Paleobiogeographical barriers within the Iapetus Ocean effectively restricted conodont faunas in Baltica from contact with those in Laurentia during Tremadocian time. Only species of Cordylodus, Paltodus, Drepanodus, Paroistodus and Iapetognathus have been reported to occur on both sides of the Iapetus Ocean. Continued studies of faunas from deeper platform and slope settings of North America (Laurentia) and generally deeper water faunas of Scandinavia (Baltica) reveal that these faunas share a few more species. One of these taxa was first described as Oneotodus variabilis Lindstrom, 1955, from upper Tremadocian beds of south-central Sweden. It now has been recovered from Nevada and Pennsylvania as well. Its apparatus appears to be seximembrate, with only minor geographic variation of elements between Baltica and Laurentia. Comparison with species of the Laurentian genus Variabiloconus Landing, Barnes and Stevens, 1986, has convinced us that 'O.' variabilis should be transferred to Variabiloconus. A closely related taxon, Variabiloconus transiapeticus n. sp., is reported from south-central Sweden, Nevada and Colorado, strengthening the connection between conodont faunas in the two paleocontinents in the Tremadocian. Moreover, a few specimens of the North American zonal index species Rossodus manitouensis Repetski and Ethington, 1983, have been found in samples from the Paltodus deltifer Zone in Scandinavia, confirming previous correlations and demonstrating that even the strictest paleobiogeographical delimitations can sometimes be overcome.
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Some trans-Iapetus conodont faunal connections in the Tremadocian