SILURIAN SEAWEEDS AND STRATIGRAPHY: NEW ALGAL-LAGERSTÄTTEN FROM THE LLANDOVERIAN OF MICHIGAN AND ONTARIO

Macroalgae (seaweeds) are critically important components of shallow marine communities, providing food, substrate, and shelter for animals. Here, new occurrences of Silurian macroalgae are reported from newly discovered algal-Lagerstätten of Llandoverian (Telychian) age located within the northern part of the Michigan Basin and the Timiskaming outlier of northern Ontario. Macroalgal taxa recovered from the latter include Thalassocystis striata. Previously, this dichotomously-branched taxon, recently redescribed as a noncalcified bryopsidalean green alga similar to living Codium (“dead man’s fingers”), was known only from the Llandoverian of northern Michigan, some 500 km to the west. Occurring in association with this new T. striata material is a new form, consisting of a creeping, runner-like stolon with numerous pinnate fronds. Thalli of this noncalcified form broadly resemble those of the living bryopsidalean alga Caulerpa (aka “killer algae”). The new Michigan Basin algal-Lagerstätte is located some 30 km to the east of the T. striata type locality. The Schoolcraft Formation at this locality, in addition to yielding well-preserved specimens of T. striata, includes a new form similar to the Late Ordovician dasycladalean green alga Archaeobatophora typa, with side branches arranged in whorls around an upright main axis. Previously proposed correlations of Llandoverian strata within and between the Timiskaming Rift and Michigan Basin and, in broader terms, the peculiar phenomenon of regionally extensive algal-Lagerstätten within the Silurian of North America, are examined in light of these findings.