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

Abstract

The new Silurian Bivalvia Goniophora thula-Mytilarca boucoti Community from a new locality east of the Hoonah Airport on Chichagof Island, Southeast Alaska, U.S.A. is composed of 9 species [?Colpomya aff. hugini Liljedahl, 1994 and ?Colpomya aff. audae Liljedahl, 1994 (Colpomyidae) . In this paper two new species are described. The Goniophora thula-Mytilarca boucoti 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 Janicula potens 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.•

Review of palaeozygopleurid gastropods (Palaeozygopleuridae, Gastropoda) from Devonian strata of the Perunica microplate (Bohemia), with a re-evaluation of their stratigraphic distribution, notes on their ontogeny, and descriptions of new taxa

“…It is now considered that this terrane most likely originated as a rifted block of the eastern Siberian palaeocontinent, probably breaking away in the later part of the Devonian (Blodgett et al 2010). Silurian Bivalvia from Chichagof Island, Southeast Alaska (Alexander terrane), seem to be very similar to the homologous and analogous late Wenlockian Bivalvia communities described from Gotland, Sweden and Baltica (Kříž et al 2011). The fauna of the Alexander terrane is altogether different in species composition from the NW part of the non-accretionary portion of North America (i.e., Laurentia), although both regions belong to the Old World Realm.…”

Strophatrypa, a new genus of Brachiopoda (Atrypidae), from upper Silurian strata of the Alexander terrane, northeast Chichagof Island, Alaska

“…1). The other paper, Kříž et al (2011), described a new bivalve community (Goniophora thula-Mytilarca boucoti Community) composed of 9 species from the same locality as Strophatrypa.…”

“…The fossiliferous zone which yielded this fauna was the only easily visible fossil-rich part of the siltstone beds exposed in the quarry. The age of this fossil horizon was indicated to be late Wenlock or early Ludlow according to the associated bivalve fauna (Kříž et al 2011). The brachiopod fauna found here indicates only a generalized late Silurian age, and occurs primarily as disarticulated external and internal moulds extracted from siltstones.…”

“…The siltstone beds near the fossil locality strike N-S and dip steeply 70-75°to the west. This rich shelly fauna consists primarily of brachiopods along with a lesser amount of gastropods and bivalves (see Kříž et al 2011 for description of the bivalves from this horizon). The fossiliferous zone which yielded this fauna was the only easily visible fossil-rich part of the siltstone beds exposed in the quarry.…”