A Silurian sphinctozoan sponge from east-central Cornwallis Island, Canadian Arctic

T. DE FREITAS Department of Geology, University of Western Ontario, London, Ont., Canada N6A 5B7

Received June 5, 1986

Revision accepted September 29, 1986

The first Silurian sphinctozoan ever recorded is reported from the Canadian Arctic. The polysiphonate sphinctozoan, Rigbyspongia catenula n. gen., n. sp., is found associated with numerous anthaspidellid sponges. Diagnostic features of the new genus are its numerous axial siphons and vertical pillars within each chamber. Although it is structurally complex for a sphinctozoan of this age and is the only Silurian representative of the order, this new genus provides few clues to the still enig- matic problem of Paleozoic sphinctozoan phylogeny.

Le premier sphinctozoaire silurien reconnu provient de 1'Arctique du Canada. Le sphinctozoaire polysiphone, Rigbyspongia catenula n. gen., n. sp., est observe en association avec de nombreuses kponges anthaspidellides. Ce nouveau genre est caractBrisk par de nombreux siphons axiaux et des piliers verticaux dans chaque chambre. M&me si ce sphinctozoaire est struc- turalement complexe pour son Ige et qu'il soit le seul repksentant silurien de l'ordre, ce nouveau genre foumit quelques Bclair- cissements sur la phylogknie des sphinctozoaires pal6ozoiques demeurant encore ambigiie.

[Traduit par la revue]

Can. 1. Earth Sci. 24, 840-844 (1987)

Introduction Ludlovian (Silurian) platformal slope sediments of the upper

Allen Bay and lower Cape Storm formations of east-central Cornwallis Island (Fig. 1) have yielded a unique and well- preserved sponge fauna. The basal portion of the slope section, approximately late Wenlockian in age, contains numerous crinoidal grainstone beds interbedded with mudstones; strati- graphically upward, however, grainstones decrease and para- conglomerates increase. The change from interbedded grainstone and mudstone to interbedded conglomerate and mudstone reflects platform-margin evolution from ramp like, with numerous crinoid shoals, to escarpment like, with active reef growth, respectively.

In this slope sequence, sponge biostromes occur in distal slope mudstones, in the transition between the interbedded grinstone and mudstone to interbedded paraconglomerate and mudstone. Although isolated sponges occur throughout the section, their abundance in the transitional beds perhaps reflects a period of slope quiescence; the short-lived, calm depositional regime and the apparent absence of any competi- tors perhaps presented optimal conditions for sponge growth in smafl but discrete biostromes.

From this, the oldest reported Silurian sponge community of the Canadian Arctic, conditions evolved such that sponges pro- liferated in bioherms of the late Ludlovian Douro Formation (Narbonne and Dixon 1984; Narbonne 1981) and of the Prido- lian Barlow Inlet Formation (Packard and Dixon 1979, 1982; Packard 1985; Graf and Dixon 1986). The Douro Formation biostromes may have reached 3 m in paleorelief and are capped by numerous stromatoporoids and corals; the latter may have reflected vertical accretion through wave base into the photic zone (?). In this study case, however, biostromes were very subdued paleotopographic features dominated by sponges, anthaspidellids in particular, and the communities thrived below wave base on the slope mudstones.

This paper documents the first reported occurrence of a Silurian sphinctozoan. The single specimen was found amongst numerous anthaspidellids in one of the slope bio- strome communities.

CLASS Calcarea Bowerbank, 1864 ORDER Sphinctozoa Steinmann, 1882

SUBORDER Porata Seilacher, 1962 FAMILY Polysiphoniidae Girty , 1908

The original description by Girty (1908, p. 86) is as follows:

Probably all types (of sponges) that could be referred to this family would have a conical or cylindrical shape, a thin outer wall, porous, possibly but without ostia, and an internal structure consisting of tabular canals, some of which run lengthwise and some in a radial direction. . . .

Girty's original diagnosis of the family, although based on one fragmentary specimen, is conformed to here in this newly dis- covered sphinctozoan. Presently, it is certain that p l y - syphonids are porate, possibly trabeculate, sphinctozoans; thus, the family diagnosis should be emended accordingly: cateniform, occasionally trabeculate, multisiphonate, porate sphinctozoans; siphons of the centrum may be interconnected by horizontal canals or pierced by endopores.

GENUS Rigbyspongia n. gen. TYPE SPECIES Rigbyspongia catenula

Diagnosis Cateniform, porate, trabeculate sphinctozoan with multi-

siphonate axial region. Shallow indistinct ventral spongocoel into which the majority of equal-sized siphons feed and con- verge; remaining siphons debouch out of exopores skirting the dorsal osculum. Numerous vertical pillars common to all chambers.

Discussion The last few years have seen a proliferation of early Paleo-

zoic sphinctozoan discoveries. The primitive, Cambrian sphinctozoan Blastulospongia (Pickett and Jell 1983) is the earliest known representative of the order. Its simple, single- chambered, asiphonate, porate character and lack of chamber fillings certainly compel one to describe this sphinctozoan as primitive. Numerous Ordovician genera include those from the Klamath Mountains of California (Rigby and Potter 1980,

Printed in Canada 1 Imprime au Canada

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FIG. 1 . Map showing the location of the present study.

1986) and from the Belubula River area of New South Wales, Australia (Webby and Rigby 1985). In contrast to the empty- chambered Early Carboniferous (Wendt 1980; Seilacher 1962; Ott 1967) and Cambrian forms (Pickett and Jell 1983) and to the multisiphonated Silurian sphinctozoan described herein, the porate, spouted, vesicular, retrosiphonate habit of the Ordovician angullongids (Webby and Rigby 1985) is certainly distinctive. An explosive radiation of the sphinctozoans in Ordovician times is perhaps indicated from an extensive suite of Ordovician sphinctozoans described from the Klamath Mountains (Rigby and Potter 1986). None of their genera are, however, allied to the sphinctozoan described herein. Silurian sponges from the Urals include Aphrosalpinx and Nematosal- pinx (Myagkova 1955; Vologdin and Myagkova 1962), but as Pickett and Jell (1983) suggested, the latter two fossils may be unrelated to the spinctozoan lineage. Indeed, the growth form of the two Russian representatives is not analogous to that of the typical sphinctozoan. Also from the Silurian of the Urals and not hitherto assigned to the spinctozoan group is the Late Silurian archaeocyathan (?) Palaeoschada crassimuralis (Myagkova 1955; Vologdin and Myagkova 1962). Although the occuI-rence of post-middle Cambrian archaeocyathans

remain dubious (Wendt 1980), the so-called archaeocyathan P. crassirnuralis is morphologically similar to the primitive sebargasiids and may very likely be assignable to this group of sphinctozoans.

Found within Devonian outcrops are Rudiothalamos unira- mosus (Pickett and Rigby 1983) and Hermospongia (Rigby and Blodgett 1983); however, the reticulate chamber fillings and the single, asymmetric siphon of Hermospongia and the radially partitioned chambers of Radiothalamos uniramosus are unlike the structure of Rigbyspongia catenula. Included within the Carboniferous sphinctozoan families are Thaumastocoelliidae, Celyphiidae, and Sebargasiidae (de Laubenfels 1955; Ott 1967; Seilacher 1962; Wendt 1980), but internally they have simple architecture and, as such, are in contrast to the complexly textured Silurian form described here.

The discovery of this "advanced" Silurian form is perhaps another key to the enigmatic sphinctozoan phylogeny. Cer- tainly, today's representatives (Vacelet 1977) will stem from the primitive Cambrian sphinctozoan of Pickett and Jell (1986), but by the Ordovician times, most of the features that designate a morphologically advanced sphinctozoan had

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842 CAN. I. EARTH SCI. VOL. 24, 1987

FIG. 2 . Rigbyspongia catenula n. gen, sp. (A) Transverse section of holotype (GSC 85330) showing the numerous vertical pillars. (B, C ) Longitudinal sections of the same (note the chain-like structures in the ventral, axial region of B; these represent incompletely preserved endo- pores of the porous siphon walls). (D) Tangential section showing filling structures and stacked chambers. (E) Exterior view of exowall pierced by numerous exopores. All figures x 3.

already amved. Thus, the Carboniferous genera did not suddenly occur, as Ott (1967, p. 57, Fig. 5), Seilacher (1961, p. 71, Fig. 8) and Wendt (1980, p. 176, Figs. 3.1 -3.5) postu- lated, but had evolved to that point, perhaps along several lines, from Cambrian precursors. From these ancestral Cam- brian forms, several separate lines have evolved from the mainstream Amblysiphonella - Blastulospongia stock (Pickett and Jell 1983; Rigby and Potter 1986). Each of the diverticula- tions of the mainstream stock perhaps accounts for the mor- phologically very different sphinctozoans of the Late Carboniferous and those of the early Paleozoic. The new early Paleozoic polysiphonate sphinctozoan described herein perhaps represents an early branching of the mainstream stock, but the contrasting structural simplicity of Middle Permian polysiphonid Polysiphon rnirabilis (Girty 1908) is somewhat enigmatic. Perhaps, with further discoveries, the polysiphon- ate lineage may be traceable from simple representatives of the

early Paleozoic; nevertheless, polysiphonates represent yet another stem of sphinctozoan phylogeny perhaps allied with, but not equivalent to, the Mesozoic Cryptocoeliidae (Stein- mann 1882) - Verticilliidae (Steinmann 1882) lineage of Seilacher (1961) and Wendt (1980) or to the porate, trabeculate sphinctozoans, stem C (the Arnblysiphonella - Cystothalamia lineage) of Finks (1970), p. 15, Fig. 11).

The multisiphonate habit (Fig. 3) of Rigbyspongia catenula is unlike that of a sponge in any other family; thus the morpho- logical distinction between this new representative and other previously described sphinctozoans is relatively clear. One sponge that may be allied to Rigbyspongia catenula, at least with respect to its chamber fillings, is Deningeria (Wilckens 1937, p. 113, P1. 13, fig. 2-5; P1. 11, fig. 3), but its pillars appear somewhat labyrinthine in longitudinal section (Pl. 11, fig. 3), and it lacks the polysiphonate character of this new genus. Polysiphon mirabilis, Girty 's type genus (Girty 1908,

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siphon

'S

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\\_/;/ \polysiphonote axial region

FIG. 3. Schematic reconstruction of Rigbyspongia showing distinc- tive structures of the genus (terminology after Finks 1983; Rigby and Potter 1986).

p. 87, P1. 16, figs. 11 - 1 lb), the only other sphinctozoan included within the family, is basically similar to Rigby- spongia catenula; however, it lacks chamber fillings and con- tains a small number of very discrete siphons and, as such, is clearly differentiated from Rigbyspongia catenula.

Etymology Rigbyspongia, named after J . K. Rigby, a distinguished

worker in sponge taxonomy.

Rigbyspongia catenula n. gen. et sp. (Figs. 2a - 2d)

Diagnosis Cateniform, polysiphonate sphinctozoan with eight or more

equal-sized siphons in longitudinal section; in transverse sec- tion at least 15 siphons visible. Siphons converge toward shallow, dorsal spongocoel and to dorsal exowall surrounding ostium. Endowall of shallow cloaca indistinct in longitudinal section because of numerous large, closely spaced endopores lining the gastral surface. Internal chambers manifested on exowall as 3 mm high ridges with wavelengths of 3.5 rnm or more. Chambers cluttered internally with vertical, occasion- ally distally bifurcating pillars.

Description The single, well-preserved, silicified specimen collected

among numerous hindiids and anthaspidellids measures 35 mm in length and 19 mm in width. Exopores are uniquitous yet variably spaced, averaging 0.50 mm in diameter and com- monly 0.30 mm apart; however, diameters ranging from 0.42 to 0.52 mm and separations ranging from 0.21 to 0.39 mm occur. The sponge surface is traversed by numerous round- crested annulations (surface manifestations of internal chambers) that project approximately 2.0 mm above constric- tions and have wavelengths of 3.0-0.5 mm.

Capping the sponge is a shallow 4.0 mm deep by 4.0 mm wide spongocoel into which many of the siphons feed. The

limited surface area of the endowall of the spongocoel can accommodate most of the siphonal openings; however, those siphonal openings that cannot be accommodated pierce the exowall skirting the margins of the shallow, dorsal osculum. Here ostia are clearly visible, ranging in diameter from 0.3 to 0.5 mm.

In longitudinal section, the cateniform organization of the chambers, the multisiphonate character, and the trabeculate chamber fillings are clearly discernible. Pillars are arranged such that they intersect the interwalls at almost 90°, although oblique angles with the bounding interwalls may occasionally be formed. Distally and, to a lesser extent, proximally dicho- tomizing pillars range in diameter from 0.25 to 0.5 mm. The latter, maximum size represents a pillar diameter at the junc- tion of a distal or proximal dichotomization. Occasionally, pillars transect the grain of the chamber fillings at angles up to 20" with the interwalls. Chambers themselves are variable in volume, decreasing in height dorsally from 3.0 mm to 1.5 mm while maintaining one width.

In longitudinal section, chambers appear as downward- sloping lobes originating at the multisiphonate centrum. Also apparent at this perspective is one siphon feeding into one chamber, but this feature is perhaps merely a function of perspective, because chambers in transverse sections have numerous siphons feeding from them. Endowalls that bound all siphons are discontinuous, perhaps due to a combination of the nature of preservation and the frequency of lateral siphonal communication. Because of the discontinuous nature of the endowalls and the linear arrangement of endopores, some siphonal endowalls appear as a chain-like structure, but this feature is often accentuated by or a manifestation of the incom- plete silicification of the original calcareous skeleton. Endo- pores are circular to elliptical pores 0.3 -0.5 mm in diameter. As discussed previously, siphons empty into the shallow, dorsal cloaca or into the exowall skirting the central ostium.

Interwalls and exowalls are well preserved in this specimen and maintain a relatively constant width of 0.4 mm throughout the specimen. However, as seen in longitudinal section, there appears to be a recurring medial, horizontal line in each inter- wall. Perhaps this structure represents the junction of two adja- cent laminated outer walls as is found in the well-preserved Ordovician sphinctozoans described by Rigby and Webby (1985).

Etymology Catenula, diminutive of catena, Latin for chain; referring to

the growth habit as well as the chain-like arrangement of the endopores of the siphons.

Material One complete, well-preserved specimen was discovered

amongst numerous anthaspidellids and hindiids of a slope biostrome on east-central Cornwallis Island (latitude 7S008'20"N, longitude 93 "54'05 "). At the type locality (Fig. I), the beds are dated as early Ludlovian according to laterally equivalent beds of the Cape Phillips Formation con- taining Neodiversograptus nilssoni (Thorsteinsson and Uyeno 1980; A. Lenz, personal communiciation, 1986). This speci- men is housed at the Geological Survey of Canada, Ottawa (GSC 85330).

Acknowledgments

Research for this paper was undertaken while the author was in the Department of Geology, the University of Western

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844 CAN. J. EARTH SCI. VOL. 24, 1987

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Loading of a large diamicton mass in glacial Lake Maumee ID sediments, southwestern Ontario

ROBERT A. STEWART Department of Earth Sciences, Iowa State University, Ames, IA 50011, U.S.A.

Received April 18, 1986

Revision accepted November 7, 1986

An 8 m long pod-shaped mass of deformed diamicton and sand occurs in Lake Maumee III glaciolacustrine sediments of the Port Stanley Drift, near Port Bruce, Ontario. Sedimentary structures observed in the diamicton mass and enclosing sands indicate large-scale loading accompanied their deposition. The lithology of the mass resembles admixed Port Stanley and Catfish Creek tills, which occur in moraines and other bodies of drift nearby. The diamicton mass may have been deposited en masse from seasonal or glacial floating ice or by subaquatic sediment gravity flow from floating ice or local moraines.

Can. J. Earth Sci. 24, 844-849 (1987)

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