DOI: 10.5593/sgem2017/11/S01.049


M. Tsinkoburova
Saturday 9 September 2017 by Libadmin2017

References: 17th International Multidisciplinary Scientific GeoConference SGEM 2017, www.sgem.org, SGEM2017 Conference Proceedings, ISBN 978-619-7105-98-8 / ISSN 1314-2704, 29 June - 5 July, 2017, Vol. 17, Issue 11, 383-392 pp, DOI: 10.5593/sgem2017/11/S01.049


The flourishing of the family Orthidae (class Rhynchonellata, order Orthida) was happening in the first half of the Ordovician, the stage known in the literature as the Great Ordovician Diversification. The problem of the establishment of the Biodiversity centers is one of the dominant problems of modern Evolution Biogeography. At the present time, there are not common criterions for the determination of biodiversity’s center in spite of the unanimous acknowledgement of the existence of local speciation’s regions. According to Naimark E.B. (2001), the biodiversification’s center is the region where one can observe high taxonomic, morphological and genetic variety, many endemics and relative increase of dimensions. In this connection the study of the Ordovician sediments of the epicontinental Baltoscandian sea containing various and full complexes of sea invertebrates is quite interesting. Describable sediments are characterised by the high degree of the litological-facial variability, in particular, the ones caused by the frequent eustatic oscillation of the Ordovician sea. For all this going on the Baltoscandian East (Leningrad region, Estonia) was in the area of shallow North Estonian confacias belt. There were different climatic changes of the paleobasin during the Ordovician (due to the migration of the continent from the area of high latitudes to the area of the low latitudes). These climatic changes influenced on the sediments and on the systematic composite of the fossil’s complex. The first finds of the Baltoscandia’s Orthidae (genus Leoniorthis) are locked in the rocks of the Vassilkovo member (Leetse formation, the second part of the Floian stage, the Lower Ordovician). This genus was well distributed in Baltoscandia in the next Volkhov time (the Dapingian age) and the Kunda time (the Dapingian and the beginning of the Darriwilian age) (Leoniorthis rubeli Hints). The transition from the Leetse formation into the Volkhov formation marks the change of the terrigenous sediments to the carbonate sediments, one should know that the representatives of the genus Leoniorthis could adapt to the different conditions during the early stage. This is evident on the example of L. robusta, which was found in the mudstones coastal sandbanks of the upper ramp and in the original primitive reefs (so-called Gekker’s hills). The Middle Ordovician epoch was the stage of the biodiversity by Orthidae, six more genera appeared besides the genus Leoniorthis. At the same time, East Baltoscandia was the region of the main development of Orthidae during the Middle Ordovician epoch. There are five genera Orthidae in the Middle Ordovician sediments of the east of Baltoscandia (from six genera Orthidae living in the Middle Ordovician epoch). One more genus Trondorthis Neuman is in the Middle Ordovician sediments of Norway (Danish-Norwegian confacias belt) (Bruton, Hurper, 1981). Moreover, the peak of biodiversity of this family was in the Kunda time (the beginning of the Darriwilian age), which corresponds to the first peak of the Great Ordovician Biodiversity. In this time, representatives of the genera Orthambonites Pander and Orthis Dalman were widespread. These two genera are characterized by the big morphological similarity (as the other related genera) (V. Jaanusson and M. Bassett, 1993), in spite of the fact that the Middle Ordovician Orthidae has had sufficiently large morphological variability. Hence, the Middle Ordovician stage of the development of Orthidae could be possibly considered as the stage of the complex (morphological and taxonomic) evolution. In conclusion, it is possible to establish that in the east of Baltoscandia in the Middle Ordovician there were as much as possible favourable conditions for the forming of the center of biodiversity of Orthidae.

Keywords: Baltoscandia, Ordovician sediments, biodiversity, brachiopoda, Orthidae

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