Using fossil "fingerprints" to circumscribe the paleogeography of two Alaskan terranes along the Uralian seaway in the late Silurian

We present an updated model for the paleogeography of Alaska's Alexander and Farewell terranes, showing how a wealth of paleontologic and geologic data allow their locations to be circumscribed to the Northern Hemisphere-specifically the Uralian Seaway-in the Late Silurian. An unusually distinctive microbial-sponge-hydroid consortium constructed substantial platform-margin reefs in the terranes during the Late Silurian (Ludlow). The reefs and associated level-bottom communities exhibit strongest paleobiogeographic affinities with marine biotas found along the western and eastern slopes of the Urals and in Salair (southwestern Siberia). The microbe-dominated reefs are a Late Silurian «fingerprint» of abnormal Uralian Seaway conditions that were induced by a shrinking ocean basin, Caledonide tectonics, and related factors in the Late Silurian.