New and biostratigraphically useful thelodonts from the Silurian of the Mackenzie Mountains, Northwest Territories, Canada

Two new species of thelodont are described. A new thelodontiform is based on articulated skeletons and isolated scales from late Telychian (Llandovery) to earliest Sheinwoodian (Wenlock) shales of the Avalanche Lake sections, Mackenzie Mountains, N.W.T., Canada. The head is blunt; it and the anterior trunk are broad and dorsoventrally flattened, while the posterior trunk and tail are more slender and laterally compressed. Unlike most other articulated thelodonts, there is no evidence of fins apart from the caudal fin. The hypocercal tail displays no evidence of rays or intermediate secondary lobes. The scales are robust, with large bases, large pulp cavities, and strong crown ridges in a stellate pattern; scale crowns are almost unornamented along the presumed ventral margin of the trunk and tail. An articulated specimen from the uppermost Telychian of Baumann Fiord, Ellesmere Island, Canadian Arctic Archipelago, is considered conspecific because of similarities in scale size and ornament, and extends the stratigraphic range of the species from the late Telychian griestoniensis-sakmaricus Graptolite Zone to the early Sheinwoodian centrifugus-insectus Graptolite Zone/Pterospathodus procerus Conodont Superzone, and the geographic range from the Selwyn Basin of the Yukon and N.W.T. to the Franklinian Basin of the Canadian Arctic Archipelago, Nunavut. A new phlebolepidiform is based on scales only. It also occurs both in the Mackenzie Mountains and the Canadian Arctic Archipelago. The two new species contribute significantly to a preliminary vertebrate faunal succession scheme for both areas and are therefore considered to be biostratigraphically useful.