During the Silurian, the same assemblages are present over North America, Europe and much of Asia.
Shelf seas covered much of North America and the Russian platform (including Gotland) and many shallow marine areas were at great distances from the nearest land, so that no land-derived sediment was deposited. Instead, limestones, consisting of shells, corals and algal debris, accumulated through biological action. In these environments, the proportion of brachiopods and other invertebrates may be locally rather different from those seen in similar habitats in areas of clastic (sand and shale) sedimentation.
In many areas of North America, the limestones have been altered to dolomite and the original organic structures are no longer present. Some rocks may show no obvious sign of fossils at all.
Changing salinity also affects marine communities. In the Late Silurian of parts of North America salt deposits indicate increased salinity in areas of higher evaporation and these more saline waters have a very reduced number of genera compared with normal marine habitats. A similar effect is seen in areas of reduced salinity.
In much of central Europe, the Silurian is entirely represented by platform mudstones. The absence of common shelf benthos has been explained by Berry and Boucot (1967) as a result of the water being too cold for them. However, we suggest that the absence of brachiopods in any particular place may be due either to too great a depth of water (i.e. greater than that under which the Clorinda Community lived) or to foul bottom conditions, which may be found at any depth. The latter may or may not have included low temperature conditions, but some brachiopods today thrive in the Arctic, and we reject cold water as a likely explanation of the structure of the central European Silurian.
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