Faunal dynamics and evolution of Ordovician conodonts on the Baltic side of the Tornquist Sea

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ABSTRACT Continental drift of Baltica from the Tremadocian subpolar latitudes to subtropical latitudes in the Katian was the main factor controlling the succession of the Ordovician Baltic conodont communities. These faunas were gradually enriched during the Floian as a result of immigrations from the regions experiencing warmer climate. Reinterpretation of quantitative data in terms of population approach to fossil assemblages shows how some of these immigrants evolved anagenetically in place, changing their contribution to the secondary productivity of the ecosystem. The composition of the fauna became surprisingly uniform, at least since the numerical domination by the presumably indigenous Baltoniodus lineage was established during the Dapingian. Baltoniodus was supplemented by another indigenous lineage of Trapezognathus-Lenodus-Eoplacognathus, which continued its subordinate occurrence during the Darriwilian. The early Sandbian transgression resulted in immigration of the Amorphognathus lineage that emerged allopatrically in an unknown region but then began evolving anagenetically until the end of the Ordovician. Conodonts with coniform apparatus elements added complexity to the general picture of immigrations and disappearances, but only the lineage of Protopanderodus rectus seems to have differentiated geographically its contribution to the biological productivity. Several brief cooling and warming episodes did not result in any long-term transformations of the conodont communities. Most intriguing was the immigration of the Yaoxianognathus lineage that probably gave rise to all of the post-Ordovician ozarkodinids. By that time, Yaoxianognathus had its close relative in the tropical North American Midcontinent, but the source area was probably in the Darriwilian of the Argentinian part of Gondwana. Forms with thin P1 elements of basal cone walls, like Scabbardella or Hamarodus, are indicators of glacial Gondwanan influences. The lineage of Sagittodontina, associated with these in the Małopolska microcontinent (with Gondwanan affinities), was subordinate in Baltica until it had been influenced by the Hirnantian glaciation that ended the Baltic conodont fauna.