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Ordovician faunal data from the Scandinavian Caledonides is tested with new geochemical information from zircons to give UPb ages and source origins of volcanic arc and ophiolite sequences. Early Ordoviaan (Arenig-Llanvim), low latitude, Toquima-Table Head faunas from the upper Upper Allochthon are associated with an island arc system formed adjacent to Laurentia. Contemporaneous mafic magmas were contaminated by crustal material during subduction and associated granites contain inherited zircons of Archaean age. The nearest source for such rocks is on the Laurentian rather than the Baltic side. Higher latitude Celtic province faunas from the upper Upper Allochthon are from one insular site accessible to forms from both Laurentia and Baltica. . The late Ordoviaan low-latitude Holorhynchus and subtropical Himantin faunas occur in overstep sequences above deeply eroded early Ordovician arc complexes. The transgression appears to be coeval with a second generation of spreading-related complexes. Single detrital zircons from sediments show sources from Archaean, Proterozoic and early Ordovician terranes. This suggests that deposition was in a basin situated along the same continental margin (Laurentia) to which the early Ordovician ophiolite/arc sequences had already become accreted. The late Ordovician faunas link both Laurentia and Baltica at a time of narrowing of Iapetus.supported by recent palaeomagnetic studies.The new geochemical data together with the faunal information is Terra Nova, 4,217-222

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