto-eurypterid Kodymirus have been shown to lie Decorah Impact Structure, a circular basin about 5.6 km in with artiopods (Lamsdell et al. 2013b), and the phylogenetic diameter in the Decorah area which is interpreted to be the position of potential stem Barachnomorph^ taxa such as result of a meteorite impact (Liu et al. 2009; McKay et al. strabopids is equivocal (Tetlie and Moore 2004). This 2011). Palaeogeographic and palaeoenvironmental studies in- changed, however, with a revision of the Xiphosura. dicate that the crater was located in marginal to nearshore Morphological reappraisal and cladistic analysis of marine conditions (Liu et al. 2007, 2009;Witzkeetal. Palaeozoic synziphosurines—Bhorseshoe crabs^ with freely 2011). The fauna is dramatically different from a normal ma- articulating opisthosomal tergites—suggested that they repre- rine shelly fossil fauna, indicating a restricted environment sent a polyphyletic grouping of crown and stem inhospitable to typical marine taxa (Liu et al. 2006, 2007, Euchelicerates (Lamsdell 2013) rather than the stem lineage 2009). to Xiphosurida as previously supposed (Anderson and Selden The specimens described here were collected from two 1997). This new interpretation is supported by new discover- sources: blocks of shale eroded during flooding (labeled ies (Selden et al. in press) and further phylogenetic analysis WL), and a 4-m section excavated from the riverbed (labeled (Garwood and Dunlop 2014), but this has not resolved the WS). The specimens are partially disarticulated individuals issue of Ordovician ghost ranges, as the majority of preserved with cuticle which survives as a brown organic film. synziphosurine taxa occur in the Silurian and Devonian, with The specimens are accessioned in the Paleontology the only Ordovician form known from the Tremadocian of Repository, Department of Earth and Environmental Fezouata (Van Roy et al. 2010). Sciences, University of Iowa (SUI). Specimen SUI 140290 Here, we describe a euchelicerate arthropod from the was photographed using a Leica DFC420 digital camera at- Middle Ordovician Winneshiek Lagerstätte of Iowa. The tached to a Leica MZ16 stereomicroscope, and SUI 140291, new taxon, Winneshiekia youngae gen. et sp. nov., possesses SUI 140289, and SUI 140288 were photographed using a 13 freely articulating opisthosomal segments, a count charac- Canon EOS 60D digital camera with a Canon EF-S 60 mm teristic of eurypterids and chasmataspidids, and a large, semi- f/2.8 Macro USM lens. All specimens were imaged dry and circular, vaulted carapace and ophthalmic ridges, features pre- under incandescent light. Image cropping and levelling was viously considered to unite synziphosurines and xiphosurids. carried out using Adobe Photoshop CS5, and interpretive This mix of characters suggests a phylogenetic position inter- drawings were prepared with Adobe Illustrator CS5, on a mediate between xiphosurids and the other euchelicerate MacBook Pro running OS X. clades. Winneshiekia fills a stratigraphic gap in the early euchelicerate fossil record and provides further support for the paraphyly of Xiphosura as traditionally defined. Results Material and methods Systematic palaeontology The specimens are from the Middle Ordovician (Darriwilian) Chelicerata Heymons, 1901 Winneshiek Lagerstätte of northeastern Iowa (Liu et al. 2006, Euchelicerata Weygoldt and Paulus, 1979 2009). They are part of an unusual fauna including well- Dekatriata Lamsdell, 2013 preserved conodonts (51 % of specimens), phyllocarids Winneshiekia youngae gen. et sp. nov. (Figs. 1, 2, 3,and4) (7.9 %), eurypterids (6.6 %), and other bivalved arthropods Etymology. The genus is named for the Winneshiek Shale, (1.6 %)—arthropods are the most important invertebrate the formation in which it is found. The species is named in group (Briggs et al. in press). Excavation of the Winneshiek honor of Jean N. Young (1933–2007), geologist at Luther Shale yielded over 5000 fossil specimens (n=5354) of which College, who played an important role in the discovery of only four represent the taxon under consideration here. the Winneshiek Shale. The Winneshiek Shale is a greenish brown to dark gray Stratigraphical range and distribution. Middle Ordovician laminated sandy shale 18 to 27 m thick (Wolter et al. 2011; (Darriwilian) Winneshiek Lagerstätte, Winneshiek Shale McKay et al. 2011) which overlies an unnamed stratigraphic Formation, Winneshiek County, IA, USA. unit of thick massive breccia. The Winneshiek Shale is Material. Holotype: SUI 140288 (part and counterpart), disconformably overlain by the St. Peter Sandstone (Liu partial carapace, and complete opisthosoma of tergites 1–13. et al. 2006, 2009). Borehole data indicate that the total thick- Paratypes: SUI 140289 (part and counterpart), SUI 140290, ness of the Winneshiek Shale is about 18 m at the outcrop SUI 140291. Sci Nat (2015) 102: 63 Page 3 of 8 63 Fig. 1 Winneshiekia youngae, SUI 140288 (holotype): a part, b counterpart, and c interpretive drawing. Solid lines represent morphological features, whereas dashed lines indicate broken margins. CA carapace, LE lateral eye, MO median ocelli, T1–T13 tergites 1–13, TE telson. Scale bars 2mm Diagnosis. Euchelicerate with broad, vaulted carapace; is highlighted by a dark organic stain in