A Trilobite Zonation of Middle Ordovician Rocks, southwestern District of Mackenzie

The upper Sunblood, Esbataottine, and lower Whittaker Formations in the southern Mackenlie Mountains, southwestern District of Mackenlie, have yielded many collections of well preserved silicifed trilobites of Middle Ordovician age (Chalyan to Shermanian/ Edenian; late llanvirnian to late Caradocian). Based on occurrences of 41 species of bathyurid, cheirurid, and encrinurid trilobites and on presumed phyletic relationships among six species of Bathyurus, eight species of CeraUl'inella, and eight species of Ceraurus (and Whittakerites), a sequence of nine assemblage lones is established. This is the first macrofaunal lonation of the Middle Ordovician interval in western North America. The lones and their ages are as follows: BathyUl'us nevadensis lone (early Chalyan, conodont Fauna 5), BathyUI'us granulosus lone (early or midChalyan, conodont Fauna 5), CeraUl'inella nahanniensis lone (late Chalyan, conodont Fauna 6), Bathyurus ulu lone (early Blackriveran, conodont Faunas 6 and 7), CeraUl'us gabrielsi lone (Blackriveran, conodont Fauna 7), CeraUl'inella longispina lone (Blackriveran/Rocklandian, conodont Faunas 7 and 8), CeraUl'inella necra lone (Rocklandian/Kirkfieldian, conodont Faunas 8 and 9), Ceraurus mackenliensis lone (Kirkfieldian/Shermanian, conodont Fauna 9), and Whittakerites planatus lone (Shermanian/Edenian, conodont Faunas 9 and ?10). Twenty new species are described. These are distributed among Bathyurus (B. ulu, B. esbataottinensis, B. granulosus, B. platyparius), CeraUl'inella (C. seriata, C. arctica, C. longispina, C. media, C. necra, C. brevispina), and CeraUl'us (C. gabrielsi, C. blussoni, C. hirsuitus, C. mackenliensis, C. maewestoides), in addition to Acanthoparypha? goniopyga, Holia anacantha, Sphaerexoehus atacius, Cybeloides anna, and Cybellela? thor.