High-resolution conodont biostratigraphy in two key sections from the Carnic Alps (Grune Schneid) and Graz Paleozoic (Trolp) - implications for the biozonation concept at the Devonian-Carboniferous boundary

摘要

Devonian-Carboniferous boundary successions are globally characterized by conodont-free siliciclastics, stratigraphic gaps, or uncertainties among the first occurrence of biostratigraphically relevant index fossils. This resulted from regionally varying palaeoenvironmental conditions caused by global major environmental changes related to the Hangenberg Crisis. The ongoing search for the ultimate causes of mass extinctions at the end of the Devonian requires a fine biostratigraphic framework, which is developed during the last half of century. This study provides new biostratigraphic implications both from the new and re-evaluated conodont material from two classical continuous pelagic limestone successions of two different geological units in Austria. The Upper Famennian and Lower Tournaisian successions at Trolp (Graz Paleozoic) and Grane Schneid (Camic Alps) have been studied in very high-resolution. This study provides the first record of a joint first occurrence of typical morphotypes of Protognathodus kuehni and Siphonodella sulcata both at Trolp and Grune Schneid in the same biostratigraphic level. This indicates a synchronous first appearance and supports the practicability of the joint sulcata/kuehni Zone previously established. The sampling resulted in a detailed record of the evolution of the biostratigraphically significant Protognathodus fauna, among them Protognathodus kockeli which is currently tested as a potential index for the new Devonian-Carboniferous boundary position. Our analysis revealed taxonomic complexity of Pr. kockeli itself, and in the lineage from its ancestor Protognathodus collinsoni, and to its descendent Protognathodus kuehni. Therefore, using the first occurrence of Protognathodus kockeli as new index fossil for the Devonian-Carboniferous boundary would face the same problem as exists with the current index Siphonodella sulcata which is under revision, and a distinctive conodont-based definition of the system boundary is, under the current taxonomic state of knowledge, difficult to maintain.