Selected cephalopods from the Ordovician of the Prague Basin (Bohemia) and Baltica (Estonia and Sweden): taxonomy, paleobiogeography and paleoecology

This dissertation thesis is a summary of five studies published in peer-reviewed, impacted scientific journals. All of the publications are taxonomic revisions of previously unknown or little known collections of fossil cephalopods from the Early Paleozoic strata of Bohemia, Estonia and Sweden. Paleogeographical and stratigraphical distributions of the respective taxa were summarized, refined and compared with contemporary fossil assemblages known from other regions. Implications on the paleoecology of the cephalopods and original environmental conditions were made. The text of the thesis is divided into three main parts. In the first part, the morphology of cephalopods is explained, stressing out the most important diagnostic characters used for their descriptions. The current systematics of the Cephalopoda is overviewed and the main cephalopod groups during the Ordovician are briefly introduced. The second part of the thesis describes the geological development and settings of the regions, from which the studied fossil cephalopods originate. The third and final part of the thesis provides a discussion and interpretation of the results of the published studies in the context of the previously published research. The genus Bactroceras Holm, 1898 and some members of the order Lituitida were studied from the Middle Ordovician of the Prague Basin. Bactroceras is unique as the earliest known representative of the order Orthocerida. The orthocerids have straight, slender shells and were the first cephalopods to have inhabited deeper-water, pelagic environments. The paleogeographic and stratigraphic distribution of Bactroceras implies that orthocerids originated during the Early Ordovician in the high latitude regions of peri-Gondwana and later, during the Middle and Late Ordovician, expanded their habitats to low latitude regions. Subsequently, the orthocerids became one of the most abundant and diverse groups of the Cephalopoda of the whole Paleozoic. The morphologically peculiar shells of another cephalopod order studied from the Prague Basin, the Lituitida, were likewise adapted to the conditions of the pelagic and relatively deep water conditions. By contrast to the orthocerids, the lituitids were, typical for the low and mid latitude regions. In the high latitude regions, such as the Prague Basin, the lituitids appeared only occasionally. The departure of cephalopods to the free water column and pelagic environments is one of the crucial events of the Global Ordovician Biodiversification Event (GOBE). Another important phase is the global expansion of metazoandominated (skeletal) reefs that more significantly progressed since the latest Middle Ordovician. Abundant and diverse cephalopod faunas inhabited these reefs. Herein, two cephalopod taphocoenoses from the late Sandbian reefal environments of Baltoscandia were studied: from the Vasalemma Formation (Estonia) and the Kullsberg Limestone Formation (Sweden). Both taphocoenoses come from roughly coeval strata that originated in the same basin (Baltic Basin) but are still taxonomically distinct. The differences in the occurrence and abundance of individual taxa probably reflect different paleoenvironmental conditions under which both cephalopod faunas lived: the reefal limestones of the Vasalemma Formation represent the more proximal and shallow water environment, while the mud-mounds of the Kullsberg Limestone Formation are interpreted as the relatively deeper-water. The Kullsberg cephalopods are in fact more similar in taxo-