New Emsian (late Early Devonian) gastropods from Limestone Mountain, Medfra B-4 quadrangle, west-central Alaska (Farewell terrane), and their paleobiogeographic affinities and evolutionary significance

摘要

A significant number of small-sized gastropods are described from Emsian (late Early Devonian) strata on the south flank of Limestone Mountain, Medfra B-4 quadrangle, west-central Alaska, providing the first detailed taxonomic inventory of Emsian gastropods from the Farewell terrane of southwestern and west-central Alaska. The fauna is distinctly of Old World Realm character, and contains not a single species in common with Emsian faunas of nearby nonaccreted rocks of western Canada and east-central Alaska ("Western Canada Province" of Blodgett et al., 2001a). The genera Balbinipleura Bandel and Fryda, 1996 and Nanochilina Fryda, 1998, as well as the subgenus Palaeozygopleura (Rhenozyga) Fryda, 2000, are reported for the first time in the Devonian of the Western Hemisphere. The gastropod fauna includes members of three (i.e., Archaeogastropoda, Caenogastropoda, and Heterobranchia) of the five modern gastropod subclasses, illustrating that these gastropod lineages were separated from each other since the Early Devonian. New taxa include the new genera Arctozone, Farewellia, and Medfrazyga, represented by the new species Arctozone cooki, Farewellia heidelbergerae, and Medfrazyga clauticae. Further new species include Quadricarina (Quadricarina?) noklebergi, Balbinipleura krawczynskii, Decorospira lepaini, Decorospira? minutula, Palaeozygopleura (Rhenozyga) reifenstuhli, and Nanochilina gubanovi. In addition, the following previously described gastropods are also discussed and illustrated: Alaskiella medfraensis Fryda and Blodgett, 1998; Alaskacirrus bandeli Fryda and Blodgett, 1998; and Kuskokwimia moorei Fryda and Blodgett, 2001a.