NORTH AMERICAN OCCURRENCES OF THE LOWER PERMIAN AMMONOID URALOCERAS

The paragastrioceratid ammonoid Uraloceras is an important biostratigraphic marker for the Lower Permian (Tastubian to Baighendzhinian) interval. Numerous species have been reported from Russia (the Ural Mountains, Novaya Zemlaya, and the Vekhoyansk Mountains), from China (Xizang, Nei Monggol, and Guangxi), from Canada (the Arctic Islands, the Yukon, and British Columbia), and from the USA (Alaska and Nevada). Species from Australia previously assigned to the genus, have been reassigned to other genera. Occurrences of Uraloceras, therefore, are restricted to northern Pangaea.

The distribution of Uraloceras from strata of northern Pangaea (Alaska, Canadian Arctic, Novaya Zemlaya, and Verkhoyansk Mountains) can be interpreted to represent Boreal occurrences. These faunas are characterized by large populations of Uraloceras but are dominated by a restricted number of species. Other associated ammonoid faunas also show very low taxonomic diversity. The Urals occurrences are more "paleo-southerly" than the Boreal occurrences. These faunas are characterized by high taxonomic diversity (over twelve described species) and extremely abundant representatives. Occurrences in Nevada, to the south and west of the Boreal occurrences, are also more diverse but populations are dominated by few species. Some taxa have somewhat restricted geographic distribution. Of the over 50 described species of Uraloceras, U. involutum is the most cosmopolitan.

Nevada occurrences of Uraloceras can be interpreted to represent a realm that is transitional between the Boreal realm to the north and the equatorial faunas to the south. Another unique feature of the ammonoid fauna from the Dry Mountain Trough of Nevada is the associated occurrence of abundant jaw apparatus interpreted to represent cephalopod mouthparts.