ALASKAN TRILOBITES REVISITED: NEW INSIGHTS TO CAMBRIAN-ORDOVICIAN PALEOGEOGRAPHY FROM FAUNAS OF THE NANOOK LIMESTONE

Uniquely Laurentian trilobites recently identified from the Nanook Limestone in the Shublik Mountains of northern Alaska refute earlier claims of a non-Laurentian (likely Siberian) origin for the North Slope Subterrane (NSS) of the Arctic Alaska Terrane. The presence of the upper Cambrian species Plethopeltis armatus (Billings) confirms that the Nanook formed as part of the Laurentian carbonate platform, most likely in proximity to the parts of that platform that are now located in the northern Atlantic region, such as Greenland and Svalbard. Lower Ordovician faunas from the Nanook provide additional support for such an origin. A moderately diverse, hystricurid-dominated fauna includes taxa strongly resembling those recently described from uppermost Skullrockian and basal Stairsian strata in Svalbard. Among these is a hystricurid previously misidentified as “Hystricurus" sainsburyi Ross, which is a species that was described from a fauna of distinctly Siberian aspect in western Alaska. The Nanook fauna also includes two species of Paraplethopeltis, the genus that dominates basal Stairsian faunas in many areas of North America, including Greenland. Although the fauna containing Paraplethopeltis lacks the other genera that typically occur with it in southern Canada and the USA, specifically Kainella and Leiostegium, positive carbon isotopic values obtained from these rocks confirm that they are basal Stairsian strata. A slightly younger fauna from the Nanook includes the first bathyurid trilobite reported from the NSS. This bathyurid species is certainly congeneric, and perhaps conspecific, with a species used to define the basal zone of the Tulean Stage in Svalbard.