PALEOBIOGEOGRAPHIC IMPLICATIONS OF FISTULELLA, A SILURIAN PROBLEMATIC HYDROID

Upper Silurian stromatolite reefs in the Ural Mountains of Russia comprise a problematic hydroid, Fistulella, preserved in association with a moderate diversity of crinoids, brachiopods, ostracodes, aphrosalpingid sponges, and calcified microbes. Fossils similar in appearance to Fistulella occur in stromatolite boundstones of comparable age and biologic composition in the Alexander terrane of Alaska. A detailed examination of these specimens from rocks exposed along the Ilych River in the Northern Urals and from Glacier Bay National Park, Alaska, was undertaken to determine if they belong to Fistulella and exhibit hydroid anatomy and lifestyle.

Burial experiments were used to assess hydroid taphonomy by studying the early stages in soft-body preservation of modern hydroids that are morphologically similar to Fistulella. These experiments confirm that most hydroids are unlikely to be preserved and that Fistulella probably was chitinous. Thin-section examination allowed the fossils from Russia and Alaska to be identified as Fistulella undosa Shuysky, 1973, on the basis of the simple, tubular shape of polyps that are undeformed and have an average diameter of 2-3 mm and an average length of 5-7 mm, rare branching, poorly preserved walls defined by microbial (or sponge) encrustations, and recrystallized interiors. The abundance of microbial laminae, calcimicrobes, associated macrofauna, and other lithologic features indicates that Fistulella formed part of a stromatolite reef that was built primarily by microorganisms at the seaward edge of carbonate platforms in both regions.

The co-occurrence of Fistulella in Upper Silurian limestones of Russia and Alaska adds new data to reconstruct paleocontinents and island terranes that bordered the Uralian Seaway. Fistulella retains its problematic status because of its poor preservation and lack of close analogy with modern hydroids, but its presence in Alaska (allochthonous terrane) and Russia (eastern Baltica) confirms that it had the ability to transmigrate between the two locations, thus providing additional evidence to support the placement of Alaska’s Alexander terrane along the seaway in the Late Silurian.