SILURIAN GASTROPODA FROM THE ALEXANDER TERRANE, PRINCE OF WALES ISLAND, SOUTHEAST ALASKA

Paleozoic gastropods have proven in the past two decades to have great utility in the study of Paleozoic biogeographic affinities and possible origins of accreted terranes along the western margin of North America. The Alexander terrane of southeastern Alaska is a major piece of the accretionary terrane collage that forms much of southeast Alaska. Ordovician, Silurian and Devonian rocks of Prince of Wales and neighboring islands 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. The mollusc-rich silicified fauna discussed here was recovered from a locality in the Heceta Limestone on Prince of Wales Island. Gastropods and calcareous algae are most abundant, followed by tabulate corals, minor brachiopods, scaphopods, bivalves, and crinoids. The contained gastropod and brachiopods indicate an undifferentiated Ludlovian age.

The limited data base on Late Silurian gastropods from Alaska and Russia, indicates strong faunal ties between the gastropod faunas of the Alexander terrane with that of the Farewell terrane of southwestern Alaska and those described from the Ural Mountains by Chernyshev (1893). The most significant co-occurrence in southeast Alaska and the eastern Urals is Medfracaulus turriformis (Chernyshev, 1893). This distinctive species with its slow rate of expansion and deep umbilicus exhibits an unusual curved axis of coiling in some specimens. The curved spire is also seen in some shells of the same genus from west-central Alaska. Medfracaulus is characteristic of Upper Silurian strata of west-central Alaska (Nixon Fork subterrane of the Farewell terrane) and southeastern Alaska (Alexander terrane). Medfracaulus and similar morphotypes such as Coelocaulus karlae are unknown from rocks located undoubtedly on the North American continent (Laurentia) during the Late Silurian.

Other Heceta gastropods with Uralian affinities include Kirkospira glacialis which appears similar to Pleurotomaria lindströmi Oehlert of Chernyshev, 1893, and a species of Retispira that is similar to Bellerophon volgulicus Chernyshev, 1893. Beraunia is only known from the Silurian of Bohemia. Pachystrophia has previously been reported only from accreted terranes of western North American (Alaska and California) and Europe.