UPPER TRIASSIC GASTROPOD BIOGEOGRAPHY OF WESTERN NORTH AMERICA

Late Triassic gastropods are found in both accreted terranes and para-autochthonous rocks of western North America. Middle Norian gastropods from central Nevada (Clan Alpine Mountains) and Sonora (Antimonio Fm.) share many species and belong to a single biogeographic province (termed here the Nevada-Sonora Province). The presence of the same fauna in both areas supports the concept of the Mojave-Sonora megashear. The Nevada-Sonora Province fauna was situated on the SW margin of the Triassic North America continent and is closely related to western Tethyan faunas, indicating that the Hispanic Corridor was already operative during Late Triassic time.

The Wallowa terrane of eastern Oregon and western Idaho has a richly diverse, tropical gastropod fauna containing many new genera. Close faunal ties exist between the Wallowa terrane and the Wrangellia terrane in its type area (Wrangell Mountains of Alaska). We believe that these two terranes were positioned near one another in the tropics of the Panthalassa Ocean. The Chulitna terrane of south-central Alaska contains a moderately diverse Norian fauna, dominated by Chulitnacula alaskana (Smith). This taxon is also present and dominant in the Farewell terrane of SW Alaska and the Alexander terrane of SE Alaska. The common occurrence of this taxon, as well as many other taxa, suggest that these three terranes were relatively close to one another during Late Triassic time. The Alexander terrane of SE Alaska contains several diverse gastropod faunas ranging in age from early Carnian to late Norian. The taxic composition of the Alexander terrane faunas is quite distinct from that of Wrangellia, and indicates that they were far removed from another in Late Triassic time. On this basis, as well as their differing stratigraphies, we believe that the Alexander and Wrangellia terranes are quite distinct from one another, and that the former should not be relegated to the status of a subterrane of the latter as has recently been suggested by others.