Silurian Bivalvia from Chichagof Island, Southeast Alaska (Alexander terrane)

The new Silurian Bivalvia <i>Goniophora thula-Mytilarca boucoti </i>Community from a new locality east of the Hoonah Airport on Chichagof Island, Southeast Alaska, U.S.A. is composed of 9 species [?<i>Colpomya </i>aff. <i>hugini </i>Liljedahl, 1994 and ?<i>Colpomya </i>aff. <i>audae </i>Liljedahl, 1994 (Colpomyidae); <i>Modiolopsis </i>aff. <i>modiolaris </i>Conrad, 1838 (Modiolopsidae); <i>Mytilarca boucoti </i>sp. nov. (Ambonychiidae); ?<i>Schizodus </i>sp. A and ?<i>Schizodus </i>sp. B (Schizodidae); <i>Sanguinolites </i>sp. (Sanguinolitidae); ?<i>Modiomorpha </i>sp. and <i>Goniophora thula </i>sp. nov. (Modiomorphinae)]. In this paper two new species are described. The <i>Goniophora thula-Mytilarca boucoti </i>Community most probably occupied a gently inclined upper slope environment lying seaward of a cyanobacterial reef front which marked the outer edge of a late Silurian carbonate platform (represented by the Kennel Creek Limestone). The fauna of this locality consists of a rich benthos, especially rich in brachiopods, but secondarily dominated by bivalves and gastropods. Cephalopods are rare, favositid tabulate corals are also present, but form only a very minor component. The new community is most closely related to the homologous and analogous late Wenlock Bivalvia community described from Gotland, Sweden, Baltica. Similar biotic ties are with the shallow water early Ludlow <i>Janicula potens </i>Community from the Prague Basin, Bohemia, Perunica. Cosmopolitanism is characteristic for the Silurian Bivalvia genera, which were dispersed in most regions of the world due to their relatively long pelagic larval life and relatively small distances between the basins, islands and continents in the Silurian. For the distribution of Bivalvia during the Silurian were more limiting physical factors like temperature, salinity, substrate, food, oxygen content, light, and depth. The fauna described here is from the Alexander terrane, an accreted terrane which appears to represent a rifted block of the eastern Siberian palaeocontinent. The latter interpretation is based on faunal affinities, notably brachiopods, but suggested by other groups such as gastropods and sponges, as well as on the basis of similarities in sedimentary succession.