Upper Silurian trilete spores and other microfossils from the Read Bay Formation, Cornwallis Island, Canadian Arctic

Upper Silurian trilete spores and other microfossils from the Read Bay Formation, Cornwallis Island, Canadian Arctic D. C. MCGREGOR Geological Survey of Canada, 601 Booth Street, Ottawa, Ont., Canada KIA OE8 AND G. M. NARBONNE Department of Geology, University of Ottawa, Ottawa, Ont., Canada KIN 6N5 Received February 8, 1978 Revision accepted April 6,1978 A diverse palynomorph assemblage of trilete spores, tetrads, acritarchs, chitinozoans, scolecodonts, and fragments of arthropods, ?hydrozoans, ?graptolites, and possibly nema- tophytalean and vascular plants occurs 20m above the base of member B of the Read Bay Formation at the type section on eastern Cornwallis Island, District of Franklin. This assemblage, here described briefly, is late Ludlow according to faunas above and below. The palynomorphs, other fossils, and stratigraphic framework indicate that the shales of member B were deposited in a sheltered near-shore marine environment. One new species of trilete spores, Retusotriletes chartulatus McGregor, is erected. Un assemblage palynomorphe diversifie de spores triletes, de tetrades, d'acritarches, de chitinozoaires, de scolecodontes, et des fragments d'arthropodes, d'?hydrozoaires, de ?grapto- lites et possiblement de plantes nematophytaleennes et vasculaires se retrouve a 20 m au-dessus de la base du membre B de la formation de Read Bay dans sa coupe-type dans I'est de 1'Ile de Cornwallis, District de Franklin. Cet assemblage qu'on decrit ici brievement date de la fin du Ludlovien selon les faunes observees au-dessus et en dessous. Les palynomorphes, d'autres fossiles et le cadre stratigraphique indiquent que les shales du membre B se sont deposes dans un milieu marin abrite, pres du littoral. On erige une nouvelle espece de spore trilete, Retusotriletes chartulatus, ~ c ~ r e g o r . Can. J . Earth Sci., 15, 1292-1303 (1978) Introduction The Late Silurian is generally considered as a time of particular significance in the transition of life from sea to land. Many authors have specu- lated, some at length, on various aspects of the marine to terrestrial evolution of early plants (e.g., Banks 1975a; Boureau and Moreau-Benoit 1973; Gray and Boucot 1977) and the oldest terrestrial arthropods (e.g., Kevan et al. 1975; Starmer 1976). However, relatively few diverse and securely dated fossil assemblages have been reported, upon which to base such s~eculation. In addition to their evolutionary significance, such assemblages may have considerable potential value for environmen- tal reconstructions and biostratigraphy. The presence in the fossil record of spores with a distinct trilete tetrad mark and resistant wall is thought to be one of the principal lines of evidence for adaptation by plants to a terrestrial environ- ment. In Devonian and younger rocks, spores of this sort are extremely abundant. They are the only fossils in some continental deposits, and thus are especially useful for correlation of nonmarine strata. In Silurian rocks, trilete spores were once thought'to'be rare, but they are now known to occur [Traduit par le journal] abundantly at some localities (Richardson and Ioannides 1973; Gray et al. 1974; Aristova and Ar- khangelskaya 1976). Nevertheless, no unequivocal record of Silurian trilete spores from Canada has been published. This account concerns an exceptionally diverse assemblage of late Ludlow age comprising 12 ap- parently distinct types of trilete spores and at least 4 kinds of tetrads, as well as acritarchs, chitinozoans, and scolecodonts. Also present are arthropod, ?hydrozoan, and ?graptolite fragments and possi- bly nematophytalean and vascular plant remains, some of which are not uncommon in palynological macerates of early Paleozoic age, although not often reported. Our aim is to illustrate and describe briefly this assemblage, to emphasize in so doing that a wide spectrum of potentially useful fossils may be en- countered in palynological preparations, to en- courage palynologists to give more attention to cer- tain types of palynomorphs that may easily be over- looked, and to add to the relatively meagre record of Silurian trilete spores. The assemblage was recovered from the type section of member B of the Read Bay Formation, C an . J . E ar th S ci . D ow nl oa de d fr om w w w .n rc re se ar ch pr es s. co m b y M cM as te r U ni ve rs ity o n 10 /1 5/ 14 Fo r pe rs on al u se o nl y. McGREGOR AN1 D NARBONNE 1293 on the south side of Goodsir Creek, eastern Cornwallis Island, Queen Elizabeth Islands (Figs. 1 and 2). The sample containing this assemblage was collected by Narbonne, who is co-author of the part of this report dealing with stratigraphy, and author of the part on depositional environment. Figured specimens bear numbers in the Geological Survey of Canada (GSC) type series, and are stored in the Palynological Type Collection of the Geological Survey of Canada, Ottawa, Canada. Stratigraphy Information on the stratigraphy and correlation of the Read Bay Formation is available in Thorsteinsson (1958), Thorsteinsson and Kerr (1968), and Gibling and Narbonne (1977). The stra- tigraphic nomenclature and biostratigraphy of the Read Bay and correlative strata of the Arctic Is- lands are being revised by Thorsteinsson and Uyeno (in preparation). The stratigraphic positions of the fossiliferous samples are shown in Fig. 3. The spore-bearing sample, a friable grey-black shale, was obtained from GSC loc. 94955, 20'm above the base of member B. Other samples, taken from the stratigraphic intervals 5.3-9.1, 135.6-149.4, 249.7-255.7, and 330.3-33 1.3 m below the top of member A (GSC locs. 94974-94977) by Narbonne, ffom 122 m below Dsvon Island * o * " d FIG. 1. Map of part of Canadian Arctic Islands, showing location of type section of member B, Read Bay Formation ('study locality') on Goodsir Creek. FIG. 2. Vertical aerial photograph of Goodsir Creek region showing stratigraphic units and fossil localities. A, B, and C = members, A, B, and C of Read Bay Formation; CS = Cape Storm Formation; m = Monograptus bohemicus tenuis; t = trilobites; s = trilete spores. the top of member A (GSC loc. 83348) by McGregor and Uyeno, and from 6.1, 10.6, and 19.7m above the base of member B (GSC locs. 83349,8413, and 8414) by McGregor and Uyeno (1968) contain ac- ritarchs, chitinozoans, and scolecodonts but no tri- lete spores or other identifiable palynomorphs. A sample obtained by McGregor and Uyeno (1968) from 15.2 m above the base of member C (GSC loc. 83350 = C-63576) contains acritarchs and rare tri- lete spores of generally similar appearance to those reported here. Seven samples collected from member D by Gibling do not contain paly- nomorphs. Thirty-one samples collected by Narbonne and i Gibling from rocks of Ludlow, Pridolian, and Gedinnian age on Somerset lsland (Leopold, Read Bay, Somerset Island, and Peel Sound Formations) did not yield spores. However, some of them, from the Leopold and Read Bay Formations, contain marine palynomorphs (chitinozoans and ac- ritarchs). Cramer and Diez de Cramer (1972) have recov- ered palynomorph assemblages consisting almost entirely of the planktonic alga Gloeocapsomorpha from the Read Bay Formation of Somerset and Cornwallis Islands, from unspecified localities. We assume that the Cornwallis Island localities re- ferred to by them are not GSC localities 83348 and 83349, in members A and B of the Read Bay, from which Cramer obtained chitinozoans according to I C an . J . E ar th S ci . D ow nl oa de d fr om w w w .n rc re se ar ch pr es s. co m b y M cM as te r U ni ve rs ity o n 10 /1 5/ 14 Fo r pe rs on al u se o nl y. CAN. J. EARTH SCI. VOL. IS, 1978 konodonts (83349) /-- Conodonts (C-63573) + Trilobites 195235) * Palynomorphs E&J Limestone a Shale 0 Sandstone OSC FIG. 3. Stratigraphic column of part of the Read Bay Forma- tion on Goodsir Creek, showing location of fossils. Numbers in brackets are GSC locality numbers. Uyeno (1977), as our preparations from these localities do not contain Gloeocapsomorpha, but do include leiospherid and acanthomorph ac- ritarchs, in addition to scolecodonts and abundant chitinozoans. Conodonts 6.1 m above the base of member B on the south side of Goodsir Creek (GSC loc. 83349) have been assigned by Uyeno (1977) to the upper part of the Polygnathoides siluricus zone of late Ludlow age. The uppermost beds of the underlying member A (GSC loc. 95235) contain a rich trilobite assemblage that according to Thomas (A. Thomas, written communication, 1977) includes " ... a new species of Hemiarges belonging to the ptyonurus- group. This species group is indicative of middle- late Ludlow, an age supported by an encrinurid species close to Frammia." Monograptus bohemicus tenuis BouEek (identified by R. Thorsteinsson, personal communication, 1977) is abundant from 129 to 168m below the top of member A (GSC loc. 95514). In Britain, M. bohemicus tenuis ranges from early to late Ludlow (Rickards 1976). Conodonts assignable to the P. siluricus zone from 122 and 2.9 m below the top of member A (GSC locs. 83348 and C-63573) also indi- cate a late Ludlow age. Beds stratigraphically above the palynomorphs, 15.2 m above the base of member C (GSC 10c. 83350 = C-63576), contain Pedavis latialata (Walliser) and other conodonts that are of late Ludlow age according to Uyeno (1977). The late Ludlow age of the palynomorphs from loc. 94955 is therefore fixed with some confidence by faunas above and below. Depositional Environment The lower 50m of member B is composed of sparsely fossiliferous grey to black silty shale. The absence of current indicators and the fine grain size suggest a low energy environment. Packard (J. Packard, personal communication, 1977) reported that certain levels in these shales contain a diverse macrofossil assemblage comprising trilobites, or- thoconic nautiloids, small articulate brachiopods (orthids and spiriferids), inarticulate brachiopods (Lingula), small bivalves, and large fragments of eurypterids and cyathaspidid ostracoderms. This assemblage suggests marginal marine conditions. Silurian eurypterids were most common in near- shore restricted marine environments (Heckel 1972, p. 284); most Silurian ostracoderms probably also lived in restricted marginal marine environ- ments (Denison 1956). The association of a diverse assemblage of trilete spores and tetrads with marine palynomorphs also favours the conclusion that the strata were deposited in a shallow near- shore environment (see Gray and Boucot 1972, 1977). This interpretation is supported by a considera- tion of the stratigraphic framework of the deposit. Underlying the shales of member B are beds of micritic limestone with a rich fossil assemblage in- cluding trilobites, corals, stromatoporoids, and crinoids. These limestones were deposited under subtidal conditions (Gibling and Narbonne 1977). Overlying the shales is a quartzarenite with desic- cation cracks, ripple-marks, planar and trough crossbeds, primary current lineation, and a sparse fauna of ostracoderms, pelecypods, and small spiriferid brachiopods. These indicators suggest a high-energy intertidal environment. The palynifer- ous shale between these two deposits probably ac- cumulated in a sheltered near-shore environment. C an . J . E ar th S ci . D ow nl oa de d fr om w w w .n rc re se ar ch pr es s. co m b y M cM as te r U ni ve rs ity o n 10 /1 5/ 14 Fo r pe rs on al u se o nl y. McGREGOR AND NARBONNE 1295 Palynomorphs The palynomorph assemblage comprises marine elements (acritarchs, chitinozoans, scolecodonts, and fragments of ?hydrozoans and ?graptolites), forms possibly of terrestrial origin (trilete spores, tetrahedral tetrads, possible vascular tissue and I nematophytalean tubes, and cellular sheets), and fragments suggesting a transitional marine-conti- nental environment (eurypterids and ?phyllo- carids). The relative abundances of trilete spores, tet- rads, and alete spheromorph palynofossils, based on a random count of 500 specimens, are: spores with distinct trilete marks, 19%; tetrads, 3%; His- panaediscus, 8%; alete leiospherid palynomorphs, 25% ( 12% CAN. J. EARTH SCI. VOL. 15, 1978 eral compression. Specimens of Hispanaediscus' sp. (PI. 1, figs. 20-22) may have been produced in tetrads similar to those shown in PI. 1, fig. 13. No isolated spores with strong apparently equatorial ridges as those of the components of the tetrad of P1. 1, fig. 15 were seen. Tetrads similar to the latter, from unspecified 'Middle Lower Llandoverian' rocks of the Niagara Peninsula, Ontario, have been illustrated by Cramer (l97la, P1.4. fig. 1). Tracheid-like Fragment Figures 16 and 17 of PI. 1 show parts of a speci- men 343 pm long and 160 pm wide, apparently un- tapered, with a dark, opaque central longitudinal region about 90 pm wide. The less opaque edges of the specimen, here illustrated, consist of alternat- ing light and dark bands about 1 ym wide and 1.5-2pm apart (PI. 1, fig. 17) and on one side, adjacent to the opaque central region, a suggestion of transversal bars between somewhat thicker and more widely spaced longitudinal bands (PI. 1, fig. 16). Evidence for suggesting this fragment may be conducting tissue of a vascular plant is tenuous, but the fragment illustrated in PI. 1, fig. 16 is at least suggestive of annularly thickened tracheids. The oldest known in situ tracheids are of uppermost Ludlow age (Edwards and Davies 1976). - Cluster of Spore-like Bodies An elongate-elliptical mass of spore-like spheromorphs was recovered, 100 x 19.5 ym in maximum width and length (PI. 1, fig. 18). The 'spores' are smooth, thin-walled, subcircular, and 26-31 pm in greatest diameter based on measure- ment of six specimens projecting from the edge of the cluster. No definite trilete marks or haptotypic features are visible, and none of the spheres shows evidence of orientation in tetrads. The shape of the cluster suggests that it may represent the more or less intact contents of a sporangium or other spore-bearing structure. However, there is no evi- dence of any enveloping wall, and the mass is rather small in comparison with known sporangia of Late Silurian and Early Devonian plants (e.g., Edwards 1970). Alternatively it could be a cluster of spheromorph acritarchs or even, conceivably, polychaete eggs (Kozlowski 1974). Hispanaediscus sp. One of the most abundant palynomorphs is the type referred here to Hispanaediscus Cramer (1966). The specimens are smooth-walled, subcir- cular, and commonly alete. Some specimens, how- ever, have a faintly discernible trilete configuration (Pl. 1, fig. 21), triradiate folds (PI. 1, fig. 20), or a thin polar wall (Pl. 1, fig. 22). The maximum equatorial diameter, based on measurement of 25 specimens, is 34-52 pm. The most characteristic feature is an abrupt change in the optical density of the wall at the inner edge of a peripheral subconcen- tric zone about 115 to 113 the radius in width. Com- monly the inner edge of the zone is marked by a narrow, opaque ring (PI. 1, figs. 21, 22), and in 1 addition on some specimens the whole zone is rela- tively dark (Pl. 1, fin. 20). similar firms have been reported previously from Late Silurian to early Emsian rocks, as Stenozonotriletes irregularis Schultz (1968, p. 32), ?S. irregularis (McGregor 1974, P1. I, fig. 4; McGregor and Uyeno 1972, P1. 1, fig. 2), ?Ar- chaeozonotriletes divellomedium Chibrikova (McGregor 1974), ?A. cf. divellomedium (Richardson and Lister 1969, p. 238), A . ? cf. divel- lomedium (Richardson and Ioannides 1973, p. 280), and "Spore no. 265 1" (Magloire 1968). ' I have found the criteria given by Cramer (1966) for distin- Acritarchs guishing Hispanaediscus from Saturnus dimcult to apply, and Acritarchsy numerous, are limited in have chosen to adopt the first described of the two, i s . , H ~ S - variety. Acanthomorph forms are most abundant, panaediscus, for the Read Bay specimens. and consist almost exclusively of Ammonidium sp. PLATE 1. x500 unless noted. (All specimens from GSC loc. 94955.) FIG. 1. Punctatisporites sp. GSC 55499. FIG. 2. Retusotriletes sp. GSC 55500. FIG. 3. Retusotriletes sp. GSC 55501. FIG. 4. Apiculiretusispora sp. GSC 55502. FIGS. 5,6. Apiculiretusispora spicula Richardson and Lister. Fig. 5, proximal view. Fig. 6, distal view, x 1000. GSC 55503. FIG. 7. Retusotriletes sp. GSC 55504. FIG. 8. "cf. Synorisporites verrucatus" of Richardson and Ioannides (1973, p. 278); specimen with faint trilete mark. GSC 55505. FIG. 9. ?Emphanisporifes protophanus Richardson and Ionnides, specimen with 'incipient ribs', but trilete mark not distinguishable. GSC 55506. FIGS. 10-12. Retusotriletes chartulatus McGregor n. sp. Fig. 10, holotype, GSC 55507. Fig. 11, specimen with well-displayed proximal wrinkles, GSC 55508. Fig. 12, same, x 1000. FIGS. 13-15. Three types of tetrads. GSC 55509-5551 1. FIGS. 16,17. Partsof tracheid-like fragment. GSC 55512. FIG. 18. Cluster of smooth palynomorphs. GSC 55513. FIG. 19. TWO spores of a tetrad, with minute conate sculpture. GSC 55514. FIGS. 20-22. Hispanaediscus sp. Fig. 20, specimen with triradiate folds and dark equatorial zone, GSC 55515. Fig. 21, specimen with faint trilete mark, GSC 55516. Fig. 22, specimen with thin polar spot, GSC 55517. FIGS. 23,24. Ammonidium sp. Fig. 24, X 1000. GSC 55518. FIGS. 25,26. Multiplicisphaeridium sp. Fig. 26, x 1000. GSC 55519. FIG. 27. Gorgonisphaeridium sp. GSC 55520. FIG. 28. Moyeria sp. GSC 55521. FIGS. 29-31. Moyeria uticaensis Thusu. GSC 55522-55524. C an . J . E ar th S ci . D ow nl oa de d fr om w w w .n rc re se ar ch pr es s. co m b y M cM as te r U ni ve rs ity o n 10 /1 5/ 14 Fo r pe rs on al u se o nl y. McGREGOR AND NARBONNE 1297 C an . J . E ar th S ci . D ow nl oa de d fr om w w w .n rc re se ar ch pr es s. co m b y M cM as te r U ni ve rs ity o n 10 /1 5/ 14 Fo r pe rs on al u se o nl y. 1298 CAN. J. EARTH (PI. 1, figs. 23, 24), Multiplicisphaeridium sp. (PI. 1. figs. 25,26), and Gorgonisphaeridium sp. (PI. 1, fig. 27). Leiospherid palynomorphs are also abundant but not all may be acritarchs. They comprise forms of varied size, wall thickness, and colour, some of which may be trilete spores with the haptotypic features obscured by folding, fracture, or corrosion of the wall. Moyeria uticaensis Thusu (PI. 1, figs. 29-31) and possibly another, smaller species of Moyeria (Pl. 1, fig. 28) are also present. Subcircular to ellipsoidal acritarchs with this rather distinctive decoration of muri occur in rocks of Ludlow age in northern Spain (Diez and Cramer 1976) and the Welsh Borderland (K. Dorning, personal com- munication, 1977), Wenlock strata of New York State (Thusu 1973), late Llandovery strata of New York State and southern Ontario (Cramer 1971b), and early Llandovery rocks of Belgium (Martin 1973). Spirally Thickened Tubes Rare specimens of thin-walled, helically thick- ened, tube-like structures were recovered. The largest fragment is illustrated in PI. 2, fig. 6. It is unbranched, 103 pm long, 18 pm wide at one end, tapered rather abruptly near the mid-point of its length to about 15 pm, then widened and again ta- pered to 15 pm at the other end. The spiral bands are about 0.5 pm wide, 1.5-2.5 pm apart, and ap- pear to be internal. The unthickened part of the wall is too thin to be measured accurately with the con- ventional microscope, but appears to be less than 0.25 pm thick. The figured specimen is closely as- sociated with a dark, hypha-like structure (see be- low) . Spirally or annularly thickened tubes much like these were found in Downtonian (=Pridolian) and Early Devonian (probably SiegenianIEmsian) rocks by Lang (1937) and Lyon (1962) respectively as part of Nematothallus and Nematoplexus, plants of unknown affinity. Similar tubes dispersed in Llandovery to early Devonian palynological preparations have been reported on several occa- sions (e.g., Banks 1975a, Llandovery; Pratt et al. 1978, ?Llandovery ; Eisenack 1971, 1972, Pridoli; Mortimer and Chaloner 1972, ?Early Devonian). Lang (1937), and others more recently, have drawn attention to the common association of resistant-walled spores and cellular sheets with these tubes, and have suggested that they may have been part of the same complex of pseudovascular land plants, possibly related to Prototaxites. Lang's proposal (1937, pp. 281, 282) that Nematothallus-Prototaxites and some commonly associated plant remains are of continental origin seems now to be the view generally held (e.g., Niklas 1976; Schmid 1976; Gray and Boucot 1977; SCI. VOL. IS, 1978 Pratt et al. 1978). That the helically-thickened tubes are related to Chitinozoa, as some have sug- gested (see Banks 1975a) seems unlikely in view of their presence in some undoubted continental strata. On the other hand, similar tubes occur with ac- ritarchs and chitinozoans in such nearshore or in- tertidal marine deposits as the Beyrichia Limestone (Eisenack 1971), the Burgsvik Beds (Gray et al. 1974), and member B of the Read Bay Formation. It seems reasonable to suppose that some or even most of them are the remains of nematophytalean plants, although this is not proven (see Eisenack 1972). If they are, it would seem that at least some of these plants could have inhabited intertidal and (or) supratidal mud-flats, close to the site of their deposition. One specimen of another kind of spirally thic- kened tube was found (Pl. 2, figs. 4 and 5). It is narrower than the type described above (4-7 pm wide excluding the height of the thickenings) and 90 pm long. The encircling spiral structure is about 1-1.5 pm wide, projects about 1 pm beyond the supporting wall, and encircles the tube at a spacing of 1.5-2 pm. Both ends of the tube appear to be broken off, suggesting that the specimen was longer when intact. The identity of this structure is not known. It resembles, perhaps superficially, the thread-like elaters comprising the capillitium of some Myxomycetes (slime-molds), particularly the Tri- chiales (Alexopoulos 1973). Evidence is not sufficient to infer a relationship to the Myxomy- cetes, however. Alternatively it may be part of the nema of a planktonic graptolite, although we know of no nemata with closely comparable structure. Other Tube-like Structures Some thin-walled, unbranched, tube-like speci- mens, about 20-25 pm wide, lacking thickened bands, are present but relatively rare (PI. 2, fig. 7). Pratt et al. (1978) found unbanded tubes of simi- lar dimensions associated with banded tubes and suggested with caution that they may be nema- tophytic. Fragments of narrower, dark brown, smooth 'tubes' are abundant in our preparations. They are commonly unbranched, but some branching specimens were found (Pl. 2, figs. 1 and 6). The commonest type is represented by the specimen illustrated on PI. 2, fig. 1, which is 178 pm long and 7 pm in maximum width. It bears three branches at obtuse angles, and toward one end two rounded lumps that may be the stumps of incipient branches. A dark, transverse mark on the main axis may be a septum (see arrow on figure), but could also be caused by aplication of the wall. No definite C an . J . E ar th S ci . D ow nl oa de d fr om w w w .n rc re se ar ch pr es s. co m b y M cM as te r U ni ve rs ity o n 10 /1 5/ 14 Fo r pe rs on al u se o nl y. McGREGOR AND NARBONNE 1299 PLATE 2. X 500 unless noted. (-411 specirnensfrorn GSC loc. 94955.) FIG. 1. Ribbon-like structure (?hydrozoan), GSC 55525, and tetrad of smooth spores, GSC 55526. FIG. 2. Sheet of cells of cuticle, x250. GSC 55527. FIG. 3. Sheet of cells or cuticle with wide rnuri, ~ 2 5 0 . GSC 55528. FIGS. 4,5. Spirally thickened tube of unknown affinity. Fig. 4, x 1000. GSC 55529. FIG. 6. Spirally thickened tube associated with unbanded, branched, hypha-like structure (?Nematothallus), X 1000. GSC 55530. FIG. 7. Tube-like structure. GSC 55531. FIG. 8. Fragment of ?graptolite theca, ~ 2 0 0 . GSC 55532. FIGS. 9-11. Chitinozoa, x250. GSC 55533-55535. FIGS. 12-15. Scolecodonts. GSC 5553655539. C an . J . E ar th S ci . D ow nl oa de d fr om w w w .n rc re se ar ch pr es s. co m b y M cM as te r U ni ve rs ity o n 10 /1 5/ 14 Fo r pe rs on al u se o nl y. 1300 CAN. J. EARTH SCI. VOL. 15, 1978 septa were seen on any other specimens. The figured specimen is twisted just above one of the branches, and the longest branch is twisted close to its junction with the main axis. Where twisted, the axis is narrow, suggesting that the structure was ribbon-like when living. This feature, and the gen- eral similarity in size and form to specimens illus- trated by Kozlowski (1959), Eisenack (1971), and others, leads us to believe that the specimens are the organic (?chitinous) remains of hydrozoan coelenterates rather than, for example, coenocytic fungi or nematophytes. A tube of somewhat different form is twined around the large, helically thickened tube shown in PI. 2, fig. 6. It is narrower than the hydrozoid struc- tures described above, apparently isodiametric, and repeatedly branched. Parts of the structure follow the contour of the larger tube in such a way that they appear to have been so associated at the time of burial. Transverse light zones resembling breaks in the tube occur at irregularly spaced inter- vals, but the abutting ends are not displaced. This narrow tube is of about the same diameter as the narrower tubes that also occur with wider, banded tubes in some specimens of Nematothallus (e.g., Lang 1937, p. 262). Cellular Sheets Sheets of cells or pseudocells are abundant in the macerate. They are predominantly of one kind, but vary in diameter of lumina and thickness of muri (Pl. 2, figs. 2,3). There is no evidence of stomata or other localized differentiation of the 'cells' except for apparently randomly scattered thinner areas (Pl. 2, fig. 2). Commonly the edges of the sheets are broken, with the breaks cutting cleanly across muri and lumina. Rarely, parts of the edge seem intact and have a smooth, slightly thickened limit. The sheets appear to consist of only one layer. The lumina are of irregular shape and varied size, and the muri between vary considerably in thickness even on one specimen. The cells comprising the sheet illustrated on P1.2, fig. 2 are among the small- est seen; the lumina are mostly 9-16 pm in greatest width, and the muri are about 1-2 pm wide at their base and tapered upward to thin, membranous crests. At the other extreme, the fragment shown on P1. 2, fig. 3 is composed of larger lumina (most 14-24 pm) and wider muri (up to 9 pm). Some of the wider muri, although darker than the lumina, seem to be partitioned off and have relatively light inter- iors, so that the distinction between lumina and muri is gradational. Various types of cuticle-like and cell-like mate- rial have been found in pre-Devonian rocks. Opin- ions as to their affinities have been set out at some length, for example by Lang (1937), Chaloner et al. (1974), Banks (1975a), Gray and Boucot (1977), and Pratt et al. (1978). It seems that the various forms could be plant tissue, mostly (at least, prior to late Ludlow) nonvascular (e.g., Nematothallus, Spongiophyton, Protosalvinia), or animal cuticles (e.g., eurypterids, annelids). From comparison with published photographs and descriptions, the Read Bay specimens are somewhat like certain forms of Spongiophyton 'cuticle' from Devonian strata(Cha1oner et al. 1974, P1. 121, fig. 2; Mortimer and Chaloner 1972, P1.111, fig. 1) or Nematothallus (Lang 1937, Fig. 64; Banks 1975b). The compari- sons admittedly are superficial and tenuous. Too little is known of the morphology and chemistry of the cuticle-like parts of most Silurian organisms, including Spongiophyton and Nematothallus, to permit these dispersed fragments to be correlated with them confidently. Arthropods Several types of eurypterid fragments, and some that may be remains of phyllocarids, are present. Most can be matched remarkably closely with pub- lished illustrations and descriptions. According to the literature, and my own observations, fragments of eurypterid cuticle are not uncommon in paly- nological preparations from lagoonal and tidal- flat marine deposits, and may have been more fre- quently encountered than reported. 1. Sheet-like fragments perforated by irregularly spaced, narrow-rimmed, subcircular to subellipti- cal holes 16-42 pm wide (Fig. 4a): see Eisenack (1955, P1.5, fig. 2; 1956, PI. 9, fig. 6), Taugourdeau (1965, PI. 3, fig. 71; 1967, PI. 2, fig. 25). 2. Sheet-like fragments with irregularly spaced, subcircular holes 1.5-2.5pm wide without rims (Fig. 4b): see Rolfe (1962, PI. 7, fig. 1, Ceratio- caris). 3. Sheet-like fragments with dark, broad-based spines (Fig. 4c): see Taugourdeau (1967, PI. 2, figs. 15, 19,23,24), Eisenack (1970, Fig. 4). 4. Long, tapered spines with a 'root' of cell-like composition (Fig. 4 4 : see Taugourdeau (1967, PI. 1, fig. 9 and p. 123). See also Richardson and Ioan- nides (1969, PI. 9, figs. 16, 17)for somewhat similar, but possibly unrelated structures. 5. Fragment with rounded, perforated, serially arranged folds or thickenings (Fig. 4e): see Holm (1898, PI. 8, fig. 4), Eisenack (1955, P1. 5, fig. 1; 1956,Pl. 8,fig. 3,Pl. 9,figs. 3-5,Pl. 10,fig. 1 b , ~ ) , Taugourdeau (1967, P1. 1, figs. 8, 10). 6. Long, narrow, gradually tapered spines with knob-like bulge at the broad end (Fig. 4j): see Holm (1898, P1. 8, fig. 2), Eisenack (1955, PI. 5, fig. 5; 1956, PI. 8, fig. 4a; 1972, P1.20, fig. 7), Taugourdeau (1967, PI. 2, figs. 21,25,27). Fragments like those illustrated in Figs. 4 a , e, f C an . J . E ar th S ci . D ow nl oa de d fr om w w w .n rc re se ar ch pr es s. co m b y M cM as te r U ni ve rs ity o n 10 /1 5/ 14 Fo r pe rs on al u se o nl y. McGREGOR AN ID NARBONNE 1301 FIG. 4. Fragments of arthropods: a and c-f, eurypterids; b, ?ceratiocarid; a, d, ~ 2 5 0 ; b , ~ 5 0 0 ; c, e, f, x 100. For details see text. a-e, GSC 55540-55544 respectively ;f, GSC 56648. were referred to Eurypterusfischeri(?) by Eisenack (1956), and specimens like that shown in Fig. 4f were prepared from megascopic remains of E. Jischeri by Holm (1898). The fragments from the Read Bay cannot be identified with confidence as E. remipes tetragonophthalmus Fischer (=E . Jischeri Eichwald) because information regarding the microstructure of the cuticle of this species and other eurypterids is too sparse. Nevertheless, the evidence of the micro-remains is sufficient at least to suggest that representatives of the E. remipes tetragonophthalmus fauna may occur in member B of the Read Bay Formation. Unidentified macro- remains of eurypterids have already been reported in members B and C of the Read Bay at Goodsir Creek (see Depositional Environment, above, and Thorsteinsson 1958, p. 68). E. remipes tetra- gonophthalmus has been found 540m below loc. 94955 at Goodsir Creek, in the Cape Storm For- mation2 (Copeland and Bolton 1960, p. 24). ?Graptolite Fragment One fragment in the macerate may be part of a graptolite theca (see Kraft 1926; Kozlowski 1971) or part of one of the little studied early Paleozoic rhabdopleurids (see Kozlowski 1970). It has the characteristic amber colour of graptolites, and al- though quite dark its semi-annular rings (?fusellae) about 20-25 pm wide are clearly evident (PI. 2, fig. 8). The structure is 440 pm long and 300 pm wide. Its surface bears an ornament of minute dots 1 pm or less in diameter and 1-4 pm apart. Chitinozoans and Scolecodonts A few intact chitinozoans and more abundant fragmentary chitinozoans were recovered. Those illustrated in PI. 2, figs. 9-11 are not necessarily representative of the kinds that are present, as the preparation method was perhaps too rigorous for good recovery of chitinozoans. Gentler treatment aimed at extraction of chitinozoans might result in specimens of greater abundance and diversity. Scolecodonts occur in considerable variety but not abundantly. Four of the smaller forms are illus- trated in P1. 2, figs. 12-15. No attempt has been made to identify those that were found. Systematics Genus Retusotriletes Naumova emend. Streel, 1964 Type species: R . simplex Naumova, 1953 (lectogenotype of Potonie, 1958) Retusotriletes chartulatus McGregor n. sp. P1. 1, figs. 10-12 1974 Retusotriletes sp., McGregor, PI. I, fig. 7. Diagnosis Spores radial, trilete. Amb subcircular, oval or elliptical. Laesurae distinct, extending 415 to 718 of the distance to the equator, simple or with narrow lips. Curvaturae perfectae defined by low, narrow ridges that may be in part confluent with the equator interradially. Contact areas with fine rugulae or wrinkles 0.5 pm or less wide, somewhat irregularly disposed but running in a generally ra- dial direction; and, on some specimens, with mi- nute closely spaced grana. Wall levigate outside contact areas, about 1 pm thick, and commonly folded. Diameter (20 specimens)-28-55 pm, mean 47 pm. ZFormerly the lower part of member A of the Read Bay Formation (see Uyeno 1977). C an . J . E ar th S ci . D ow nl oa de d fr om w w w .n rc re se ar ch pr es s. co m b y M cM as te r U ni ve rs ity o n 10 /1 5/ 14 Fo r pe rs on al u se o nl y. 1302 CAN. J. EARTH SCI. VOL. 15, 1978 Occurrence COMBAZ, A. 1967. Un microbios du tremadocien dans un son- GSC loco 94955, Goodsir Creek, Cornwallis 1s- dage d'Hassi-Messaoud. Actes de la Socittk Linntenne de Bordeaux, 104B, No. 29, pp. 1-26. land' District of 20m above base of COPELAND, M. J., and BOLTON, T. E. 1960. Canadian fossil Inember B, Read Bay HO1Otype, Arthropoda, Eurypterida, Phyllocarida and Decapoda. Geo- fig. 10, slide 94955- 1 ; figured specimen, PI. 1, figs. logical Survey of Canada, Bulletin 60.84 p. 11, 12, slide 94955-2. CORN& 0 . 1970. Plant remains in the Ordovician of the Bohe- mian Massif. Geologicky Zbornik, Geologica Carpathica, 21, Com~arison DD. 183-186. L . ~ -~ ~ ~ ~ h ~ ~ i ~ ~ ~ ~ i ~ ~ ~ D of ~ i ~ h ~ ~ d ~ ~ ~ and loan- CRAMER, F. H. 1966. 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