Late Cambrian (Furongian) to mid-Ordovician euconodont events on Baltica: Invasions and immigrations

The stratigraphic distributions, taxonomy, diversity, evolutionary lineages and events of late Cambrian to Middle Ordovician euconodonts of Baltica are briefly reviewed and re-evaluated. The euconodonts of the late Furongian (Stage 10, late Cambrian) to Middle Ordovician successions of Baltica record innovation, immigration and two invasions of short duration and one major extinction. The innovation event with an origin on Baltica is the rise of the Proconodontus serratus lineage in the late Furongian (Stage 10). The immigration events comprise the Cordylodus immigration to Baltoscandia in latest Cambrian and this phylogenetic lineage persisted to mid- Tremadocian (Early Ordovician) and vanished at the global mid-Tremadocian extinction event. The late Floian – basal Dapingian transition is a significant event on Baltica. It is characterized by the immigration of the important genera Baltoniodus and Microzarkodina and upon their arrival these taxa remained on the Baltica palaeocontinent and stayed for a long period forming evolutionary lineages. The basal Darriwilian immigration event by the genus Lenodus and coeval prominent sea-level lowstand were caused by a short global icehouse stage related to the expansion of the polar ice sheets. The extinction event caused the disappearance of the characteristic Cordylodus fauna. Most events are related to changes in the palaeoclimate resulting from the movement of the Baltica palaecontinent and associated eustatic sea-level changes. However, the cause of the global early Tremadocian extinction event is not yet completely clear and remains unsolved. Perhaps abrupt and significant palaeoceanographic changes on a global scale (i.e., plate tectonic movements causing sudden change of ocean currents and climate from warm to cold) were the cause of this significant extinction in the Early Ordovician.