graptolites from Arctic Canada
Lenz1 and Anna Kozlowska–Dawidziuk2
1Department of Earth Sciences, University of Western Ontario, London, Ontario N6A 5B7, Canada. E–mail: firstname.lastname@example.org
2 Institute of Paleobiology, Polish Academy of Sciences, Twarda 51/55, 00-818 Warsaw, Poland.
Key words: Graptolites. Ludlow. Pridoli. Biostratigraphy. Arctic.
Ludlow and Pridoli graptolite-bearing strata are widespread in Arctic Canada. They occupy the upper part of the Cape Phillips Formation and, locally, the Bathurst Island Formation, and range up to 500 m in thickness. Graptolites are preserved as both flattened films on shale surfaces, and as uncompressed, three–dimensional forms in nodular limestone from which they can be released by acid dissolution. Flattened forms typically occur in shales or calcareous shales in the deep–water basin or slope regions (from which 7 sections were sampled), whereas uncompressed forms are common in the nodular limestones that occur along, or near, the platform edge environments (from which 5 sections were sampled).
Twenty–six species of monograptids, and at least 10 species of retiolitids occur in the Ludlow, of which most of the former, and all of the latter, occur in the lower and middle Ludlow. In comparison with the graptolite sequences of Poland/Baltic and Ukraine (East European Platform), the Bohemian Massif, and Kazakhstan/Central Asia, the Ludlow monograptid diversity in northern and Arctic Canada, particularly in the upper Ludlow, is remarkably low. Notably rare are most representatives of neolobograptids, cucullograptids and neocucullograptids. Specifically, while a few species of these subgroups occur in the Lower Ludlow, they appear to be totally absent from the Upper Ludlow. Related to the much lower species diversity of the Arctic, only five biozones are recognized for the entire Ludlow (in comparison to seven in the Czech Republic and 14 from the Baltic region of Poland!). The Arctic biozonal sequence comprises (from older to younger) the progenitor, scanicus, linearis, praecornutus–tenuis, and formosus biozones.
Nine species of monograptids are found in the Pridoli of the Arctic. This number, though small, is about normal for Pridoli species diversity elsewhere in the world, and permits the recognition of six biozones, in spite of the absence of Monograptus perneri and its associates. The biozonal sequence comprises the parultimus, ultimus, branikensis, bouceki, transgrediens, and birchensis biozones. The birchensis Biozone appears to range across the Silurian–Devonian boundary.
The low diversity in the Ludlow, particularly the upper Ludlow, is puzzling, since Arctic Canada lay very close to the Silurian paleo–equator, and the high diversity proto–continent Baltica was in close proximity to eastern Laurentia. It is suggested that the long, northeast–southwest oriented (and periodically positive?) shield areas lying between Baltica and Arctic Canada, and involving Greenland, might have formed a partial barrier to free exchange from the east. Added to that, the presumed north–derived cold water oceanic currents moving generally
southward along the northwestern side of Laurentia (Wilde, et al., 1991), coupled with apparent (local?) shallowing during the upper Ludlow, may have resulted in a considerable stress on the deeper water, graptolite habitats.
Wilde, P., Berry, W. B. N. and Quinby–Hunt, M. S. 1991. Silurian oceanic and atmospheric circulation and chemistry. p. 123–143. In. Bassett, M.G., Lane, P.D. and Edwards, D. (eds.). The Murchison Symposium: Proceedings of an International Conference on The Silurian System. Special Papers in Palaeontology, 44. London.
Received: February 15, 2003
Accepted: June 15, 2003