A New Eurypterid (Chelicerata) from the Upper Ordovician of Manitoulin Island, Ontario, Canada
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J. Paleont., 79(6), 2005, pp. 1166±1174 Copyright q 2005, The Paleontological Society 0022-3360/05/0079-1166$03.00 A NEW EURYPTERID (CHELICERATA) FROM THE UPPER ORDOVICIAN OF MANITOULIN ISLAND, ONTARIO, CANADA CHRISTOPHER A. STOTT,1 O. ERIK TETLIE,2 SIMON J. BRADDY,3 GODFREY S. NOWLAN,4 PAUL M. GLASSER,5 AND MATTHEW G. DEVEREUX6 1Department of Earth Sciences, The University of Western Ontario, London, Ontario N6A 5B7, Canada ,[email protected]., 2Department of Geology and Geophysics, Yale University, PO Box 208109, New Haven, Connecticut 06520-8109, ,[email protected]., 3Department of Earth Sciences, University of Bristol, Wills Memorial Building, Queen's Road, Bristol BS8 1RJ, United Kingdom, 4Geological Survey of Canada, 3303-33rd Street N.W., Calgary, Alberta T2L 2A7, Canada, 5School of Dentistry, Faculty of Medicine & Dentistry, The University of Western Ontario, London, Ontario N6A 5C1, Canada, and 6Faculty of Science, Of®ce of the Dean, The University of Western Ontario, London, Ontario N6A 5B7, Canada ABSTRACTÐA new genus and species of eurypterid (Eurypterida: Chelicerata) is described as Orcanopterus manitoulinensis from the Upper Ordovician Kagawong Submember (Upper Member) of the Georgian Bay Formation, Manitoulin Island, Ontario, Canada. The material comprises several partial specimens in addition to disarticulated carapaces, appendages, metastomas, opisthosomal segments, and telsons. Associated fossils include rare bryozoans, a conularid, ostracodes, and conodonts. A restricted marine lagoon, or very shallow subtidal to intertidal environment is inferred. This assemblage, perhaps representing an accumulation of molted exuviae, was apparently preserved as the result of rapid burial by carbonate muds and silts during a storm event. O. manitoulinensis shares a number of traits with both the Hughmilleriidae and the Carcinosomatidae. Diagnostic features include curved preabdominal segments, a petaloid A metastoma with deep anterior emargination, spiniferous appendages of Carcinosoma type, paddle with enlarged, symmetrical po- domere 9, and a xiphous telson. It is only the fourth (the ®rst Canadian) well-documented Ordovician eurypterid genus, and provides the oldest reliable record of the Hughmillerioidea to date. INTRODUCTION form differs markedly from the new material described herein. URYPTERIDS ARE Paleozoic predatory chelicerates. They are Eurypterid appendages and telsons were also reported by Way E rare as fossils, due to their thin, unmineralized chitinous cu- (1936), from a 15 cm thick, recessive, shaly dolostone unit im- ticle, which was subject to rapid postmortem disarticulation and mediately below typical lithologies of the basal Manitoulin For- bacterial decay (Plotnick, 1999), although they are locally abun- mation in the Kagawong West Quarry (our locality 2; Fig. 1). dant in some strata of Late Silurian age. The evolutionary acme Copeland and Bolton (1960, 1985) assigned this eurypterid-bear- of the group occurred during the Late Silurian and Early Devo- ing stratum, occurring at a slightly higher stratigraphic level than nian, most of the known species originating in the Pridolian± the eurypterid horizon we report herein, to the Manitoulin For- Lochkovian interval (Plotnick, 1999). Reports of pre-Silurian eu- mation and consequently inferred an Early Silurian age for this rypterids are few; most species from the Ordovician of New York fauna. Subsequent biostratigraphic work (Barnes and Bolton, State are particularly dubious (see Tollerton and Landing, 1994; 1988; Nowlan, 2001) has revealed that this recessive unit contains Braddy et al., 2004; Tollerton, 2004). Thus, their early evolution- the stratigraphically highest Ordovician conodont fauna in local ary history is obscure, and any new material from this interval is boundary sections, suggesting continuity with immediately un- potentially signi®cant. derlying lithologies of the Kagawong Submember, Upper Mem- Canadian eurypterids are primarily known from Silurian strata ber, Georgian Bay Formation containing the eurypterid horizon of Ontario (see Copeland and Bolton, 1960, 1985). Rare reports reported herein, and the potential for additional, as yet undiscov- from other provinces include an equivocal specimen, apparently ered, eurypterid layers in the uppermost Ordovician strata of the an eurypterid opisthosomal fragment, from the Upper Ordovician region. (Richmondian) Stony Mountain Formation of southern Manitoba In this paper, we report on a new eurypterid fauna from two (Elias, 1980), an unidenti®ed stylonurid from the Battery Point localities on Manitoulin Island, Ontario, Canada. This fauna was Formation (Middle Devonian) of GaspeÂ, QueÂbec (Jeram, 1996), uncovered during excavation of a 2±3 cm thick dolostone bed in as well as fragmentary remains of probable Megalograptus Miller, the uppermost Kagawong Submember (Upper Member) of the 1874 from Richmondian strata of the Nicolet River Formation, Georgian Bay Formation, slightly below its contact with the Man- RivieÁre des Hurons, southern QueÂbec (Chartier et al., 2002) itoulin Formation, at the West Bay Indian Reserve road cut (lo- Recent reports from Arctic Canada include: Stylonurus sp. from cality 1; Fig. 1), central Manitoulin Island, as well as from a the Snowblind Bay Formation (Early Devonian) of Cornwallis similar, if not identical, stratigraphic level at the Kagawong West Island, Nunavut (Plotnick and Elliott, 1995), and a fauna com- Quarry section (locality 2), 13 km northwest of locality 1. The prising Erieopterus microphthalmus (Hall, 1859), Drepanopterus initial discovery was made at locality 1 by S. Donato, then of the sp., and Carcinosoma sp. from the Bear Rock Formation (Early Department of Earth Sciences, University of Western Ontario, in Devonian) of the Northwest Territories (Braddy and Dunlop, the company of CAS, PMG, and J. Jin. Signi®cant collections 2000). Additional discoveries from Ontario include a putative On- were made during the initial visit, additional material being un- ychopterella carapace from the Whirlpool Formation (early Llan- earthed on a subsequent collecting trip to locality 1 undertaken dovery) near Georgetown (J. Waddington, personal commun., by PMG and MGD. These strata are reliably dated as Late Or- 2003), and material from the Eramosa Member of the Guelph dovician, very close to the Ordovician±Silurian boundary, based Formation (late Wenlock) of the southern Bruce Peninsula which on conodonts (see below). has been tentatively assigned to the pterygotid genus Erettopterus Salter, 1859 (D. Rudkin, personal commun., 2002). Previously reported eurypterids from Manitoulin Island include GEOLOGICAL SETTING AND AGE OF THE FAUNA Carcinosoma libertyi Copeland and Bolton, 1960 from the Early The eurypterid-bearing lithology consists of thinly bedded, Silurian St. Edmund Formation (mid-Llandovery), although this ®nely crystalline hypidiotopic to xenotopic dolostone, probably 1166 STOTT ET AL.ÐNEW ORDOVICIAN EURYPTERID 1167 FIGURE 1ÐMap of eastern and central Manitoulin Island, showing locations of two Ordovician±Silurian boundary sections at which eurypterid remains have been recovered from the uppermost Kagawong Submember, Upper Member, Georgian Bay Formation. representing recrystallization of primary micritic limestone, sug- et al., 1976; Sweet, 1979a, 1979b; Leatham, 1984; Norford et al., gestive of a relatively low-energy depositional environment. Rare, 1998; Armstrong and Owen, 2002). Low-energy, restricted envi- branching bryozoans and one conularid were the only other mac- ronments are likely to have been present landward of laterally rofossils found in this bed. Probable ostracodes were observed in extensive wave-baf¯ing stromatoporoid-tetradiid-bryozoan car- thin sections of the eurypterid-bearing lithology, as were rare bonate buildups represented by the Mudge Bay and Maple Point bryozoans. The sparse macrofauna suggests that this environment biostromes (see Copper, 1978; Copper and Grawbarger, 1978). was marine, although highly restrictive, possibly the result of the The latter biostrome occurs in the interval between 1.5 and 4 m development of hypersaline, variable salinity, or anaerobic/dys- below the Ordovician±Silurian boundary at locality 1 (Fig. 2). aerobic conditions. These structures are estimated to have risen up to 1 m above the Conodont faunas recovered from the eurypterid-bearing bed surrounding sea¯oor, their tops being only marginally submerged and immediately higher and lower strata (samples M1±4 through during the closing phase of local Ordovician sedimentation (Cop- M1±6; Fig. 2) are dominated by Rhipidognathus symmetricus per and Grawbarger, 1978). Low-energy lagoons in the lee of Branson, Mehl, and Branson, 1951 (Nowlan, 2001). The abun- these biostromes, characterized by poorly aerated waters, would dance of this species indicates the shallowest Late Ordovician provide ideal conditions for the preservation of uncalci®ed ar- conodont biofacies (Sweet and BergstroÈm, 1984), and very shal- thropod cuticle, given suf®ciently rapid burial and anoxia in the low subtidal to intertidal, and possibly hypersaline, conditions bottom waters (see Brett et al., 1997; Plotnick, 1999). Burial was (Kohut and Sweet, 1968; Barnes and FaÊhraeus, 1975; Le FeÁvre almost certainly the product of the deposition of carbonate muds 1168 JOURNAL OF PALEONTOLOGY, V. 79, NO. 6, 2005 FIGURE 2ÐChronostratigraphy and lithostratigraphy of the Upper Ordovician to lowermost Silurian sequence of Manitoulin Island. The bracket adjacent to the lithology column