HERMANN JAEGER'S PROPOSAL TO DEFINE THE BASE OF THE DEVONIAN ON A SIGNIFICANT SPECIATION IN MONOGRAPTID EVOLUTION

The base of the Monograptus uniformis graptolite Zone in the Klonk stratigraphic section in the Barrandian area, Czech Republic, was approved by the International Commission on Stratigraphy to be the base of the Devonian System at the 1972 International Geological Congress. That approval followed from the recommendation of the International Committee on the Silurian-Devonian Boundary after more than a dozen years of activity. McLaren (1977) reviewed the field studies and discussions of the Committee, pointing out that the majority of the members favored selection of the base of the Monograptus uniformis Zone for the base of the Devonian System relatively early in the deliberations. Although most committee members favored use of a Zone for the base of the Devonian, Hermann Jaeger argued that the boundary should be based on a significant speciation in the evolution of monograptid graptolites. To him, the appearance of M. uniformis and its close relative, M. uniformis angustidens, which denote the base of the M. uniformis Zone, reflect a unique evolutionary development among monograptid graptolites. Jaeger (1977), in a review of the work of the Committee, summarized his position: “the base of the uniformis Zone coincides with a distinctive step in graptolite evolution.” “In the Lower Devonian, Monograptus is represented exclusively by biform to uniform species having hooked thecae of the type of M. uncinatus at least in the proximal part of the rhabdosome, .i.e. the thecal apertures are covered by an unpaired mesial shield with fusellar structure. Viewed against the background of graptolite evolution, the Early Devonian Monograptus association appears to be the most monotonous and uniform monograptid fauna which ever inhabited the seas, thus demonstrating the ‘programme evolution’ (or Zeitsignature) in graptolite history. “ Biform monograptids that occur stratigraphically below M. uniformis have extensions on their thecal apertures and appear superficially somewhat similar to M.uniformis, especially in crushed specimens. These and other monograptids became extinct before appearance of M. uniformis with its distinctive thecal apertures.