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Paleoenvironmental and Taphonomic Implications of Trace Fossils in Ordovician Kukersites
In an examination of the trace fossils of the Upper Ordovician Yeoman Formation of southeastern Saskatchewan, close relationships were observed between dispersed organic matter (DOM) or macerals (kerogen) and biogenic sedimentary structures. Macerals in these sediments are dominantly composed of the microfossil, Gloeocapsomorpha prisca alginite. To date, no agreement has been reached concerning the life habit of this organism, or the mechanism for accumulation of beds rich in G. prisca. The more popular view is that G. prisca was a non-photosynthetic, prokaryotic, benthic algal mat; others believe it was a photosynthetic, eukaryotic or prokaryotic, planktonic organism, which bloomed episodically throughout the Ordovician. Differing evidence and conclusions may be explained by the fact that the alga was preserved at various stages in its life cycle; i.e. it could be either planktonic or benthic. Depending on the nature of G. prisca, these algal microfossils were consumed and processed differently by burrowing infauna. Fluctuations in abundance of G. prisca are represented by fluctuations in feeding habits represented by the trace fossils and by fluctuating abundance of G. prisca preserved in the sediment. Periods of lowered G. prisca availability are represented by early life cycle, small disseminated type A G. prisca thinly lining the burrows of suspension-feeders. When conditions shifted and G. prisca was more abundant, larger, vegetative, disseminated type B G. prisca are also found in the burrow linings and fills. At maximum blooms, kukersites (beds rich in disseminated type B G. prisca) were preserved. In all phases of the bloom, G. prisca is absent from the fill of deposit-feeding Planolites, but occurs in the linings of suspension-feeding Palaeophycus. Based on the distribution of G. prisca within trace fossils in the Yeoman Formation, a planktonic nature for this microfossil better explains the accumulation of kukersite beds rich in G. prisca that occur in southeastern Saskatchewan. Although conditions causing these blooms and the scale of cyclicity need to be further examined, a restriction of the sea-floor is suggested by the limited trace fossil suite as well as dominance of a single species of phytoplankton, G. prisca.
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