PROLIFERATION OF SILURIAN STROMATOLITE REEFS IN THE URALIAN SEAWAY

Temporal and geographic evidence from modern and ancient sites indicates that, despite stromatolite decline in the Late Precambrian, microbes were important ecologically in many reefs and associated habitats throughout the Phanerozoic. They were critical agents in reef colonization, stabilization, and lithification. As the ultimate "survivors," they were capable of weathering environmental disturbances that suppressed the reef-building activities of metazoans. Applying what is now known about Phanerozoic stromatolites to the Silurian yields important insights into the circumstances that promoted stromatolite proliferation, persistence, and preservation.

During the Late Silurian, stromatolite reefs formed along the margins of the Uralian Seaway in areas that include present-day Alaska, the Ural Mountains, and the Salair region of Siberia. Like many other post-Cryptozoic stromatolites, these were not restricted to the intertidal zone; calcified cyanobacteria, algae, and microproblematica were the primary frame-builders in offshore barrier reefs strengthened by penecontemporaneous marine precipitates. Sphinctozoan sponges and problematic hydroids were important volumetrically and shared a specialized ecologic relationship. Environmental factors affected microbial paleoecology in ways similar to reef metazoans. Differences in composition and biofabrics from other Phanerozoic stromatolites imply evolution and diversification of microbial-algal-invertebrate communities from ancestral stocks, not a resurgence of "disaster" taxa in "anachronistic facies." Environmental conditions favoring microbial reef development and fossilization along the Uralian Seaway were probably due to late-stage Caledonide activity: increased terrestrial runoff, elevated nutrient concentrations that fueled microbial-algal "blooms," and relaxed ecological landscapes after the regional decline of metazoan competitors.