BORN IN CONTROVERSY - DEVELOPING THE DEVONIAN PERIOD

Born as a “problem child,” the Devonian Period is biostratigraphically delineated by a variety of pelagic and benthic fossils. Sir Roderick Murchison, mapping rocks beneath the Coal Measures, Old Red Sandstone and “Kulm” rocks in Britain, was quite perplexed by the “graywackes” beneath. Debates between Murchison, Adam Sedgwick and their rival Henry De la Beche gave Murchison an urgency to present this new geologic period in 1839. Subsequent work in the Rhineland, and recovery of what is now a relatively cosmopolitan Devonian index fossil (Stringocephalus), allowed Murchison with Sedgwick and de Verneuil to change their earlier opinions that it was Silurian under the recognizable “Kulm chalk”in Germany. In fact confirmation of conspicuously non-Silurian or Carboniferous fossils within the “graywackes” in the Rheinischen Schiefergebirgen confirmed the new period. Devonian stages have only recently been finalized, as their provenance is from four countries. Early Devonian stages, originally Gedinnian, Siegenian and Emsian (based on the Belgian-German section) were replaced by Prague Basin nomenclature (Lochkovian, Pragian and Zlichovian) after Chlupac followed Joachim Barrande’s suggestion that the Czech section was more easily mapped than the Rhine stratigraphy. Yet the Czech work was originally included into the Silurian, since graptolites occurred with seemingly “Devonian” shelly fossils. Complicating the Early Devonian correlations is distinct faunal provincialism. The Bohemian shelly faunas of the Prague Basin stood in contrast to Rhenish faunas in the Jeseník Mountains near the Polish frontier. Consensus has established the Lower Devonian to include the Lochkovian, Pragian and Emsian stages. Middle Devonian stages (Eifelian and Givetian) were correlated by palynomorphs, conodonts, goniatitic cephalopods and select benthic fossils. Late Devonian stages are also well-established (Frasnian and Famennian). Middle and Late Devonian stages are best referenced in Germany, Belgium and France. Latest Devonian events, dominated by glaciation, shut down the prolific Frasnian carbonate factory, and may have contributed to the end Frasnian faunal extinction. The index fossil Retispora lepidophyta is more ubiquitous than coeval marine acritarchs (e.g., Umbellasphaeridium saharicum), thereby suggesting restricted marine circulation.