LOWER SILURIAN T H E L O D O N T S FROM PRINCE O F WALES ISLAND, NORTHWEST TERRITORIES

SUSAN TURNER AND JAMES DIXON

Turner, Susan & Dixon, J.: Lower Silurian thelodonts from Prince of Wales Island, Northwest Territories. Lethaia, Vol. 4, pp. 385-392. Oslo, October 15th, 1971.

Thelodont scales, determined as Thelodus laevis (Pander), Logania martinssoni Gross, and Phlebolepida gen. et sp. indet., and other agnathan remains have been found in the dolomite-limestone sequence of the Transition Beds between the Allen Bay and Read Bay Formations. This thelodont association is typical of the Middle and Lower Upper Silurian of &el and Gotland, and its presence therefore provides evidence for the upper age limit of the transition beds.

Susan Turner, Department of Geology, University of Reading, White-knights, Reading RG6 2AB, England; James Dixon, Department of Geology, University of Ottawa, Ottawa 2, Canada; December 8th, 1970.

New vertebrate faunas are being found in the Siluro-Devonian rocks of Arctic Canada (Dineley 1966; Broad, Dineley & Miall 1968). Thorsteinsson (1967) first reported thelodonts from the Cape Phillips Formation of Cornwallis Island. An expedition from the University of Ottawa during the field season of 1969 to Prince of Wales Island led to the discovery of a new fossil fish site. This flaggy, dolomitic band is 70 m above the base of the Transition Beds, which lie between the dolomitic Allen Bay Forma- tion (Ordovician in part) and the overlying limestones of the Read Bay Formation (Upper Silurian in part). The outcrop is in an unnamed stream which flows eastward into Young Bay, 8 km south of Savage Point (see Fig. 1). The band is 200 mm thick, the greatest concentration of fish remains being in the middle 75 mm.

Material

Specimens of the dolomite, which weathers to a grey buff colour, were treated with 20% acetic acid and a microfauna was looked for in the residues. The thelodont scales found are all very small, less than 1 mm in length, but are unbroken and only slightly waterworn. About 100 scales have been examined, and these will be lodged at the National Museum of Canada, Ottawa.

Fauna

Three thelodont species have been determined. Thelodus laevis (Pander) is very common (see Fig. 3). Gross (1967, 1968) dealt with this form and there is little to add to his description. The neck ornament on the slightly

Fig. 1. Locale of fish band on Prince of Wales Island. Inset map :Position of island in Canadian Arctic.

waterworn scales is usually absent. All stages of base development are found, so that the width of the pulp cavity varies. Crown lengths vary from 0.3 mm to 0.75 mm. There are numerous scales of Loganiu martinssoni Gross, both head and body scales, but the forms with root-like extensions of the base looking ‘like a little mammal-tooth’ (Gross 1968), have not been seen. Body scales, average length 1.0 mm, vary in the width of the crown, but the characteristic median spine beneath the posterior apex is present (see Fig. 4). In some cases the other spines have been worn away. Transitional scales of the sort shown by Gross (1968) in Fig. 3H are common. The head scales (see Fig. 5) are smaller than the body scales, on average 0.5 mm in length. Some tend to be symmetrical, with a rounded bifurcated anterior crown which has two posteriorly directed notches on each side, about one-third scale length behind the anterior edge; others have an asymmetrical crown with a deeply notched perimeter. There are gradations between these end forms (see Fig. 6).

Only a few scales of the third member of the assemblage have been found. One of these is well-preserved and resembles Phlebolepis elegans Pander (see Fig. 7), but is smaller, 0.8 mm in length, and more delicate than those from the type area of Saaremaa (Osel) in Estonia. A new species may be represented, but this cannot be determined until more material is available. Until then, it is assigned to Phlebolepida gen. et sp. indet.

LOWER SILURIAN THELODONTS 387

Fig. 2. Generalised section of strata 8 km south of Savage Point.

Fig. 3. Thelodus laevis(Pander), 0.5 mm length. . .

- letre:

720

600

480

360

240

120

0

-

READ BAY FORMATlOf

SF---- TRANSITION BEDS Fish bond

Dolomites with stromatoliles Ccrol beds Slumping and sliding or base of unit

. - - - - - - - - - - -

_ _ _ _ _ - - _ _ _ -

ALLEN

Dolomites with stromatolites

Coral bed

BAY

FORMATION

Block shale

3uortz sandstones, shales and dolomite

. - - - - - - - - - - -ault contact against 3r01erozoic rocks

The other agnathans are represented by heterostracan and osteostracan plates. All are very fragmentary and so specific identification is not pos- sible. As in other localities in the Canadian Arctic, the heterostracans are more typical of a Dittonian freshwater assemblage as known from European sites (White 1950a). Here, however, they are present in a much older horizon.

The following is a summary of the fish band fauna: Thelodus laevis, Logania martinssoni, Phlebolepida gen. et sp. indet., traquairaspidids, cyathaspidids, Anglaspis sp. indet., Corvaspis sp. indet., Osteostraci gen. et sp. indet.

Stratigraphy

T h e detailed stratigraphy of the Transition Beds has only recently been studied and the age of the sequence is still uncertain. There is a Middle or Upper Ordovician fauna in the underlying Allen Bay Formation on Prince of Wales and the adjacent Somerset Island (Christie 1967). At the time of Christie’s study there was no indication of missing strata between these fossiliferous horizons and the Read Bay Formation, which had been dated as Middle Silurian to early Downtonian (Dineley 1965, 1966).

388 SUSAN TURNER AND JAMES DIXQN

Fig. 4 . Logania martinssoni Gross, body scales. A, A' 0.5 mm; B,0.3 mm.

Fig. 5. Logania martinssoni, head and transitional scales. A - C, 0.5 mm; D, 0.75 mm.

Ages of the Allen Bay Formation from Cornwallis Island, the type sec- tion, give an upper limit of Middle Silurian, and the top of this formation may be as young as lower Upper Silurian in places (Thorsteinsson 1958; Thorsteinsson & Kerr 1968). On Devon Island, two separate 'faunas have been recognised in the Allen Ray Formation, the upper one being dated as Lower Silurian (Fortier et al. 1963, p. 226). The upper boundary of this formation must be diachronous on the evidence of the enclosed faunas and therefore lateral equivalents of the Transition Beds should be delineated in these other areas. From the regional setting the age of the upper boundary of the Transition Beds could be between Upper Ordovician and Upper Silurian - a very long time span!

A study of the cephalopods from the Read Bay Formation on Prince of Wales and Somerset Island suggests an Upper Llandovery to Lower

LOWER SILURIAN THELODONTS 389

Fzg. 6. Loganza martinssonz, gradation of scale forms. All approximately 0.5 mm.

Wenlock (Clinton) age and that the Transition Beds should be the basal unit of this formation (S. R. Williams pers. comm.). The age of the fish band, therefore, may be Upper Llandovery or older.

Correlation of thelodont faunas

Thelodus laevis was first described from the Rootsikula Stage (Kl), Osel Island (Saaremaa; Pander 1856). Recently, fine sampling of the Silurian sequence on Saaremaa has demonstrated its presence in the Maasi Beds of the Wenlockian Jaagarahu Stage. I t also occurs higher in the Lud- lovian Paadla Stage (K2; Mark-Kurik & Noppel 1970). Thelodus laevis is common in the 9g Beds of Ringerike, Norway, which may be Lower Lud- lovian (Martinsson, personal communication in the Ludlow Research Group Bulletin, 1969; Heintz 1969), and in the Wenlockian Halla Beds from Gothemshammar on Gotland (Gross 1968). The latter is the best locality for Logania martinssoni which was first recorded from the K1 Beds of Saaremaa. It is also recorded from the Maasi and K2 Beds, and is quite common in the 9g Beds of Ringerike. Phlebolepis elegans is associated with these two forms in the K2 Saikla and Himmiste Beds but is known princi- pally from the K l Beds (Mark-Kurik & Noppel 1970). It is also known from the Middle Ludlovian Hemse Beds at Gogs on Gotland (Gross 1968b). The presence of these thelodonts in Baltic sites, therefore, is indicative of Wenlockian to Middle Ludlovian age, and if older rocks are searched, it is reasonable to extrapolate that one or more of these species will be found in association. It may be anticipated here that the associated ostra- codes suggest Wenlockian age, or possibly even older, for the Transition Beds.

This is true of the Canadian Arctic. Thorsteinsson (1967) described Thelodus laevis and a Thelodus sp. from the early 1,udlovian M . vulgaris zone of Cornwallis Island, and, more important, he claims to have articu- lated material ‘representing a new genus and species of thelodont that may constitute a relative of Phlebolepis elegans Pander’ from an Upper Llandov-

26 - Lethaia 4 : 4

390 SUSAN TURNER AND JAMES DIXON

Fig. 7. Phlebolepid scale, 0.8 mm.

erian or early Wenlockian site. Therefore, the assemblage of Thelodus laevis, Logania martinssoni (which may be equivalent to Thorsteinsson’s Thelodus sp.) and a phlebolepid from Prince of Wales Island is not unex- pected, and certainly an Upper Llandoverian date for the fish bands is not excluded. If the fauna is Llandoverian or older, the effect on agnathan migrations can be considered. It is feasible that the thelodonts migrated from the Canadian Arctic region to the Baltic area by Wenlockian time and not vice versa (Turner 1970). Similarly, from the evidence of the heter- ostracan fauna, the absence of the traquairaspided-corvaspided-anglaspided group from ‘Europe’ before Dittonian times might be explained if they originated in, and migrated from, the Ordovician and Silurian seas then covering the north of America.

Environment

The fish band occurs in a sequence of stromatolites with evidence of evapor- ites, intraformational breccias, and an increasing number of limestones between dolomites. The stromatolites are either laminar or domal structures (type LLH, Logan, Rezak & Ginsburg 1965). The former are dominant, laterally giving way to domes, which are isolated features 150 mm to 1 m in diameter, with a maximum height of 225 mm. The intraformational breccias represent penecontemporaneous erosion of the stromatolites or reworking of stromatolite plates formed by desiccation (Logan 1961). Des- iccation polygons, however, have not been found. Casts of evaporite minerals, mostly gypsum, occur associated with very finely laminated stromatolites. The interbedded limestones contain a fauna of gastropods, brachiopods, and ostracodes, suggesting a subtidal environment (S. R. Williams pers. comm. 1970). Preliminary petrographic study of the dolomites suggests early diagenetic, if not penecontemporaneous, dolomitisation, which has been noted in intertidal and supratidal environments (Illing, Wells & Taylor 1965; Shinn, Ginsburg & Lloyd 1965).

LOWER SILURIAN THELODONTS 391

The Transition Beds are thought to represent the supratidal, intertidal, and shallow water conditions with frequently very high salinities - a period of fluctuation which occurred during the change from the restricted marine environment of the Allen Bay Formation to the more widespread marine conditions of the Read Bay Formation. As the thelodont scales are slightly waterworn and the other remains fragmentary, the environment may not strictly represent the life habitat of the fish. More likely they were living in the subtidal zone, and their remains were washed into the nearby intertidal zone. The character and number of ostracodes and gastropods relative to the number of taxa suggest a low subtidal enviroment.

Acknowledgements. - The authors are most grateful to their respective Departments of Geology, especially to that of the University of Ottawa, for supplying specimens which were collected with the aid of grants from the National Research Council of Canada and the Department of Indian Affairs and Northern Development. Particular thanks are given to Professor D. L. Dineley, Dr. 0. A. Dixon, Dr. L. B. Halstead, Mr. S. R. Williams, Mr. M. J. Jackson and Mr. J. Weir for their help and discussion. Mr. J. Smith and Mr. J. Watkins were most helpful with the preparation of the figures.

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