LUDLOVIAN (LATE SILURIAN) GASTROPOD FAUNA OF THE ALEXANDER TERRANE, SOUTHEASTERN ALASKA, AND THEIR PALEOBIOGEOGRAPHIC SIGNIFICANCE

Late Silurian (Ludlovian) gastropods are both abundant and diverse in the Alexander terrane of southeastern Alaska where they have been described from the Heceta Limestone on the west side of Prince of Wales Island and the Willoughby Limestone in Glacier Bay. They typically are found in lagoonal settings in shallow-shelf carbonates. Taxa found in these strata include Retispira aff. R. vogulica (Chernyshev, 1893). Pachystrophia gotlandica (Lindström, 1884), oriostomatid operculum, operculum of Oriostoma sp., Beraunia bohemica (Perner, 1903), Beraunia bifrons (Perner, 1903), Hecetastoma gehrelsi Rohr and Blodgett, 2008, Kirkospira glacialis Rohr and Blodgett, 2003, Bathmopterus liratus Kirk, 1928, Spinycharybdis krizi Rohr, Blodgett, and Frýda, 2008, Spinycharybdis boucoti Rohr, Blodgett, and Frýda, 2008, Haplospira craigi Rohr, Blodgett, and Frýda, 2008, Australonema sp., Morania wagneri Rohr, Blodgett, and Frýda, 2008, Medfracaulus turriformis (Chernyshev, 1893), Coelocaulus karlae Rohr and Blodgett, 2003, Medfrazyga gilmulli Rohr, Blodgett, and Frýda, 2008, and Baichtalia tongassensis Rohr and Blodgett, 2008. None of the above-mentioned species is known from Laurentia. The closest Silurian gastropod faunas to these are known from the Urals and Bohemia, as well to the Farewell terrane of SW Alaska. The Alexander terrane Silurian platform carbonates are also typified by banks of the large, endemic bivalve genus Pycinodesma, reported to date only from SE Alaska. Associated brachiopod and sphinctozoan sponges from the Late Silurian of the Alexander terrane are likewise typically Siberian-Uralian (not Laurentian) in aspect, and at the species level the brachiopods are most similar to those of northeastern Siberia (especially the Omulevka terrane). The gross Silurian-Lower Devonian stratigraphy and close biogeographic affinities of the brachiopods suggest a probable rift origin of the Alexander terrane from NE Siberia (see accompanying abstract by Blodgett and Boucot).