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Lower Paleozoic palynomorphs show a large morphologic diversity and are, generally, extremely abundant in unmetamorphosed marine sediments, yet the stratigraphic ranges and regional distribution of most taxa still are poorly known. Only now are data becoming available to permit the determination of the distribution of palynomorphs in the Silurian System and, to a much smaller extent, in the Upper Ordovician and Lower Devonian as well. An embryonic palynostratigraphy is being constructed for the Silurian. Its zones are based on (1) the appearance of miospores and on their increasing morphologic complexity, and (2) the ranges of selected acritarch and chitinozoan taxa. Megafossil evidence, mainly from graptolites, fixes these ranges. The palynostratigraphic system, crude as it may be, appears to be valid for the areas bordering the Atlantic Ocean.
Several contrasting, "worldwide," acritarch biofacies existed in the regions bordering the Atlantic, in Arctic Canada, and in Siberia during the Silurian. From the megafossil evidence these biofacies are judged to be contemporaneous; they are regularly and predictably time-transgressive. On a regional scale the facies are not significantly correlative with such local differences in sedimentary realm as are expressed in changes in lithology, but because lineations based on compositional differences in acritarch spectra seem to be roughly parallel with lithotope boundaries, a causal relation between them is suspected.
On a Wegenerian palinspastic reconstruction of Atlantic Pangea, the parallelism of biofacies lineations, lithotopes, and perhaps even paleomagnetic latitudes is apparent. This parallelism is interpreted as reflecting regional differences of paleotemperature. For example, Silurian acritarch biofacies boundaries would be paleoisotherm-parallel, and therefore paleolatitude-parallel. Arguments are: (1) on Atlantic Pangea there was an epicontinental sea with a minimum width of at least 45°. In such a sea the latitudinal temperature gradient must have been quite pronounced; (2) biofacies regionally are continuous and have a simple and regular geometry; (3) lithotopes and biofacies are parallel and their boundaries follow small circles; (4) regional biofacies are independent from such s ort-living factors as islands, troughs, and local lithology changes; (5) biofacies form a cross-continental chronologic and regional homotactic arrangement; and (6) the biofacies show a time-transgression which follows the polar trajectory as extrapolated from Devonian and Ordovician pole positions.
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