THE AULACOPLEURID TRILOBITE OTARION, WITH NEW SPECIES FROM THE SILURIAN OF NORTHWESTERN CANADA

The genus Otarion Zenker, 1833, first appears in the Wenlock simultane ously with Cyphaspis Burmeister, 1843, as the oldest known species of each occur together in the southern Mackenzie Mountains of the Canadia n Northwest Territories. The genera are unambiguous sister groups, a r elationship supported most compellingly by a uniquely derived and dist inctive pattern of juvenile cephalic spines, shared also with the Carb oniferous genus Namuropyge Richter and Richter, 1939. This sister grou p relationship permits the development of a robust and stratigraphical ly correlated hypothesis of relationship among the adequately known sp ecies of Otarion. Otarion, Cyphaspis, and Namuropyge constitute the tr ibe Otarionini. The Mississippian genus Dixiphopyge Brezinski, 1988, m ay also belong to Otarionini. Namuropyge is a paedomorph, likely deriv ed from a Degree Six or Seven meraspid of an older species of uncertai n position in the Otarion-Cyphaspis clade. Three Wenlock species of Ot arion occur in stratigraphic succession in a single section in the Mac kenzie Mountains. Analysis of morphological change with time, and of s equential ontogenies, suggests that the Silurian history of the genus was dominated by incidents of peramorphosis. New species are Otarion h uddyi, O. beukeboomi, and O. coppinsensis. Otarion brauni Perry and Ch atterton, 1979, is revised.