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A THELODONT FROM THE ORDOVICIAN OF CANADA

Thelodonts are one of the most enigmatic groups of extinct agnathans. Despite a number of publications describing their scale structure, histology, body shape, and aspects of their internal anatomy, there is still disagreement over whether they form a monophyletic group (Turner, 1991; Karatajūtė-Talimaa, 1978), although it is now generally accepted that they are closely related to the gnathostome crown group (Janvier, 1996; Wilson and Caldwell, 1998). A historical lack of well-preserved, articulated material has focused study on individual scales, and biostratigraphic schemes have been erected on the basis of changes in scale morphology that are particularly important through the Late Silurian and Early Devonian when the group reaches its acme. However, recent discoveries of articulated material from the Late Silurian and early Devonian of the Yukon and Canadian Arctic (Wilson and Caldwell, 1998; Soehn et al., 2001) and reinterpretation of existing specimens (for examples see Märss and Ritchie, 1998, and Donoghue and Smith, 2001) has resulted in a wealth of additional information on the interrelationships and anatomy of some thelodont groups. Ordovician thelodonts are comparatively rare, having only hitherto been recognized from three areas (Colorado, TimanPechora, and Severnaya Zemlya; see discussion below) and therefore any further records are of critical importance in documenting the early history of the clade.