Early evolution of the genus Amorphognathus and updated Sandbian (Upper Ordovician) conodont biostratigraphy in Baltoscandia
Date
2023-09-26
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Abstract
Planeet Maa ajaloo jaotamine geoloogilisteks ajastuteks võimaldab planeedi ajaloo suuremaid sündmusi järjestada. Geoloogilised ajastud hõlmavad endast kümneid miljoneid aastaid. Detailsemaks dateerimiseks, aastamiljonid või sajad tuhanded, on kasutusel eluslooduses toimunud muutused. Evolutsioonilisi muutusi loomade ja taimede arengus kajastavad fossiilid, mille ilmumiste ja kadumiste põhjal määratakse lühemaid ajavahemikke eristavaid biotsonaalseid üksuseid (biotsoonid), mis on põhialuseks igale geoloogilisele uuringule ning määravad ühtlasi geoloogiliste kaartide täpsuse.
Eesti Ordoviitsiumi ja Siluri ajastute olulisimad biotsonaalsed üksused põhinevad konodontidel, mikroskoopilistel (valdavalt alla 2 mm) varieeruva väliskujuga hambalaadsetel, väljasurnud keelikloomadelt pärinevatel fossiilidel. Käesolev uuring täpsustab perekonna Amorphognathus liigilist koosseisu, levikut ja varajast evolutsiooni Eesti ja Rootsi läbilõigetes. Täpsustatud andmetel puudub Läänemere regioonis biotsonaalne liik A. inaequalis ja biotsonaalne liik A. tvaerensis jaguneb kaheks erinevaks liigiks. Töö peamiseks tulemuseks on neist uutest andmetest lähtuv Põhja-Atlandi regiooni konodontide biotsonaalsuse põhjalik revisjon, mis mõjutab märkimisväärselt nii edasisi teadusuuringuid kui geoloogilist praktikat. Töö sisaldab Baltoskandia regiooni vanema Ülem-Ordoviitsiumi tugiläbilõigete täpsustatud korrelatsiooni skeemi ning täpsustab kivimiüksuste korrelatsiooni, mis võimaldab edaspidi parandada Eestis koostatavate geoloogiliste kaartide kvaliteeti. Lisaks parandab saadud tulemus teadusuuringute omavahelist seotust Läänemere regioonis ja võimaldab neid paremini võrrelda mujal tehtavate uuringutega.
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.
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.
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Keywords
conodonts, biostratigraphy, Late Ordovician, Baltoscandia, Estonia