The cyanophytic alga Girvanella in the Lower Ordovician of Hälludden, northern Öland, Sweden

Traces of microscopic filaments of the cyanophytic alga Girvanella have been found in the inferred regressive Lower Ordovician limestone sequence at Hälludden, N. Öland. The algae and their mode of occurrence, as aggregates or as encrustations, are described. Their vertical range is discussed in relation to water depth and water energy. A correlation between the presence of Girvanella and limited water depth is demonstrated. The distribution of Girvanella thus supports the idea of a shallowing in the A. expansus zone in the upper part of the section. An observed association between sponge spiculae and encrusting blue-green algae, is discussed briefly.