Description:
<jats:p>In the South Nahanni River area of the District of Mackenzie, the Middle Ordovician Sunblood Formation comprises mainly limestones and dolostones of intertidal and shallow-subtidal origin, as indicated by the presence of desiccation polygons, fenestral fabric, and oncolites. Faunas of well-preserved, silicified trilobites from a low-diversity, nearshore, <jats:italic>Bathyurus</jats:italic>-dominated biofacies are compositionally distinct from faunas in correlative strata around North America that represent different shelf to upperslope biofacies. A temporal biostratigraphy applicable to nearshore biofacies through much of the Whiterockian Series consists of five zones, in ascending order: <jats:italic>Bathyurus mackenziensis, B. sunbloodensis, B. margareti, B. nevadensis</jats:italic>, and <jats:italic>B. granulosus.</jats:italic> Twenty-six species are assigned to 18 genera, of which <jats:italic>Ludvigsenella</jats:italic> is new. Three new species of <jats:italic>Bathyurus</jats:italic> are <jats:italic>B. mackenziensis, B. sunbloodensis</jats:italic>, and <jats:italic>B. margareti.</jats:italic></jats:p>