Lower to Middle Ordovician trilobite faunas along the Ural border of Baltica

HELJE PÄRNASTE & JAN BERGSTRÖM Recent revision of the Ölandian trilobite faunas in Baltoscandia and the Ural Mountains throws new light on the development of the trilobite faunas in Baltica and possible migration links to surrounding terranes.The trilobite assemblages of 104 genera on the Uralian side of Baltica show different development patterns for the south and north through the Tremadocian to the Darriwilian.The oldest Uralian trilobites -disputably of latest Cambrian or earliest Ordovician age -arrived probably from the Siberian and Kazakh terranes being represented by mostly endemic genera such as Kujandaspis and Jdyia, but also with pandemic Akoldinioidia and Micragnostus.The following Kidryasian, Kolnabukian and Kuagachian faunas change gradually to show increasing difference between the sections in the South Urals and those in the northern Polar Urals or Pay-Khoy.In Kidryasian the olenids dominate in the South Urals as they do in many other regions during the early Tremadocian.The Kolnabukian trilobites represent the most diverse trilobite association in the region, and are comparable to the Ceratopyge fauna.The Kuagachian fauna contains a few additional elements, increasing the difference between south and north but with reduced generic diversity.The routes of faunal exchange are modified too.Thus during the Early Ordovician migration between the Uralian side of Baltica and the Baltoscandia, Kazakh and Altai-Sayan terranes becomes more important than that between the Uralian side of Baltica and the Siberia, North and South China plates.The Darriwilian Karakoľ-Mikhailovskian faunal association shows a clear separation between north and south Urals with the former region, as in Baltoscandia, dominated by asaphids, while in south a reefal illaenid-cheirurid association of Laurentian genera occurs.This is rather intriguing given the widely accepted palaeogeographical disparate position and latitude of Laurentia at the time.•