TROLLING FOR SILURIAN FOSSIL “FINGERPRINTS:” NEW RAYS OF EVIDENCE FOR CALEDONIDE INFLUENCE ON EVOLUTIONARY ECOLOGY IN THE ALEXANDER TERRANE

The Alexander terrane (AT) originated as a Neoproterozoic island arc and accreted to North America in the Mesozoic. Debates for three decades have focused on whether the terrane resided in the Paleozoic near Australia or at northern latitudes near Baltica. Silurian deposits in the AT contain species that serve as diagnostic “fingerprints” to constrain the paleogeographic setting of the AT.

During the Early-Late Silurian, the 3000 m-thick Heceta Limestone accumulated in the AT within 14° of the equator. An expanding carbonate platform, built on top of a volcanic edifice, was colonized by diverse, normal-marine benthic species. Deposition of thick conglomerate interrupted limestone sedimentation during mid-Silurian orogenic pulses. As limestone formation resumed in the Late Silurian, Pycinodesma megalodont bivalves—infilled with clay that reflects the influx of continental sediment—formed dense concentrates associated with nearshore microbial communities. At the platform margin, an unusual microbial-sponge consortium—Hecetaphyton, Ludlovia, and Sphaerina (calci-microbes) in association with Aphrosalpinx (sponge) and Fistulella (problematic hydroid)—built extensive stromatolite reefs. As orogenesis progressed in the Late Silurian, the shelf margin foundered and the reefs collapsed, generating enormous underwater landslides revealed as thick breccias along the coast and in new roadcuts on Prince of Wales Island.

The AT shares the distinctive microbial-dominated biotas in common with only a few sites (Urals, western Siberia, and northern Laurentia). Thus those “fingerprint” species anchor the AT to the southern margin of the Uralian Seaway in the Silurian. Proximity to the Caledonides explains the enormous ecological change recorded in AT biotas. Caledonide orogenesis generated significant terrestrial influx thereby reducing habitats for normal-marine metazoans, elevating nutrient loads, and, ultimately, fueling extensive clam and microbial “blooms.”