Abstract

Graptolite zonation of the Upper Silurian and Lower Devonian of the northern Canadian Cordillera is revised and expanded with recognition of the following zones: Lobograptus progenitor, Saetograptus fritschi linearis, and Bohemograptus bohemicus tenuis (Ludlow); Monograptus formosus, Pristiograptus ultimus, P. chelmiensis, M. bouceki, and P. transgrediens praecipuus (Pridoli); M. uniformis and M. hercynicus (Lochkov); and M. fanicus, M. thomasi (questionable), and M. yukonensis (Prag and lowest Ems). Total species diversity of the Upper Silurian of the region is low in comparison with that of USSR, Poland and Czechoslovakia.

The following species, morphologically related to M. yukonensis, are recognized from the region: M. yukonensis, M. craigensis, M. pacificus (rare), M. telleri, M. thomasi and M. langgunensis. Simple biometric analyses of M. yukonensis and the closely related species M. craigensis and M. thomasi show some gradation and some arbitrariness among them. Almost all yukonensis Zone forms, however, share many common characteristics.

The age of the yukonensis Zone is concluded to be Pragian and early Emsian, but probably older than Zlichovian (approximately mid-Emsian).

Strongly bioturbated shales and mudstones, and rocks showing evidence of extensive soft-sediment deformation, are commonly devoid of graptolites in the Richardson Mountains. This fact may, in part, help explain some of the low diversity.