PALEOBIOGEOGRAPHIC AFFINITIES OF EMSIAN (LATE EARLY DEVONIAN) GASTROPODS FROM LIMESTONE MOUNTAIN, MEDFRA B-4 QUADRANGLE, WEST-CENTRAL ALASKA (FAREWELL TERRANE)

Recent study has provided the first detailed taxonomic inventory of Emsian gastropods from the Farewell terrane of southwestern and west-central Alaska, based on a highly diverse, silicified gastropod fauna from the south flank of Limestone Mountain, Medfra B-4 quadrangle. This fauna is distinctly of Old World Realm character, and contains not a single species in common with Emsian faunas of nearby non-accreted rocks of western Canada and east-central Alaska ("Western Canada Province" of Blodgett et al., 2001) or with that of the Great Basin (Nevada Province). The Western Canada Province was part of the Old World Realm during Emsian time, while the Nevada Province belonged to the Eastern Americas Realm during Pragian-early Emsian time, but whose gastropod fauna subsequently became more Old World Realm in character during later Emsian time. At the generic or subgeneric level only a few connections can found between the Farewell terrane and the latter provinces (i.e., Decorospira and Quadricarina (Quadricarina) are also present in the Eifelian strata of the Nevada Province). On the other hand, about half the generic or subgeneric taxa found in the Limestone Mountain fauna are known from Emsian and/or Middle Devonian age strata of the Rhenish-Bohemian Region of the Old World Realm. These taxa include Euomphalopterus, Straparollus (Eleutherospira), Quadricarina (Quadricarina), Decorospira, Balbinipleura, Palaeozygopleura (Rhenozyga), and Nanochilina. The latter three genera are reported for the first time in the Devonian of the Western Hemisphere. On the basis of previous studies of various Early and Middle Paleozoic (Cambrian-Devonian) age faunas from the Farewell terrane, it was suggested that this terrane represents a continental margin sequence that has been rifted away from the Siberian continent. Our recent data fit with this interpretation and support the Eurasian origin of the Farewell terrane.