Abstract

The psammosteid ostracoderms provide perhaps the most useful zonal indices of the Old Red Sandstone facies during Middle and Late Devonian times. The standard zonal succession has been established in the Baltic province, where the gradual evolution of this group can be traced in considerable detail. In other provinces there is no evidence of evolution having taken place, but the same zonal scheme can be applied in such areas as the Timan and Scotland, where the same species occur. There is evidence of periodic waves of migration from the source region to other provinces such as the Donbas, the Urals, Spitsbergen, Greenland and Ellesmereland. However the further the distance from the source area, the less exact the correlations become. This is exemplified by the situation in Spitsbergen where a primitive pycnosteid arrived in late Narova times, became established and continued unchanged until late Burtniecki times. On the basis of the psammosteids the top of the Spitsbergen sequence could easily be correlated with the bottom of the Baltic one.

This illustrates the dangers of using ‘zonal indices’ uncritically when correlating sequences from one province to another.

The evolutionary radiation of a group of organisms in a single major province, from which there are successive waves of migration, appears to be a fairly general pattern, for which, as yet, there is no adequate explanation. It does, however, account for the lack of evidence of evolution, in the majority of faunal provinces throughout the fossil record.