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ABSTRACT-
A 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 podomere 9, and a xiphous telson. It is only the fourth (the first Canadian) well-documented Ordovician eurypterid genus, and provides the oldest reliable record of the Hughmillerioidea to date.
INTRODUCTION
EURYPTERIDS ARE Paleozoic predatory chelicerates. They are rare as fossils, due to their thin, unmineralized chitinous cuticle, which was subject to rapid postmortem disarticulation and bacterial decay (Plotnick, 1999), although they are locally abundant in some strata of Late Silurian age. The evolutionary acme of the group occurred during the Late Silurian and Early Devonian, most of the known species originating in the PridolianLochkovian interval (Plotnick, 1999). Reports of pre-Silurian eurypterids are few; most species from the Ordovician of New York State are particularly dubious (see Tollerton and Landing, 1994; Braddy et al., 2004; Tollerton, 2004). Thus, their early evolutionary history is obscure, and any new material from this interval is potentially significant.
Canadian eurypterids are primarily known from Silurian strata of Ontario (see Copeland and Bolton, 1960, 1985). Rare reports from other provinces include an equivocal specimen, apparently an eurypterid opisthosomal fragment, from the Upper Ordovician (Richmondian) Stony Mountain Formation of southern Manitoba (Elias, 1980), an unidentified stylonurid from the Battery Point Formation (Middle Devonian) of Gaspé, Québec (Jeram, 1996), as well as fragmentary remains of probable Megalograptus Miller, 1874 from Richmondian strata of the Nicolet River Formation, Rivière des Hurons, southern Québec (Chartier et al., 2002)
Recent reports from Arctic Canada include: Stylonurus sp. from the...