Paleontological Society A New Synziphosurine (Chelicerata: Xiphosura) from the Late Llandovery (Silurian) Waukesha Lagerstätte, Wisconsin, USA Author(s): Rachel A. Moore, Derek E. G. Briggs, Simon J. Braddy, Lyall I. Anderson, Donald G. Mikulic, Joanne Kluessendorf Source: Journal of Paleontology, Vol. 79, No. 2 (Mar., 2005), pp. 242-250 Published by: Paleontological Society Stable URL: http://www.jstor.org/stable/4094976 . Accessed: 20/08/2011 04:27 Your use of the JSTOR archive indicates your acceptance of the Terms & Conditions of Use, available at . http://www.jstor.org/page/info/about/policies/terms.jsp JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact [email protected]. Paleontological Society is collaborating with JSTOR to digitize, preserve and extend access to Journal of Paleontology. http://www.jstor.org J. Paleont., 79(2), 2005, pp. 242-250 Copyright ? 2005, The Paleontological Society 0022-3360/05/0079-242$03.00 A NEW SYNZIPHOSURINE (CHELICERATA:XIPHOSURA) FROM THE LATE LLANDOVERY (SILURIAN) WAUKESHA LAGERSTATTE, WISCONSIN, USA RACHEL A. MOORE, DEREK E. G. BRIGGS,2 SIMON J. BRADDY,1 LYALL I. ANDERSON,3 DONALD G. MIKULIC,4 AND JOANNE KLUESSENDORF5 'Departmentof EarthSciences, Universityof Bristol,Wills MemorialBuilding, Queen's Road, Bristol BS8 1RJ,United Kingdom, <[email protected]>;2Department of Geology and Geophysics,Yale University,P.O. Box 208109, New Haven,Connecticut 06520- 8109; 3Departmentof Geology and Zoology, NationalMuseums of Scotland,Chambers Street, Edinburgh EH1 1JF,United Kingdom;4Illinois State Geological Survey,Champaign 61820 (publishedwith permissionof the Chief of the Illinois State Geological Survey);and 5WeisEarth Science Museum,University of Wisconsin-FoxValley, Menasha54952 ABSTRACT-A new synziphosurine(Chelicerata:Xiphosura) is described from the Late Llandovery(Silurian) Konservat-Lagerstditte of Waukesha,Wisconsin, USA. Venustuluswaukeshaensis n.gen. and sp. is characterizedby a semicircularcarapace with a slightly procurvedposterior margin lacking genal spines and an opisthosomacomposed of 10 freely articulatingsegments, divided into a preabdomenof seven segments with blunt pleuraeand a postabdomenof three segments lacking pleurae.The tail spine is shortand styliform.This is the earliest known unequivocalsynziphosurine, extending their fossil recordfrom the Wenlockto the Llandovery, and only the second species to be describedwith prosomalappendages; the presenceof six pairs (a pair of cheliceraeand five pairsof walkinglegs) contrastswith the seven in the synziphosurineWeinbergina opitzi, but is comparableto the numberin modem horseshoe crabs. V. waukeshaensisn. gen. and sp. is not assignedto a family here pendinga widerrevision, but it bearsmost resemblanceto the Weinberginidae. INTRODUCTION During the Aeronian-early Telychian, Burnt Bluff and older Silurian strata in the northern Basin were be- YNZIPHOSURINESARE basal members of the phylum Chelicer- Michigan uplifted, came and were to erosion and karstification S ata, which includes xiphosurans (horseshoe crabs), eurypter- emergent, subjected and Mikulic and ids, chasmataspids, arachnids (e.g., spiders and scorpions) and (Kluessendorf Mikulic, 1998; Kluessendorf, The southern of this karstified which has a possibly the pycnogonids (sea spiders) (Dunlop, 1999; Waloszek 1999). edge ridge, relief of more than 50 is as an and Dunlop, 2002). Chelicerates are characterized by a pair of regional m, expressed 8-m-high at Wisconsin and chelate preoral appendages and a body divided into two tagmata, paleoscarp Waukesha, (Kluessendorf Mikulic, features the prosoma and opisthosoma. The closest living relatives of the 1996, 1998). Epikarstic (e.g., kamenitzas, kluftkarren, broad and runnel on the in- synziphosurines are the xiphosurids, which are often described as scalloping, notching, troughs) scarp dicate formation on carbonate strata within a coastal living fossils due to their extreme morphological conservatism unvegetated under humid conditions with at least abundant since the Mississippian (Waterston, 1985; Siveter and Selden, setting seasonally rainfall and BBF strata that 1987). There are four extant species of xiphosuran, the best (Kluessendorf Mikulic, 1996, 1998). the here were in an intertidal as known being Limulus polyphemus (Linnaeus, 1758), which is onlap scarp deposited setting, indicated fenestral interference rill and wrin- found along the Atlantic coast of North America (Shuster, 1982). by porosity, ripples, kle and tidal channels and Mik- Synziphosurines form a paraphyletic stem group of xiphosurans marks, microkarst, (Kluessendorf the toe of the beneath the is a chert characterized by an opisthosoma composed of nine to 11 freely ulic, 1996). Along scarp, BBF, to m thick that formed as a soil articulating tergites (Anderson and Selden, 1997). They range conglomerate up 1 probably residuum and was reworked as a from the Silurian to the Upper Devonian and their paleogeograph- lag deposit during transgression and The BBF out ic range includes the USA, Europe, Australia, and South America (Kluessendorf Mikulic, 1996). wedges against the more laminated and it is (Dunlop and Selden, 1997). scarp, becoming finely argillaceous; absent north of Waukesha. The nonmineralized cuticle of chelicerates requires special con- The Waukesha biota occurs within a laminated mudstone ditions for preservation such as those that prevailed in the Kon- finely and dolomite the lowest 2 m of the Brandon servat-Lagerstaitteof Waukesha County, Wisconsin (Mikulic et al., overlying Bridge strata. Fossils are uncommon and scattered the 1985a, 1985b). Silurian are comparatively scarce in throughout layer, Lagerstitten but are more abundant in certain et the geological record (Kluessendorf, 1994; Briggs et al., 1996), bedding planes (Mikulic al., to a accumulation of so this locality also provides important data for reconstructing the 1985a). They appear represent taphonomic molts within a sediment on the diversity of Silurian organisms. primarily arthropod deposited trap and Preservation of the fauna In this paper, we describe a new synziphosurine from the Wau- scarp (Mikulic Kluessendorf, 2001). is related to conditions that occurred at the The kesha Lagerstaitte.Following a summary of the Waukesha biota scarp. presence limited bioturbation and absence of de- by Mikulic et al. (1985a, 1985b) and the discovery of new ma- of fine-scale lamination, terial from this site, the taxon is described in detail and assigned cay in the strata adjacent to the scarp suggest rapid deposition in to a new genus and species. an anoxic environment. The fossiliferous, dolomitized limestone beds containing the GEOLOGICAL SETTING AND ASSOCIATED FAUNA Waukesha biota are limited to only two localities in Wisconsin. The Waukesha biota occurs within the Brandon Bridge For- The biota is dominated by arthropods and includes trilobites (13 mation (late Telychian-early Sheinwoodian) which outcrops in the genera), phyllocarids and ostracods, the earliest reported repre- Milwaukee, Wisconsin area (Mikulic and Kluessendorf, 1998). sentative of the class Thylacocephala, and at least three arthropods The Brandon Bridge Formation (BBF) is typically a thinly bed- of uncertain affinity (Mikulic et al., 1985a, 1985b). In addition, ded, light-green to pinkish-gray argillaceous dolomite that uncon- at least four wormlike taxa have been discovered (Mikulic et al., formably overlies the Kankakee Dolomite or Burnt Bluff Group 1985a). The absence of shelly fossils such as brachiopods, mol- (Aeronian) (Mikulic and Kluessendorf, 1998) and grades upwards luscs, corals, bryozoans, and echinoderms, which are common into the Waukesha Dolomite (Mikulic et al., 1985a). organisms in other Silurian biotas, is noteworthy. Those shelly 242 MOORE ET AL.--A NEW LOWER SILURIAN SYNZIPHOSURINE 243 Ri Db ~F Ph 10 Mr FIGURE1-Venustulus waukeshaensisn. sp., Lower Llandovery,Waukesha, Wisconsin, USA, holotype,UW 4001/21, ventralview. 1, Cameralucida drawing; 2, photograph. Db = doublure, F = fracture, Ph = phosphatic material, P1 = pleura, Ri = ridges, Mr = median ridge, numbers 1-10 = opisthosomal segments. Scale bar = 1 mm. fossils that are present are commonly decalcified. Also absent are PRESERVATION their occurrence at other Silurian eurypterids, despite arthropod- Of the eight specimens, three are almost complete and five rich such as the late Silurian Bertie Waterlime localities, (Pi•idolf) consist only of the prosoma and part of the opisthosoma. All of New York State and (Clarke Ruedemann, 1912; Kluessendorf, specimens are flattened dorsoventrally and in most cases splitting A new of has been 1994). genus dasycladalean algae recently occurred between the tergites and sternites. Preferential decay described from the Waukesha but occurs at a Lagerstditte separate around the margins has resulted in poorly preserved lateral re- level to those which the rest of the biota et yield (LoDuca al., gions in all specimens. In some cases, the carapace margin, the 2003). opisthosomal pleurae, and distal regions of the tail spine are ab- The entire BBF has been dated as late Wen- Llandovery-early sent. Localized conditions resulted in preferential preservation of lock based on conodonts and however the Waukesha graptolites, the carapace in some