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 MATTHEW G. DEVEREUX6
1 Department of Earth Sciences, The University of Western Ontario, London, Ontario N6A 5B7, Canada ,[email protected]
, Department of Geology and Geophysics, Yale University, PO Box 208109, New Haven, Connecticut 06520-8109, ,[email protected]
, 3 Department 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, Office of the Dean, The University of Western Ontario, London, Ontario N6A 5B7, Canada 2
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.
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 Pridolian– Lochkovian 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 Gaspe´, Que´bec (Jeram, 1996), as well as fragmentary remains of probable Megalograptus Miller, 1874 from Richmondian strata of the Nicolet River Formation, Rivie`re des Hurons, southern Que´bec (Chartier et al., 2002) Recent reports from Arctic Canada include: Stylonurus sp. from the Snowblind Bay Formation (Early Devonian) of Cornwallis Island, Nunavut (Plotnick and Elliott, 1995), and a fauna comprising Erieopterus microphthalmus (Hall, 1859), Drepanopterus sp., and Carcinosoma sp. from the Bear Rock Formation (Early Devonian) of the Northwest Territories (Braddy and Dunlop, 2000). Additional discoveries from Ontario include a putative Onychopterella carapace from the Whirlpool Formation (early Llandovery) near Georgetown (J. Waddington, personal commun., 2003), and material from the Eramosa Member of the Guelph Formation (late Wenlock) of the southern Bruce Peninsula which has been tentatively assigned to the pterygotid genus Erettopterus Salter, 1859 (D. Rudkin, personal commun., 2002). Previously reported eurypterids from Manitoulin Island include Carcinosoma libertyi Copeland and Bolton, 1960 from the Early Silurian St. Edmund Formation (mid-Llandovery), although this
form differs markedly from the new material described herein. Eurypterid appendages and telsons were also reported by Way (1936), from a 15 cm thick, recessive, shaly dolostone unit immediately below typical lithologies of the basal Manitoulin Formation in the Kagawong West Quarry (our locality 2; Fig. 1). Copeland and Bolton (1960, 1985) assigned this eurypterid-bearing stratum, occurring at a slightly higher stratigraphic level than the eurypterid horizon we report herein, to the Manitoulin Formation and consequently inferred an Early Silurian age for this fauna. Subsequent biostratigraphic work (Barnes and Bolton, 1988; Nowlan, 2001) has revealed that this recessive unit contains the stratigraphically highest Ordovician conodont fauna in local boundary sections, suggesting continuity with immediately underlying lithologies of the Kagawong Submember, Upper Member, Georgian Bay Formation containing the eurypterid horizon reported herein, and the potential for additional, as yet undiscovered, eurypterid layers in the uppermost Ordovician strata of the region. In this paper, we report on a new eurypterid fauna from two localities on Manitoulin Island, Ontario, Canada. This fauna was uncovered during excavation of a 2–3 cm thick dolostone bed in the uppermost Kagawong Submember (Upper Member) of the Georgian Bay Formation, slightly below its contact with the Manitoulin Formation, at the West Bay Indian Reserve road cut (locality 1; Fig. 1), central Manitoulin Island, as well as from a similar, if not identical, stratigraphic level at the Kagawong West Quarry section (locality 2), 13 km northwest of locality 1. The initial discovery was made at locality 1 by S. Donato, then of the Department of Earth Sciences, University of Western Ontario, in the company of CAS, PMG, and J. Jin. Significant collections were made during the initial visit, additional material being unearthed on a subsequent collecting trip to locality 1 undertaken by PMG and MGD. These strata are reliably dated as Late Ordovician, very close to the Ordovician–Silurian boundary, based on conodonts (see below). GEOLOGICAL SETTING AND AGE OF THE FAUNA
The eurypterid-bearing lithology consists of thinly bedded, finely crystalline hypidiotopic to xenotopic dolostone, probably
STOTT ET AL.—NEW ORDOVICIAN EURYPTERID
FIGURE 1—Map 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, suggestive of a relatively low-energy depositional environment. Rare, branching bryozoans and one conularid were the only other macrofossils found in this bed. Probable ostracodes were observed in thin sections of the eurypterid-bearing lithology, as were rare bryozoans. The sparse macrofauna suggests that this environment was marine, although highly restrictive, possibly the result of the development of hypersaline, variable salinity, or anaerobic/dysaerobic conditions. Conodont faunas recovered from the eurypterid-bearing bed and immediately higher and lower strata (samples M1–4 through M1–6; Fig. 2) are dominated by Rhipidognathus symmetricus Branson, Mehl, and Branson, 1951 (Nowlan, 2001). The abundance of this species indicates the shallowest Late Ordovician conodont biofacies (Sweet and Bergstro¨m, 1984), and very shallow subtidal to intertidal, and possibly hypersaline, conditions (Kohut and Sweet, 1968; Barnes and Fa˚hraeus, 1975; Le Fe`vre
et al., 1976; Sweet, 1979a, 1979b; Leatham, 1984; Norford et al., 1998; Armstrong and Owen, 2002). Low-energy, restricted environments are likely to have been present landward of laterally extensive wave-baffling stromatoporoid-tetradiid-bryozoan carbonate buildups represented by the Mudge Bay and Maple Point biostromes (see Copper, 1978; Copper and Grawbarger, 1978). The latter biostrome occurs in the interval between 1.5 and 4 m below the Ordovician–Silurian boundary at locality 1 (Fig. 2). These structures are estimated to have risen up to 1 m above the surrounding seafloor, their tops being only marginally submerged during the closing phase of local Ordovician sedimentation (Copper and Grawbarger, 1978). Low-energy lagoons in the lee of these biostromes, characterized by poorly aerated waters, would provide ideal conditions for the preservation of uncalcified arthropod cuticle, given sufficiently rapid burial and anoxia in the bottom waters (see Brett et al., 1997; Plotnick, 1999). Burial was almost certainly the product of the deposition of carbonate muds
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FIGURE 2—Chronostratigraphy and lithostratigraphy of the Upper Ordovician to lowermost Silurian sequence of Manitoulin Island. The bracket adjacent to the lithology column illustrates the portion of the local Ordovician and Silurian stratigraphy represented by the West Bay section. The precise stratigraphic level of the eurypterid horizon within the West Bay (locality 1) section is shown in relation to the Kagawong Submember/ Manitoulin Formation contact, as well as to conodont sample levels (M1 series) throughout the boundary interval. O 5 sample yielded Ordovician conodonts; S 5 sample yielded Silurian conodonts.
and silts during a single storm event. Despite pervasive dolomitization of the host lithology, thin-section examination of a 5 mm interval of sediment overlying visible eurypterid integument has revealed an upward-fining trend characteristic of tempestite deposits and the presence of fine silt-sized detrital quartz grains in a lithology apparently otherwise free of such grains. The latter may be indicative of the short-term introduction of terrestrially derived fine clastics into nearshore carbonate environments by returning flows of strong storm waves (see Byerley and Coniglio, 1991). Organic-rich carbonate intervals noted in the West Bay exposure (Fig. 2) and in other outcrops of uppermost Ordovician strata on Manitoulin Island may constitute evidence for periodic seafloor anoxia, most probably occurring in stagnant zones landward of the wave-baffling biostromes. Evidence of bioturbation within the eurypterid-bearing bed appears to be confined to narrow, predominantly horizontal, tubelike trace fossils, the general
absence of large and deep burrows suggesting the prevalence of anoxic conditions immediately below the sediment-water interface. In the absence of escape traces or other indications of the postburial conditions of the animals, the true nature of the eurypterid assemblage remains uncertain. The partial specimens may represent animals overcome by local restrictive conditions and/or storm deposition, or may merely represent exuviae. As suggested by Plotnick (1999), this determination is not easily made on the basis of the fossils alone as all portions of the preservable integument may potentially be sufficiently well represented in molts. Evidence of incipient preburial disarticulation in the form of abundant dissociated skeletal elements favors the latter interpretation. The suggestion that lagoonal environments were not regularly inhabited by eurypterids but were sought out as areas where molting could occur under current- and predator-free conditions, and in
STOTT ET AL.—NEW ORDOVICIAN EURYPTERID which preservation potential was relatively high (Caster and Kjellesvig-Waering, 1964; Plotnick, 1999; Braddy, 2001), is a possibility that should be considered in future taphonomic interpretations of eurypterid faunas. Micropaleontological samples obtained from the eurypterid horizon and immediately under- and overlying strata have yielded a conodont fauna of Late Ordovician aspect comprised by the taxa Aphelognathus grandis Branson, Mehl, and Branson, 1951, Oulodus robustus (Branson, Mehl, and Branson, 1951), Panderodus aff. P. panderi (Stauffer, 1940), Panderodus gibber Nowlan and Barnes, 1981, Panderodus gracilis (Branson and Mehl, 1933), Pseudobelodina? aff. Pseudobelodina? dispansa (Glenister, 1957), Pseudobelodina inclinata (Branson and Mehl, 1933), and Rhipidognathus symmetricus. This conodont fauna is typical of Richmondian (approximately equivalent to the British Cautleyan– lower Rawtheyan stages) assemblages of the eastern North American Midcontinent (Fauna 12). To date, no Atlantic Faunal Region conodonts have been reported from this high in the Georgian Bay Formation. Amorphognathus superbus (Rhodes, 1953), Periodon grandis (Ethington, 1959), and Icriodella superba Rhodes, 1953 are known from the lowermost 40 m of the Georgian Bay Formation on Manitoulin Island (Barnes et al., 1978; Goldman and Bergstro¨m, 1997), suggesting assignment of the lower portion of the formation to the A. superbus Zone. On the basis of available graptolite and shelly fossil biostratigraphies, the A. superbus–A. ordovicicus Zonal boundary on Manitoulin Island is estimated to occur some 50–60 m above the base of the Georgian Bay Formation within the claystone-dominated Lower Member; this zonal boundary is regarded to occur in the lower portion of the Richmondian Stage within the upper Amplexograptus manitoulinensis Zone (Fig. 2; Goldman and Bergstro¨m, 1997). Lowermost strata of the Manitoulin Formation outcropping ;20 cm above the eurypterid horizon at locality 1 yield conodonts suggestive of a Silurian age, including Icriodella discreta Pollock, Rexroad, and Nicoll, 1970, Kockelella manitoulinensis (Pollock, Rexroad, and Nicoll, 1970), Ozarkodina hassi (Pollock, Rexroad, and Nicoll, 1970), and Spathognathodus comptus Pollock, Rexroad, and Nicoll, 1970 sensu formo (Nowlan, 2001). Observed occurrence of the atrypoid brachiopod Zygospiraella planoconvexa (Hall, 1852), an index of Llandovery A age (Copper, 1978, 1982) within 20 cm of the base of the Manitoulin Formation at this locality, confirms the Rhuddanian age of the Manitoulin Formation. Consequently, the eurypterid fauna occurs just below the local Ordovician–Silurian boundary. As in much of the North American Midcontinent, this boundary appears to be defined by a disconformity, the result of local nondeposition and/ or subaerial exposure during the latest Ordovician glacioeustatic regression (see Dennison, 1976; Kobluk, 1984; Barnes and Bolton, 1988; Byerley and Coniglio, 1991). Overall, a mid- to late Richmondian (middle Ashgill) age is indicated for this eurypterid assemblage. Although developed at a similar stratigraphic level, this fauna does not appear to be related to the Upper Ordovician eurypterids of Ohio, characterized by an extremely well-preserved Megalograptus fauna (Caster and Kjellesvig-Waering, 1964). MATERIALS AND METHODS
Bedding-plane exposure of the eurypterid-bearing stratum near the top of the West Bay (locality 1) section permitted a collection over approximately 10 m2. The eurypterid fossils are preserved in positive relief in a dark gray, poorly laminated marl matrix, although in places this matrix has weathered to a pale brown color. Eurypterid cuticle is well preserved as black to maroon organic films and accompanying underlying molds at locality 1. Preservation at locality 2 is comparatively poor. The eurypterid remains, ranging from fragments to near-complete individuals,
were present in patchy distributions characterized by localized high abundance. Several partial eurypterids were recovered in addition to dissociated carapaces, appendages, metastomas, opisthosomal segments, and telsons. Drawings were made using a camera lucida. All measurements are given in millimeters. Morphological terminology and systematics for the higher eurypterid taxa follow Tollerton (1989). The material described is deposited in the collections of the Royal Ontario Museum (ROM), Toronto, under the repository numbers ROM 56449–56463, 57017, 57018, and 57026. Additionally, more poorly preserved and less informative material is stored in the ROM under the original label 03D1. SYSTEMATIC PALEONTOLOGY
Phylum CHELICERATA Heymons, 1901 Order EURYPTERIDA Burmeister, 1843 Superfamily HUGHMILLERIOIDEA Kjellesvig-Waering, 1951 Discussion.The new form shares a number of features with two of the constituent families of the Hughmillerioidea, namely the Hughmilleriidae and the Carcinosomatidae (see below), but does not fit easily within the diagnosis of either family (see Tollerton, 1989; Braddy et al., 2002), indicating that this form has an intermediate hughmilleriid-carcinosomatid affinity, and is possibly a basal hughmillerioid. We hesitate herein to propose a new family, pending cladistic analysis of the entire Eurypterida, which is beyond the scope of this study. Genus ORCANOPTERUS new genus Type species.Orcanopterus manitoulinensis n. gen. and sp., by monotypy. Diagnosis.Hughmillerioid with parabolic carapace, antelateral intramarginal eyes, curved preabdominal segments, petaloid A metastoma with deep anterior emargination, a unique paddle with an enlarged, symmetrical podomere 9, and a xiphous telson with a median keel. Etymology.A composite of Ordovician, Canada, and ptero (greek)–wing, nom. trans. opterus, the traditional suffix for eurypterid genera. ORCANOPTERUS
Figures 3–5 Unnamed hughmilleriid; STOTT ET AL., 2001, p. 46–47, fig. 3. Eurypterida indet.? new genus; BRADDY ET AL., 2004, p. 256, table 25.1. New genus and species (?); TOLLERTON, 2004, p. 238, table 2.
Diagnosis.As for genus. Description.The carapace is known from four undistorted specimens (only two figured: ROM 56450, Fig. 3.1; 56451, Fig. 3.2) with length to width ratios ranging from 0.88 to 0.94, averaging 0.89. The lateral angles vary between 908 and 1058, giving a parabolic carapace shape. The eyes are intermediate between reniform and oval (Fig. 5), positioned antelaterally and intramarginally. They are separated from the margin by the relatively broad marginal rim, which is widest anteriorly and narrows towards the posterolateral angles, and is ornamented with three to four lines converging posteriorly. The ocelli are known from one specimen (ROM 56450, Fig. 3.1); they are situated slightly posterior to the posterior margin of the eyes, and three-eighths the length of the carapace from the anterior margin. The carapace is ornamented anteriorly with numerous terraced folds (Fig. 3.3) following the contours. Anterior to and between the eyes, these folds split up into large lunate scales. The suture of the doublure was originally reported (Stott et al., 2001) to be of Hughmilleria type, however the doublure could not be found and it would appear that the dorsal marginal rim was originally misinterpreted as the ventral doublure. The chelicerae are unknown, but the spiniferous prosomal appendages II–V are known from one specimen (ROM
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STOTT ET AL.—NEW ORDOVICIAN EURYPTERID 57018; Figs. 3.4, 4.3). We interpret the spines on the podomeres as directed anteriorly, giving the identity of the appendages as labeled in Figure 4.3. Appendage II is poorly preserved and only the outlines of three or four podomeres are distinguished. Appendage III has four short and stout podomeres preserved, and traces of two or three anteriorly oriented spines are evident. Appendage IV has three very stout podomeres and there is evidence for anteriorly oriented spines on the two distal podomeres. Appendage V has the four distal podomeres preserved; there is a massive spine on the penultimate podomere, and the distal podomere is also a long spine. The spiniferous appendages are also known from two coxal morphotypes, here tentatively assigned to appendages IV and V. Coxa IV (ROM 56455; Fig. 3.5) bears at least eight very long and slender curving teeth. Coxa V (ROM 56454; Fig. 3.6) also has eight teeth, but these differ from those of coxa IV in having one blunt, large anterior tooth, and seven smaller posterior teeth. Appendage VI is known from a coxa (ROM 56453; Fig. 3.7) and two other specimens showing the distal podomeres of the paddle. The coxa has a continuous gnathobase of 22 small teeth, all of similar size (0.5 mm long), and an ornament of broad lunules ventrally. The rest of appendage VI is known from podomeres 6, 7, 7a, 8, and 9 (ROM 56452; Fig. 3.8) and 7, 7a, 8, and 9 (ROM 57017; Fig. 4.2). The most conspicuous feature of the paddle is the large size of podomere 9, which makes this a unique type of appendage VI (see below). Five dissociated metastomas are present in the collection. Four of these are morphologically similar and can be confidently assigned to O. manitoulinensis as one of the four is associated with the holotype (ROM 56459), and appears to be the correct size to belong with it. The only complete metastoma of this type (ROM 56456; Fig. 3.9) is widest at one-third of the distance along its length from the anterior margin, and has a length-width ratio of 1.91. The specimen also has rounded shoulders and a broad and deep-rounded anterior emargination, with an angle of cordation of 888. The convex sides converge towards the truncated posterior margin, with a lateral angle of 758. The shape is most similar to petaloid A (Tollerton, 1989, fig. 5.7), but the deep emargination is more similar to that of the obovate shape (Tollerton, 1989, fig. 5.3). The metastoma has a doublure (Fig. 3.9, 3.10), but no discernible ornament on the dorsal side, and an extremely coarse ornament of broad angular scales on the ventral side (ROM 56456; Fig. 3.9, 3.11). A single small metastoma (ROM 57026) (22 mm long, 10 mm wide at widest point; length-width ratio of 2.20) exhibits an apparently different morphology in which the position of greatest width occurs just past the midlength point on the posterior half of the element. It is similar to the larger morphotype, figured herein, in having a truncated posterior margin and a deep anterior notch with rounded shoulders anterolaterally. The similarities in these key characters suggest that this could represent the metastoma of a juvenile O. manitoulinensis, however, we choose to withhold judgment on the latter pending future discoveries concerning eurypterid ontogeny. As in most eurypterids, the first visible opisthosomal segment
FIGURE 4—Orcanopterus manitoulinensis n. gen. and sp.; 1, genital appendage type B, ROM 56459; 2, distal podomeres (7, 7a, 8, and 9) of appendage VI, ROM 57017; 3, appendages II–V, ROM 57018. Scale bars 5 10 mm.
is reduced, being approximately half the length of the other mesasomal segments. Several specimens show the opisthosoma to have a gradual transition between the pre- and postabdomen. However, the holotype has what appears to be epimerae (same type as those present in Hughmilleria socialis Sarle, 1903) on the ventral side of segment 7, and as such, a midsection second-order differentiation is present (Tollerton, 1989). The individual segment boundaries of the mesosoma are highly convex (cf. carcinosomatids) as compared to the straight boundaries of the metasoma (Fig. 3.12), but there is no clear-cut third-order differentiation as in mixopterids. No fourth-order differentiation is present. The genital operculum is composed of two segments. Although no discrete boundary can be seen, the ornament and coloration of the two segments are different on the holotype. The genital appendage of the holotype (ROM 56459; Figs. 3.13, 4.1) was originally interpreted as being a type A, but is here reinterpreted as
← FIGURE 3—Orcanopterus manitoulinensis n. gen. and sp. 1, Parabolic-shaped carapace with visible ocelli, ROM 56450; 2, carapace mold showing antelaterally positioned reniform eyes, ROM 56451; 3, dorsal anterior margin of carapace showing terraced fold ornamentation, ROM 56462; 4, prosomal appendages II–V, ROM 57018; 5, coxa of prosomal appendage IV?, ROM 56455; 6, coxa of prosomal appendage V?, ROM 56454; 7, coxa of prosomal appendage VI, ROM 56453; 8, prosomal appendage VI with swimming paddle, ROM 56452; 9, complete petaloid A metastoma with deep anterior cordation, ROM 56456; 10, incomplete petaloid A metastoma with well-developed dorsal doublure, ROM 56457; 11, ventral surface of metastoma exhibiting prominent crescentic scales, ROM 56456; 12, counterpart and part of partial specimen showing curving of preabdominal segments, ROM 56458 A, B; 13, ventral surface of partial eurypterid showing type B genital appendage with furca, ROM 56459; 14, ventral surface of partial xiphous telson, ROM 56461; 15, dorsal surface of incomplete telson exhibiting prominent central keel, ROM 56460; 16, distorted nearcomplete specimen, ROM 56449 (scale bar 5 50 mm). Scale bars 5 10 mm unless otherwise indicated.
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a type B, and is composed of two segments and the furca. There are no deltoid plates visible on the genital operculum, and spatulae are not present. Furthermore, the holotype (ROM 56459; Fig. 3.13) also shows that only the first pair of sternites (posterior to the genital operculum) are unfused. The three following sternite pairs are apparently fused, although no sutures are visible. The ornament of the dorsal opisthosoma and the ventral part of the mesosoma is composed of overlapping lunate and chevron-shaped scales, and the ventral metasoma has an ornament of scattered, raised, narrow lunules. This ornament is most notably expressed in smaller specimens on the dorsal middle third of each segment, but in larger specimens, the entire segment has this type of ornament dorsally. Larger specimens also tend to show an ornament of three to four lines parallel to the anterior segment margins on the dorsal side of the mesosoma; laterally these lines curve posteriorly to become parallel to the lateral margins of the segments. Metasomal segments reveal dentate posterior margins. The telson is incomplete but appears to be xiphous-shaped (ROM 56461; Fig. 3.14). There is a narrow marginal rim present, but no marginal ornament. Another specimen (ROM 56460; Fig. 3.15) indicates that a median keel was present on the dorsal side of the telson. Etymology.After Manitoulin Island, Ontario, where the specimens were found. Type.Holotype, ROM 56459, Royal Ontario Museum, Toronto, Canada. Other material examined.ROM 56449–56458; 56460–56463; 57017–57018; 57026 and many additional specimens in the ROM all retaining the original labeling (03D1), repository as above. Occurrence.Kagawong Submember (Upper Member) of the Georgian Bay Formation, Upper Ordovician (Richmondian). West Bay Indian Reserve road cut and Kagawong West Quarry section, Manitoulin Island, Ontario, Canada. Discussion.All well-preserved specimens can reliably be assigned to O. manitoulinensis, which on the basis of the largest partial specimen (Fig. 3.16) and proportions of dissociated elements is estimated to have had a total mature body length approaching 60 cm. A reconstruction of O. manitoulinensis is provided in Figure 5; the number of spines on each podomere of the spiniferous appendages is presumed to be two, as in hughmilleriids and most carcinosomatids. The variable opisthosomal ornament initially suggested that two forms were present in the fauna. However, one specimen (ROM 56458) shows that the ornament of the dorsal opisthosoma and the ventral mesosoma is composed of overlapping large lunate and chevron-shaped scales, and the ventral metasoma has an ornament of smaller, scattered, raised, narrow lunules more similar to that of Salteropterus abbreviatus (Salter, 1859). Fragments with an ornament of lunate and chevron-shaped scales mixed with fragments with other ornament do not necessarily imply that 1) a pterygotid is present in the fauna, nor that 2) there are two different forms present in the fauna. The spiniferous appendages are all apparently of Carcinosoma type (Tollerton, 1989), with much longer spines than those present in the Hughmilleria type. It also appears that the spines were anteriorly oriented, typical for carcinosomatids. The swimming appendage (VI) is unique. The large podomere 9 can only be compared to that of Dolichopterus Hall, 1859 and those of the carcinosomatids Carcinosoma Claypole, 1890 and Paracarcinosoma Caster and Kjellesvig-Waering, 1964, although this podomere is larger than the Carcinosoma type and more symmetrical than the Dolichopterus type. Podomeres 7 and 8 closely resemble the Mixopterus type. Coxa VI is interesting as it has a typical shape for hughmilleriids with a very bulbous distal end and a proximal, long, continuous gnathobase with 22 teeth. Coxa VI in Hughmilleria socialis has around 18–20 teeth (Sarle, 1903), while mixopterids (16 teeth for Lanarkopterus dolichoschelus Ritchie,
FIGURE 5—Reconstruction of Orcanopterus manitoulinensis n. gen. and sp. with a type B genital appendage (5male). 1, Dorsal view; 2, ventral view. The chelicerae are hypothetical and reconstructed according to its closest relatives.
STOTT ET AL.—NEW ORDOVICIAN EURYPTERID 1968) and pterygotids (12–13 teeth; Clarke and Ruedemann, 1912) have a reduced number of teeth on this element. Another character shared between O. manitoulinensis and hughmilleriids are a series of lines parallel to the anterior segment margins on the mesosomal segments. This has been reported from, for example, Parahughmilleria hefteri Størmer, 1974. Another putative character shared with hughmilleriids is the dentate posterior metasomal segment margins previously reported from P. hefteri (e.g., Størmer, 1974). The type B genital appendage in O. manitoulinensis is more similar to that described in Kokomopterus longicaudatus (Clarke and Ruedemann, 1912) by Kjellesvig-Waering (1966) and Størmer (1974) than those from established hughmilleriids, while type B appendages are poorly known from carcinosomatids. The implications of this similarity are presently unknown; K. longicaudatus is a stylonurid eurypterid, and thus would seem to be only distantly related to O. manitoulinensis on the basis of other characters. The telson has a marginal rim, but does not have any marginal ornament of sclerotized scales, as is the case in the pterygotids, as well as the genera Slimonia Page, 1856, Salteropterus Kjellesvig-Waering, 1951, and the species Hughmilleria banksii (Salter, 1856) (OET, personal observation). This suggests that similarities between the ornament of O. manitoulinensis developed elsewhere, and that of the pterygotids are a product of convergence and do not reflect a trait inherited from a common ancestor. Some characters are consistent with those of the family Hughmilleriidae Kjellesvig-Waering, 1951, such as the carapace shape, midsection second-order opisthosomal differentiation, coxal shape, the telson keel, and some of the ornamentation. However, other characters, such as the spinosity of appendages II–V, the form of podomeres 7–9 of appendage VI, the metastomal shape, and the curvature of the preabdominal segments, are more consistent with the Carcinosomatidae Størmer, 1934. A phylogenetic analysis of the entire Eurypterida, including this new form, is required to determine whether the new species belongs to the carcinosomatid clade, the ‘‘hughmilleriids,’’ or another group, but this work requires various systematic revisions and is beyond the scope of this contribution. ACKNOWLEDGMENTS
S. Donato, formerly of The University of Western Ontario (UWO) (currently at McMaster University, Ontario), is thanked for the initial discovery and recognition of this material and his assistance in the field. D. Tetreault (formerly at UWO, now at University of Windsor, Ontario), D. Rudkin (ROM), and R. A. Moore (University of Bristol) offered constructive and helpful discussions. A. D. McCracken (Geological Survey of Canada) reviewed the manuscript before final submission. OET thanks J. Waddington (ROM) for making this material available for study. The paper was reviewed by B. Kues and one anonymous reviewer. Field work and laboratory preparation of the fossils were funded through a NSERC research grant to J. Jin (UWO). OET acknowledges funding from the NFR (Norwegian Research Council) grant 145565/432 and the University of Bristol, United Kingdom. REFERENCES
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