LATE TRIASSIC GASTROPOD BIOGEOGRAPHY OF SOUTHERN ALASKAN ACCRETED TERRANES

Gastropods are relatively common in Late Triassic (especially Norian) age shallow-water strata in all major terranes (Farewell, Chulitna, Alexander, Wrangellia, and Peninsular terranes) of southern Alaska. Early Norian gastropods are quite diverse and abundant in the Cornwallis Limestone of Kuiu Island (Alexander terrane), southeast Alaska. A large number of new genera and species are present in this fauna, elements of which are also known from the Screen Islands further to south in the same terrane. Early Norian gastropods are also abundant in strata of the Chitistone Limestone exposed at Green Butte in the Wrangell Mountains (Wrangellia terrane). The Chitistone gastropod fauna is nearly identical in taxonomic content with the coeval Spring Creek fauna of the Wallowa terrane (formerly considered part of Wrangellia) in Oregon. Although the Chitistone and Cornwallis faunas are contemporaneous, only a single species, Spinidelphinulopis whaleni Blodgett, Frýda, and Stanley, is shared between the two terranes. The quite differing character of the Alexander and Wrangellia terrane gastropod faunas, together with their differing stratigraphies, and lithofacies (i.e., abundant evaporate deposits in Wrangellia, but absent in Alexander terrane) suggest that they both may have been situated in tropical to subtropical settings in the eastern Panthalassa Ocean, they were separated far enough that only gastropods with teleplanic larvae (i.e., like the neritimorph S. whaleni) were shared. Nearly all the remaining gastropods of both faunas belong to groups characterized by direct development or a short larval stage, and thus were not capable of long-distance dispersal across broad, deep oceanic expanses. Late Norian gastropods are known from the Farewell, Chulitna, and Alexander terranes. The species Chulitnacula alaskana (Smith) is commonly found in all three terranes, together with a number of other (mostly new) gastropod taxa. The common occurrence of these taxa indicates that these three terranes were relatively close to one another during Late Triassic time.