Skolithos in the Upper Cretaceous Cardium Formation: an ichnofossil example of opportunistic ecology

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Abstract
In recent ecological studies, opportunistic species have been defined as those species that can respond rapidly to an unexploited niche. Such species are characterized by a lack of equilibrium population size, a density independent mortality, an ability to increase abundance rapidly, a relatively poor competitive ability, high dispersal ability, and devoting a high proportion of resources to reproduction. The occurrence of Skolithos in the Cardium Formation (Turonian) provides ichnofossil evidence for opportunistic behavior in the geological record. The Cardium Formation in the Pembina oilfield area of west-central Alberta [Canada] consists predominantly of mudstones, which coarsen upward into fine-grained sandstone beds; the sequence is interpreted as a storm-dominated shelf deposit. The mudstones contain a deposit-feeding ichnofossil assemblage indicative of deposition in low energy conditions: Thalassinoides, Chondrites, Zoophycos, Terebellina, Rhizocorallium, Planolites, Teichichnus and Asterosoma. These mudstones are interrupted by frequent sharp-based thin sandstone beds, which commonly contain abundant Skolithos and which are the result of rpaid storm deposition. Such storm activity would be capable, through scouring and rapid deposition, of completely destroying the normal benthic fauna. Modern studies of opportunistic recolonization have shown that the most common opportunistic colonists in open marine settings are tube-dwelling or shallow-burrowing suspension-feeding polychaetes. Opportunistic forms are frequently smaller and found in higher densities than the equilibrium counterparts. The Skolithos from the storm sandstones of the Cardium Formation are thus inferred to represent the activities of shallow-burrowing opportunistic colonists following storm depopulation.