Tagasi otsingusse
Aubrechtová, 2018

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

Aubrechtová, M.
Aasta2018
Pealkiri originaalVybraní hlavonožci (Cephalopoda) ordoviku pražské pánve a Baltiky (Estonsko a Švédsko): taxonomie, paleobiogeografie a paleoekologie
KirjastusCharles University, Faculty of Science
Kirjastuse kohtPrague
Leheküljed1-259
Tüüpdoktoritöö / kandidaaditöö
Keelinglise
Id8528

Abstrakt

Abstract 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. Paleogeogra- phical 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 envi- ronmental 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 Litui- tida 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 stra- tigraphic 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 environ- ments is one of the crucial events of the Global Ordovician Biodiversification Event (GOBE). Another important phase is the global expansion of metazoan- dominated (skeletal) reefs that more significantly progressed since the latest Mi- ddle Ordovician. Abundant and diverse cephalopod faunas inhabited these reefs. Herein, two cephalopod taphocoenoses from the late Sandbian reefal environ- ments 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 un- der which both cephalopod faunas lived: the reefal limestones of the Vasalemma 1 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 taxonomic composition to those of the previously described, stratigraphically younger (late Katian – early Hirnantian) and also deeper-water mud-mounds of the Boda Limestone Formation (Sweden). On the species level, both the Vasa- lemma and Kullsberg cephalopods are uniquely diverse and highly endemic. On the genus- and higher taxonomic levels, however, they show similarities to fossil assemblages from distant regions, such as Laurentia. Additionally, the earliest member of the order Ascocerida from the Late Silurian strata of Estonia is reported herein. During the Silurian, the ascocerids are mainly known from the Swedish island of Gotland and the Prague Basin. Their occurrence in Estonia is thus an important addition to our knowledge on the paleogeographical pattern of the group and an example of the use of fossil cephalopods for paleogeographical purposes also in later phases of the Paleozoic.

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