BIOSTRATIGRAPHIC REFINEMENTS IN THE LOWER ORDOVICIAN OF EASTERN ALASKA: EMERGING EVIDENCE OF AN EASTERN SUBPROVINCE OF THE LAURENTIAN FAUNAL PROVINCE?

Seven distinct trilobite faunas recovered from the uppermost 65m of the informal Hi-Yu member of the Jones Ridge Formation in east-central Alaska are assigned by associated conodonts to the basal Ordovician Iapetognathus and Cordylodus angulatus biozones. One or more species of Symphysurinadominate(s) each of the seven trilobite faunas, but other genera occur as minor components in four of the faunas, as do agnostoid arthropods. Brachiopods, both calcitic and phosphatic, are common in all collections.

Although most or all of the Symphysurina species from the Jones Ridge are new, some strongly resemble established species in coeval faunas from Greenland. In particular, species from the lowermost and uppermost collections from the Symphysurina Zone in the Hi-Yu member are very similar to S. elegans and S. porifera (respectively) in the Antiklinalbugt Formation of Greenland. In contrast, none of the Alaskan species compares closely with any of the numerous species of Symphysurina that occur in the Iapetognathus and C. angulatus zones in the Snowy Range Formation in Wyoming and Montana, or with any species reported so far from the rich silicified faunas in the Great Basin of the western USA. This apparent similarity of Symphysurina Zone taxa in faunas across northern North America, as opposed to those from the western USA, suggests the existence of a distinct eastern Laurentian trilobite subprovince within the Laurentian faunal province in the earliest Ordovician.

Additional evidence for distinctly different trilobite faunas within the Symphysurina Zone in eastern and western Laurentia (Ordovician coordinates) is provided by a species of the dimeropygid genus Tulepyge in the highest collection from the Hi-Yu member. Like the associated species of Symphysurina, the Alaskan Tulepyge species most closely resembles a species from northeastern North America (southeastern Laurentia). It displays virtually all the unique features of Tulepyge paucituberculata (Fortey), whose only known occurrence is in olistoliths in the Symphysurina Zone in the Green Point Formation of western Newfoundland. A rigorous test of this paleobiogeographic hypothesis, however, awaits detailed description of basal Ordovician trilobite faunas in several other areas of Laurentian North America, particularly those of Arctic Canada.