Biogeography of Late Silurian and Devonian acritarchs and prasinophytes

Abstract

The palaeobiogeography of Late Silurian-Devonian acritarch and prasinophyte microfloras is assessed using qualitative data, similarity indexes and cluster analysis. Based on the recent palaeogeographic reconstructions, the northwards movement of Gondwana initiated during the Ludlovian with the progressive narrowing of the Rheic ocean, resulted in North Africa and south European regions being closer to the equatorial zone and hence in a warmer climate. An apparent homogeneity of Late Silurian assemblages from regions of southern Baltica and the North Gondwanan margin could be explained by such a configuration. The phytoplanktonic associations could also have benefited from the equatorial currents system for trans-Rheic connections. In addition, the qualitative study and the cluster analysis allow for delineation of a biogeographic unit, including part of South America, North Gondwanan and perigondwanan regions, which is comparable to the cold Malvinokaffric realm of the southern hemisphere, based on invertebrates. During the same time, major differences recognized in the distribution of acritarchs from near shorelines to deep oceans, emphasize the potential of mazuelloids (previously referred to as acritarchs) as indicators of deep-water facies and outer margin sites. The Early Devonian shows a geographic restriction of several acritarch genera and species, with a pronounced lateral differentiation between North Gondwanan and eastern North American microfloras, that seems to have been controlled by physical barriers rather than climatic differences. The similarities in acritarch assemblages between northern Europe and the North Gondwanan margin argue for a decrease in acritarch provinciality on both sides of the Rheic ocean, and precludes the presence of a wide Devonian ocean north of western Gondwana.