Early and Middle Ordovician events of Baltoscandia

The Baltoscandia region was a wide platform covered by an epicontinetal sea on the Baltic Craton during Early to Middle Ordovician. In the same period the Baltica palaeoplate moved rapidly from high southern latitudes towards low southern latitudes. This northern directed movement crossed climate zones and the platform sedimentary succession changes from predominantly siliciclastic sedimentary rocks with minor carbonate in the late Cambrian to Early Ordovician to increasingly carbonate-rich rocks: first by cool, temperate shallow-water limestone (Orthoceras Limestone) and then by warmer temperate shallow-water carbonate and marl accumulation at the end of the Middle Ordovician (Darriwilian). True conodont faunal endemism within Baltica has not been observed. Instead Early and Middle Ordovician multielement conodonts from Baltoscandia record a series of faunal events comprising (1) conodont faunal phylogenetic developments, (2) migration events, (3) immigration events and (4) significant faunal turnovers. The late Cambrian and earliest Ordovician (early Tremadocian) record of the conodont faunas is incomplete for the Baltic Platform, because of pronounced global sea-level lowstand. The conodont phylogenic evolution starting from late Tremadocian (Early Ordovician) comprises the significant lineage of Paltodus, ‘Acodus’ (= Acodus deltatus sensu stricto), Trapezognathus and to Lenodus and from the top Floian the lineages of Baltoniodus and Microzoarkodina. Several migration events include genera in the Tremadocian and the significant Oepikodus evae acme in the Floian. The dispersal of the bi-membrate genus Eoplacognathus (sensu stricto) in the Darriwilian was a significant migration event from deeper-water setting at the margin of the craton and onto the platform. Visitors (i.e. Yangtzeplacognathus) immigrating from other palaeocontinents also reached the Baltica plate. These stayed only for a short period and disappeared equally fast as they appeared. However, the precise geographic source region for invaders and the palaeoceanographic conditions that facilitated dispersal into the Baltoscandia platform is still poorly understood. Biogeographic analysis indicates that in the Early Ordovician communication exclusively with Laurentia prevailed. This line of communication was disturbed at the beginning of the Middle Ordovician (Dapingian), where communication with Peri-Gondwana faunas also appeared on the Baltic platform. The Dariwillian invasions were multidirectional that involved taxa immigrating into the Baltoscandian region from both Laurentia and peri-Gondwana plates that approached continental margins of the Baltica palaeocontinent. The late Darriwilian–early Sandbian conodont faunas of Baltica show close similarities to faunas in the oceans, at margins of Laurentia, South China, and other adjacent tectonic plates and terranes, which indicate their semi-cosmopolitan distribution. The synchronicity of the geologic, climatic and oceanographic events supports the influence of geologic events on conodont evolution on Baltica. Causal links between the faunal turnovers includes predominantly (1) sea-level changes (2) tectonics and (3) global climate change. The trends also correlate with the perturbations in the carbon cycle, with the d13C minima corresponding to the low diverse fauna and faunal turnover and the positive values generally occurring during radiations.