TAPHONOMY AND PALEOECOLOGY OF SILURIAN ALGAL-LAGERSTÄTTEN

Lagerstätten dominated by the remains of noncalcified (“fleshy”) macroalgae are known from a number of Silurian stratigraphic units. Aspects of sedimentology and biotic composition indicate that these “Algal-Lagerstätten” formed in bodies of shallow marine water characterized by poor circulation, muddy substrates intermittently disturbed by storms, and low grazing pressure. In these settings, anoxia, obrution, and carbonate sediment composition contributed to exceptional preservation, the latter facilitating early diagenesis. Algal material is preserved as carbonaceous compressions, resembling in this regard Cambrian Burgess Shale-type preservation. Differences exist, however, including occasional three-dimensional preservation of thallus elements, elevated carbonate content of the sediment, and apparent lack of taphonomically associated aluminosilicification and pyritization. From a paleoecological perspective, nearly all of the algal taxa that comprise Silurian Algal-Lagerstätten correspond to the Filamentous and Foliose functional groups. Corticated macrophytes are rare, and the Leathery and Articulated Calcareous functional groups are lacking, despite the fact that calcareous macroalgae are known as allochems in contemporary limestone units associated with higher-energy depositional settings, including reef-related facies. In modern seas, macroalgal floras established in shallow, poorly circulated settings have a community structure dominated by the Filamentous and Foliose functional groups, analogous to that observed for Silurian Algal-Lagerstätten. Thus, because of the particular environmental conditions required for exceptional preservation, in combination with a tight relationship between algal functional group and environment in this setting, it can be inferred that Silurian Algal-Lagerstätten convey only a select subset of macroalgal diversity from this timeframe. Consequently, it is possible that Leathery macrophytes (i.e., heavily corticated forms analogous to kelp and Fucus with a capacity to achieve large size) were abundant in better-circulated parts of Silurian oceans, but are beyond the view afforded by the available taphonomic windows.