Early Devonian eurypterids from the Northwest Territories of Arctic ...

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Abstract: A new eurypterid fauna from the Lower Bear Rock Formation (Early Devonian, Emsian) of Anderson River, in the Northwest Territories of the Canadian ...
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Early Devonian eurypterids from the Northwest Territories of Arctic Canada Simon J. Braddy and Jason A. Dunlop

Abstract: A new eurypterid fauna from the Lower Bear Rock Formation (Early Devonian, Emsian) of Anderson River, in the Northwest Territories of the Canadian Arctic, is described. The material comprises an almost complete specimen and five isolated carapaces of Erieopterus microphthalmus; an incomplete carapace and telson referred to Drepanopterus sp.; and an isolated prosomal appendage of Carcinosoma sp. Associations include actinopterygian, sarcopterygian, and acanthodian fish, as well as lingulids, conchostracans, ostracodes, coprolites, and plant material. A nearshore marine environment is inferred. This assemblage provides the first Canadian record of Drepanopterus and the youngest Canadian occurrences of erieopterid and carcinosomatid eurypterids. Résumé : Cet article décrit une nouvelle faune euryptéridée de la Formation Bear Rock inférieure (Dévonien précoce, Emisien) de la rivière Anderson, aux Territoires du Nord-Ouest, dans l’Arctique canadien. Le matériel comprend un spécimen presque complet et cinq carapaces de Erieopterus microphthalmus, une carapace incomplète et un telson que l’on attribue à Drepanopterus sp., et un appendice prosomal isolé de Carcinosoma sp. Les associations comprennent des poissons actinoptérygiens, sarcoptérygiens, et acanthodiens, des Lingulidés, des Conchostracés, des Ostracodes, des coprolithes et du matériel végétal. On conclut qu’on est en présence d’un environnement marin littoral. Cet assemblage est le premier Drepanopterus canadien enregistré et les plus jeunes indices canadiens d’euryptéridés erieopteridés et carcinosomatidés. [Traduit par la Rédaction]

Braddy and Dunlop

Introduction Eurypterids (Chelicerata: Eurypterida) are Palaeozoic predatory arthropods. Although rare as fossils, they are known from many localities around the world, particularly in eastern North America. Their fossil record in the Canadian Arctic is poor. Copeland and Bolton (1960) reviewed the distribution of eurypterids from Canada and provided a further review of material from Ontario (Copeland and Bolton 1985), from where most Canadian eurypterids are known. More recently, Plotnick and Elliott (1995) reported Stylonurus sp. from Cornwallis Island (see below) and Jeram (1996) noted an unidentified stylonurid from the Battery Point Formation (Middle Devonian) of Gaspé, Quebec. Eurypterids from the Northwest Territories of Canada are known from only a few localities. Baltoeurypterus tetragonophthalmus (Fischer 1839) and Erieopterus laticeps (Schmidt 1883) were reported from the Read Bay Formation (late Wenlock) of Cornwallis Island by Thorsteinsson (1958). Carcinosoma sp. was noted by Copeland (1962), from the Allen Bay Formation (Wenlock) of Devon Island. Further fragments of Eurypterus or Baltoeurypterus were noted by McGregor and Narbonne (1978) from the Read Bay Formation (late Wenlock) of Cornwallis Island. Received November 2, 1999. Accepted February 23, 2000. Published on the NRC Research Press Web site on July 28, 2000. S.J. Braddy.1 Department of Earth Sciences, University of Bristol, Wills Memorial Building, Queen’s Road, Bristol, BS8 1RJ, United Kingdom. J.A. Dunlop. Institut für Systematische Zoologie, Museum für Naturkunde der Humboldt – Universität zu Berlin, Invalidenstraβe 43, D-10115, Berlin, Germany. 1

Corresponding author (e-mail: [email protected]).

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Eurypterus sp., Baltoeurypterus sp., and Rhinocarcinosoma sp. were recorded from the Leopold Formation (late Ludlow) of Somerset and Leopold Islands by Jones and Kjellesvig-Waering (1985). Plotnick and Elliott (1995) reported Stylonurus sp., from the Snowblind Bay Formation (Early Devonian) of Cornwallis Island. Other putative eurypterids from this area include the enigmatic Angustidontus, known from the Late Devonian Ireton Formation of Yukon Territory and the lowermost Carboniferous of Alberta. This form, which resembles the gracile chelicerae of a pterygotid eurypterid, was compared with the second maxilliped of the stomatopod Squilla by Copeland and Bolton (1960), and hence rejected from the Eurypterida by Kjellesvig-Waering (1964). However, the alternating pattern of short and long teeth in Angustidontus (e.g., Copeland and Bolton 1960, Pl. 10, fig. 3) is unknown from the dactylus of the second thoracopod of any known fossil or Recent stomatopod (C. Hof, personal communication, 1999). Briggs (1979) suggested that the affinities of Angustidontus may lie with the eurypterids. Perhaps there is cause for the inclusion of Angustidontus back in Eurypterida. The trackways of eurypterids are also known from Canada, supplementing our knowledge of their stratigraphic distribution in this region. Trackways described by Greiner (1972), from the Jacquet River Formation (Early Devonian) of Chaleur Bay in northern New Brunswick, were referred to Palmichnium antarcticum (Gevers et al. 1971) by Braddy and Milner (1998). Their unusual morphology (short length and narrowing series) may be due to the animals responsible (?stylonurid eurypterids) launching themselves off the substrate into the water. Another “launching” P. antarcticum trackway, from the Battery Point Formation (Middle Devonian) of the Gaspé peninsula of Quebec, was described by © 2000 NRC Canada

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Fig. 1. Erieopterus microphthalmus Hall. Anderson River, Northwest Territories, Canada. (a) MBA 931. Part. x 1. (b) CMN 51120a. Counterpart. x 1.

© 2000 NRC Canada

Braddy and Dunlop Fig. 2. Erieopterus microphthalmus Hall. Anderson River, Northwest Territories, Canada. Interpretative drawings. (a) MBA 931. Part. (b) CMN 51120a. Counterpart. Scale bar equals 2 cm.

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quence of about 2 m, and the eurypterid material was distributed throughout the middle part of this sequence. This assemblages provides the youngest occurrence of Erieopterus, Drepanopterus, and Carcinosoma from Canada.

Materials and methods The materials described here were collected in 1997 as part of an expedition led by Prof. H.-P. Schultze (Berlin) and S. Cumbaa (Ottawa). The eurypterid fossils are preserved in positive relief in a dark grey, poorly laminated marl matrix, although in places this matrix has weathered to a pale brown colour. Cuticle is often preserved as a brown film. Drawings were prepared using a camera lucida. All measurements are given in millimetres. Morphological terminology and systematics for the higher eurypterid taxa follow Tollerton (1989). The material has been divided between the palaeontology collections of the Museum für Naturkunde, Berlin (MBA) and the Canadian Museum of Nature, Ottawa (CMN). Specimen CMN 51120 has three separate fossils occurring on the same slab, which are denoted by the letters “a,” “b,” and “c,” respectively.

Systematic palaeontology Order Eurypterida Burmeister, 1843 Superfamily Eurypteroidea Burmeister, 1843 Family Erieopteridae Tollerton, 1989 Genus Erieopterus Kjellesvig-Waering, 1958 Erieopterus microphthalmus microphthalmus (Hall 1859) (Figs. 1–3, 4c)

Braddy and Milner (1998) and attributed to a large scorpion or stylonurid eurypterid. More P. antarcticum like trackways have recently been discovered in the Whirlpool Formation (Llandovery) from near Georgetown, Ontario (J. Waddington, personal communication, 1999). In this example, many overlapping trackways are preserved, associated with several subparallel medial impressions, although in places discrete series of three oval tracks, each 4–5 mm in diameter, arranged in opposing near-oblique chevrons, are apparent (external width 91 mm, internal width 32 mm, average stride 38 mm). They are reportedly associated with Onychopterellalike eurypterids, their potential producer. In this paper, we report on a new Early Devonian eurypterid fauna from the Bear Rock Formation at Anderson River, in the Northwest Territories of the western Canadian Arctic, about 330 km east of Inuvik and 150 km southsouthwest of Paulatuk (68°11′06″N, 125°49′72″W). The strata at this locality have been dated as Early Devonian (Emsian), based on the presence of the ostracode Moellerita (H.-P. Schultze, personal communication, 1999). The palaeoenvironment most likely represents a nearshore marine environment based on sedimentology and associations. Associated fossils include fish (actinopterygians, specifically Dialipina, sarcopterygians, and small acanthodians), as well as lingulids, conchostracans, ostracodes, coprolites, and (?algal) plant material. The fossils were collected from a se-

Material Almost complete specimen, part (MBA 931) and counterpart (CMN 51120a). Five isolated carapaces, each with part only: MBA 932, MBA 933, CMN 51120c, 51121, and CMN 51122. Horizon and locality Lower part of the Bear Rock Formation, Early Devonian (Emsian). Standard Oil Company locality Number AR.25.58 (Mr. Ben Moore’s Anderson River traverse in summer 1958), 68°11′06″N, 125°49′72″W, Anderson River, about 330 km east of Inuvik and 150 km south-southwest of Paulatuk, Northwest Territories, Canada. Description Carapace known from six specimens (see Table 1 for proportions). MBA 931 (part; Figs. 1a, 2a) and CMN 51120a (counterpart; Figs. 1b, 2b) has a semicircular carapace, a poorly preserved lateral eye tubercle on the right side of the part, and the distal two podomeres of appendage VI preserved on left side of part. MBA 932 (Fig. 3a) consists of a partial (posterior margin broken and both posterolateral corners missing), distorted, semicircular carapace with small, reniform lateral eyes, and a tiny median eye (ocelli) tubercle, located centrally between the lateral eyes. CMN 51121 (Fig. 3b) is a semicircular carapace (anterior margin broken), with reniform lateral eyes and a small cuticle fragment to the left of the carapace, probably representing a partial isolated © 2000 NRC Canada

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Can. J. Earth. Sci. Vol. 37, 2000 Table 1. Proportions of the six specimens of Erieopterus micropthalmus micropthalmus Hall 1859. Prosomal proportions Specimen

Length

Width at base

Eye length

Eye width

MBA MBA CMN MBA CMN CMN

32 35 41 46 37 42

54 59 68 91 (distorted) 61 46a

4 5 7 7 7 6

3 3 3 3 5 3

931/CMN 51120a 932 51121 933 51122 51120c

Proportions (length/width) of opisthosomal segments, in specimen MBA 931–CMN 51120a 1 5/54

2 6/55

3 6/54

4 7/53

5 8/51

6 8/49

7 6/41

8 9/27

9 8/24

10 16/17a

11 11/14

12 18/13

Note: All measurements are given in millimetres. a Preserved width.

Fig. 3. Erieopterus microphthalmus Hall. Anderson River, Northwest Territories, Canada. Carapaces. (a) MBA 932. x 1.08. (b) CMN 51121. x 1.2. (c) MBA 933. x 1.18. (d) CMN 51122. x 1.11.

podomere of a prosomal appendage. MBA 933 (Fig. 3c) consists of a distorted (stretched laterally, posterior margin concave) carapace, the right margin broken, with lateral eye tubercles. CMN 51122 (Fig. 3d) consists of the left half of a semicircular carapace, posterior margin broken in places,

left lateral eye well preserved, and possible median eye tubercle faintly preserved on mid-line. CMN 51120c (Fig. 4c) consists of a partial semicircular carapace, the posterior left corner and right half broken, with a well-preserved left lateral eye. © 2000 NRC Canada

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Fig. 4. Drepanopterus sp. (a-d) and Erieopterus microphthalmus Hall (c). Anderson River, Northwest Territories, Canada. (a) CMN 51123. Carapace part. x 0.78. (b) CMN 51123. Carapace partial counterpart. x 0.95. (c) CMN 51120b (Drepanopterus telson part) and CMN 51120c (Erieopterus carapace). x 0.85. (d) MBA 934. Telson counterpart. x 0.85.

Opisthosoma known only in MBA 931 (part; Figs. 1a, 2a) and CMN 51120a (counterpart, broken into two pieces; Figs. 1b, 2b). Total reconstructed length of this specimen is 183, including telson (the specimen curves slightly to the left). See Table 1 for proportions of opisthosomal segments. Preabdomen broadly oval, length 46, but broken on right side. Median region of second tergite superimposed by impression of proximal portion of genital appendage, length 6, width 4. Tergite 7 produced into posterolateral flange. Postabdomen total length 62, excluding telson. Telson lanceolate, length 43, slightly bulbous proximally, width 9, ta-

pering to a sharp point. Cuticle preservation patchy over whole body surface, but details of slight tuberculation apparent where preserved. Remarks Kjellesvig-Waering (1958) diagnosed the genus Erieopterus for eurypterids with a rounded prosoma, small reniform centrimesial eyes, and the distal podomere of appendage VI enlarged into a short spur (claw). Although the most diagnostic character of this genus, the distal podomere of appendage VI, is lacking from our material, the propor© 2000 NRC Canada

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tions of the carapace and position and relative size of the lateral eyes indicate that it can confidently be referred to Erieopterus. Kjellesvig-Waering (1958) recognised 10 species of Erieopterus, although E. pustulosus was subsequently transferred to Buffalopterus by Kjellesvig-Waering and Heubusch (1962), on the basis of its broad telson. Of the remaining nine species, some are based on only a single specimen and diagnosed on their carapace proportions (see Kjellesvig-Waering 1958 for a review). Erieopterus brewsteri (Woodward 1864), for example, a monotypic form from the Midland Valley assemblage in Scotland (the only record of this genus from the British Isles) is known from only a single isolated carapace. The position and shape of the eyes of E. brewsteri, and the form of the median ridge between them, is similar to Tarsopterella scotica (Woodward 1872) from the same assemblage; these similarities indicate that E. brewsteri may be a juvenile T. scotica (Braddy, in press). E. chadwicki (Clarke and Ruedemann 1912) and E. hudsonicus (Ruedemann 1934) are also known from monotypic and distorted specimens. In comparison with the known species of Erieopterus, our undeformed material is most consistent with E. microphthalmus (Hall 1859), in the shape of the carapace and the position and proportions of the lateral eyes. Three subspecies of E. micropthalmus are known: (i) E. microphthalmus microphthalmus (Hall 1859), from the Rondout Formation (dolomites; late Prídolí) of Syracuse county, New York and the Manlius Formation (limestones; Early Devonian) of Herkimer county, New York; (ii) E. microphthalmus eriensis (Whitfield 1882), from the Bass Islands dolomites (Prídolí) of south Bass Island, Ohio; and (iii) E. microphthalmus turgidus Stumm and Kjellesvig-Waering 1962, from a borehole in the Bass Islands dolomite (Prídolí) of Detroit, Michigan. They are all identical in most respects, but differ slightly in the proportions of their carapace and paddle and the degree of ornamentation (Kjellesvig-Waering 1958; Stumm and Kjellesvig-Waering 1962). Clarke and Ruedemann (1912) considered that E. microphthalmus eriensis and E. microphthalmus microphthalmus were identical. The present authors agree and suggest that such slight differences may be due to preservational factors. The material described here corresponds most closely with E. microphthalmus microphthalmus. E. microphthalmus microphthalmus was reported by Ciurca (1982) from Silurian strata near Hagersville and Cayuga, Ontario, although Copeland and Bolton (1985) were unable to confirm this. Erieopterids have previously been reported from the Northwest Territories. Copeland and Bolton (1960, pl. VII, fig. 6) recorded E. laticeps, from the late Wenlock of Cornwallis Island, but the material described here is larger, with smaller and more closely positioned eyes. Indeed, this new material is somewhat larger than previously described Devonian examples of Erieopterus (e.g., Størmer 1936; Kjellesvig-Waering 1958). Superfamily Stylonuroidea Diener 1924 Family Drepanopteridae Kjellesvig-Waering 1966 Genus Drepanopterus Laurie 1892 Drepanopterus sp. (Fig. 4)

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Material One carapace, part and partial counterpart (both CMN 51123). One telson, part (CMN 51120b) and counterpart (MBA 934, on the reverse of the same slab as MBA 931). Horizon and locality As above. Description The carapace (CMN 51123) consists of a part (Fig. 4a) and a partial counterpart (Fig. 4b), restricted to the area around the right eye. Carapace part large, anterior margin well rounded, with a broad marginal rim, 6 wide near the mid-line, but becoming gradually narrower laterally. Marginal rim has distinct ornament comprised of three ridges, which follow the outline of the carapace. Large lateral eyes, broadly oval, length 19, width 18. Median eye tubercle indistinct. Large oval structure near the centre of the carapace may be a taphonomic artefact (?clast impression), although its size (length 22, reconstructed width 14) and shape, with a cordate anterior margin, suggest that it is an impression of the metastoma, displaced forwards and rotated. The telson consists of part (CMN 51120b; Fig. 4c) and counterpart (MBA 934; Fig. 4d) of the distal portion only. Telson relatively broad, lanceolate, preserved length 102, proximal width 20, tapering to a blunt point. Telson has distinct median ridge (keel) and, in places, the lateral margin has a serrated edge, the serrations becoming smaller distally. A poorly preserved Erieopterus carapace also occurs on the part (see above). Remarks Although very incomplete, this material shows various characteristics of the genus Drepanopterus: (i) the wellrounded shape of the anterior carapace, (ii) the broad marginal rim of the carapace, (iii) the oval shape, large size, and centrimesial position of the lateral eyes, (iv) the oval-shaped metastoma, with a cordate anterior margin, and (v) the broad, tapering, styliform telson. Seven species of Drepanopterus are known, mostly from the Silurian, although two are known from the Devonian. D. abonensis Simpson 1951, from the Upper Old Red Sandstone (Late Devonian) of Portishead, near Bristol, England, is a large form with an unusual, elongated body. D. struvei Størmer 1976, from the Alken assemblage (Early Devonian) of Germany is a much smaller form. The material described here is similar both in size and proportions to D. abonensis, although the incompleteness of the material ensures that it is cannot be referred to this, or any other species with confidence. Based on Simpson’s (1951, Fig. 4) reconstruction, our material probably represents an animal almost 70 cm long. Stylonurid eurypterid body fossils (Copeland and Bolton 1960, appendix; Plotnick and Elliot 1995; Jeram 1996) and trace fossils (see above; Braddy and Milner 1998) are known from Canada, but this is the first record of Drepanopteridae and Drepanopterus in Canada. Superfamily Hughmillerioidea Kjellesvig-Waering 1951 Family Carcinosomatidae Størmer 1934 Genus Carcinosoma Claypole 1890 Carcinosoma sp. (Figs. 5–6) © 2000 NRC Canada

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Fig. 5. Carcinosoma sp. Claypole. Anderson River, Northwest Territories, Canada. Distal podomeres of large, spinose prosomal appendage. (a) MBA 935. Part. x 1.05. (b) CMN 51124. Counterpart. x 1.05.

Fig. 6. Carcinosoma sp. Claypole. Anderson River, Northwest Territories, Canada. Distal podomeres of large, spinose prosomal appendage. (a) MBA 935. Part. (b) CMN 51124. Counterpart. Scale bar equals 2 cm.

Material One prosomal appendage (III or IV), part (MBA 935) and counterpart (CMN 51124). Horizon and locality As above. Description Part (MBA 935; Figs. 5a, 6a) and counterpart (CMN 51124; Figs. 5b, 6b) comprising the distal five podomeres of a large prosomal appendage. Of these podomeres, the proximal four each have a single, long, slightly curved spine projecting from their distal margins, each spine ornamented with narrow grooves running along their length. Podomere 8 is itself a long spine. Extensive cuticle preserved on the counterpart, but podomere boundaries indistinct. Total preserved length 75. The spines of the proximal two preserved podomeres are broken distally. Next podomere partially disarticulated from preceding podomere, with spine broken distally. Next podomere shows spine, length 35. Distal podomere forms a single large spine lying adjacent to the spines of the preceding podomeres, length 38, basal width 7, tapering distally to a blunt point. Distal podomere also ornamented with parallel grooves.

Remarks Caster and Kjellesvig-Waering (1964) recognised eighteen species within Carcinosomatidae, but as many of these are diagnosed on poorly preserved material, four generic groups are more useful for taxonomic purposes: Carcinosoma, Eocarcinosoma, Paracarcinosoma and Rhinocarcinosoma. Carcinosoma, containing the majority of species, accommodates medium to large forms with a triangular carapace and strongly developed prosomal appendages II–V, orientated anteriorly with long spines, and a large paddle-shaped appendage VI, with prominently developed articulations. The remaining genera are smaller and have more weakly developed prosomal appendages (II–V) which, where known, are turned downwards. Although only a single incomplete appendage of this form is known, the morphology of MBA 935–CMN 51124 is clearly of Carcinosoma type (cf. Tollerton 1989, fig. 8). By comparison with more complete specimens (e.g., Clarke and Ruedemann 1912, Pls. 37–39), the specimen represents the distal podomeres of appendage III or IV of a large individual, with an estimated body length of nearly 60 cm. The size, robustness of the podomeres, and length of the spines are all most consistent with the genus Carcinosoma, although there is insufficient data to refer it to a species. © 2000 NRC Canada

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Some comparisons with previously reported Canadian carcinosomatids (see Copeland and Bolton 1960, 1985 for review) may be made. Carcinosoma sp. was reported in the Richmond Formation (Ordovician), from near Chambly, Quebec. C. libertyi Copeland and Bolton 1960 was described from the basal beds of the St. Edmund Formation (Wenlock), of Manitoulin Island, Ontario, although this form apparently did not possess spines as large and numerous as C. newlini (Claypole 1890) and the material described herein. C. logani (Williams 1915) is known from poorly preserved fragments from the Amabel Formation (Wenlock), from south of Eramosa and the Bruce peninsula in Ontario. Carcinosoma sp. nov. (Copeland and Bolton 1985, fig. 19) was also recorded from the Amabel Formation, west of Wiarton. From the Northwest Territories, Rhinocarcinosoma sp. was noted by Jones and Kjellesvig-Waering (1985) from the Leopold Formation (late Ludlow) of Somerset and Leopold Islands. Carcinosoma sp. was also noted from the Allen Bay Formation (Wenlock) of Devon Island (Copeland 1962, Pl. 12, fig. 1). This material consists of an isolated large anterior appendage, with long spines, somewhat similar to the material described herein. The new Early Devonian material described here represents the youngest record of the Carcinosomatidae in Canada.

Acknowledgements We thank Prof. Hans-Peter Schultze (Humboldt Museum, Berlin) for making this material available to us and for information on the geological setting. The collection of this material was supported by a German Science Foundation (DFG, grant Schu 212/10-1+2 to H.-P. Schultze), the National Geographic Society (grant 5742-96 to H.-P. Schultze), and the Canadian Museum of Nature (grants to S. Cumbaa). Dr. Cees Hof (University of Bristol) advised on stomatopods. Dr. Janet Waddington (Royal Ontario Museum) provided information concerning the Ontario trackways. Vera Heinrich assisted with Figs. 1, 3, 4, and 5. Prof. Derek E. G. Briggs, Dr. Derek J. Siveter, and one anonymous referee reviewed this paper. SJB gratefully acknowledges funding from the Leverhulme Trust (Grant F/82/AZ, to Prof. D. E. G. Briggs, University of Bristol).

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