The Ordovician conodont Semiacontiodus cornuformis (Sergeeva, 1963) and related species in Baltoscandia

Semiacantiadus cornuformis (SERGEEVA, 1 963) was probably derived from Laurentian forms such as Semiacontiodus nogamii MILLER, 1969, or Teridontus nakamurai (NOGAMI, 1967). It had a seximembrate apparatus with Pa, Pb, Sa, Sb, Sc and Sd elements. Its oldest occurrences are encountered at the base of the Microzarkodina parva Zone. The elements of its inferred ancestors, appearing immediately below this level, are morphologically simpler, resembling S. nogamii. Elements of the lineage then rapidly attained the typical morphology of S. cornuformis which spread over the Baltoscandian shelf during the late Arenig. In the early Kunda it was joined by Semiacantiodus davidi n. sp. which initially also seems to have resembled S. nogamii and is probably a descendant of a new immigrant to the area. Less specialized taxa of Laurentian origin apparently established themselves a number of times in Baltoscandia and evolved into taxa adapted to the deep and cold environment on that platform. Later Ordovician representatives of this conodont group are less widely known in Baltoscandia, but they also appear to differ from Laurentian forms of similar age, thus confirming the general picture of Baltoscandia as a local centre of evolution for the groupntly established themselves a number of times in Baltoscandia and evolved into taxa adapted to the deep and cold environment on that platform. Later Ordovician representatives of this conodont group are less widely known in Baltoscandia, but they also appear to differ from Laurentian forms of similar age, thus confirming the general picture of Baltoscandia as a local centre of evolution for the group.