Early evolution of the genus Amorphognathus and updated Sandbian (Upper Ordovician) conodont biostratigraphy in Baltoscandia

Geological periods that divide Earth history into tens of millions of years allow us to sequence major events in its past. More detailed studies, requiring an accuracy of million or even hundreds of thousands of years, are based on evolutionary changes recorded in the remains of past living beings or their tracks and traces preserved in rocks, referred to as fossils. With the appearance and disappearance of species, biozonal stratigraphic units (biozones) are determined, that are used to define and characterize the age relationships between different rock bodies. This approach forms the foundation for every geological study, determines accuracy of obtained results and geological maps. The most important biozonal units used in the Ordovician and Silurian of Estonia are based on conodonts, microscopic (mostly less than 2 mm) tooth-like fossils with variable morphology, derived from extinct early chordates. The present study addresses the species composition, distribution and early evolution of the genus Amorphognathus in Estonian and Swedish sections. According to the detailed study, the suggested occurrence of the biozonal species A. inaequalis could not be confirmed in the Baltic region and the biozonal species A. tvaerensis actually represents two different species. The main result of the theses is a comprehensive revision of the conodont biozonation of the North-Atlantic conodont province which will significantly influence any further scientific research on Ordovician geology. The work includes an updated correlation scheme of the Upper Ordovician key sections in the Baltoscandia region and justifies the correlation of rock units, improving the quality of geological maps prepared in Estonia. Additionally, the obtained result facilitates the integration of geological study results within the Baltic Sea area and comparisons with research results from other regions.