PALEOGEOGRAPHIC AFFINITIES OF SILURIAN GASTROPODS FROM SOUTHEASTERN ALASKA

Gastropods have shown to be useful in the study of the origins of Paleozoic terranes of Alaska because of their limited larval distribution in shallow marine environments. The Alexander terrane of southeastern Alaska is a major piece of the accretionary terrane collage that forms much of western North America. Ordovician, Silurian and Devonian rocks of southeastern Alaska have been interpreted to be the remnants of an island-arc that was rifted from near the Urals or Siberia and later sutured to North America; however, the arc's relative position to these entities is still unclear. The paleobiogeographic affinities of gastropods and other taxonomic groups (sphinctozoan sponges, pentamerid brachiopods, and rugose and tabulate corals) from the terrane consistently show no similarities with North American (Laurentian) faunas, but rather demonstrate Siberian, Uralian and eastern European affinities. The shelly faunas (mollusks and gastropods) are poorly known from Alaska, as well as Siberia, and the Urals. The study of the Alaskan fossils may predict the composition of coeval shelly fauna existing in those areas of Russia as well.

Gastropods occurring in the Silurian (Ludlovian) Heceta Formation on Prince of Wales Island that are not known from cratonic North America include Retispira, Pachystrophia, Beraunia, Kirkospira, Bathmopterus, Medfracaulus, Coelocaulus, and Goniostropha. Of these genera, Beraunia, Coelocaulus, and Goniostropha as well as members of subfamily Spinicharybdiinae occur in the Silurian of Bohemia. Pachystrophia has previously been reported only from accreted terranes of western North America (Alaska and California) and Europe. Spinicharybdis is the only endemic taxon with an affinity to the Canadian Arctic Islands. The occurrence of Medfrazyga and Medfracaulus suggests a faunal link between the Alexander and Farewell terranes of Alaska.