Middle ordovician conodonts from the Atlantic faunal region and the evolution of key conodont genera

Sammanfattning: This thesis consists of seven separate articles that are mainly concerned with Middle Ordovician conodonts. The studied conodont specimens were obtained from three Middle Ordovician sections in south China and four Middle Ordovician sections in Sweden. The new genera, Baltoplacognathus and Yangtzeplacognathus, and the new species, Pygodus lunnensis and Pygodus protoanserinus, were erected. Yangtzeplacognathus crassus and Eoplacognathus suecicus, which are very important species for international stratigraphic correlation, were described and discussed in detail. One conodont zone, the Y. crassus Zone, and two subzones, the Pygodus lunnensis and P. protoanserinus Subzones, were established. The study shows that there are four evolutionary lineages in the family Polyplacognathidae and two evolutionary branches in the late stage of the Pygodus lineage. The geographical distribution of Scolopodus spinatus suggests that it is a reliable index of deep water environment. The Middle Ordovician Guniutan Formation in south China has been demonstrated to be closely similar to the "Orthoceratite limestone" in Baltoscandia as regards facies, stratigraphic development, and conodont biostratigraphy. Features shared between the Chinese and Baltoscandian carbonate facies are frequently occurring, burrows and furrows, mineralized discontinuity surfaces, buckled beds (mini-mounds), perched cephalopod conchs, thin seams of stromatoids, and biocalcarenitic components dominated by arthropods and echinoderms. Over 90% of the conodont species from three sections in Yangtze platform, south China are recorded from the Baltic area. Only 80% of the species from the Baltic area known to be present are shared with in south China.