Katian conodonts from the Portrane Limestone: the first Ordovician conodont fauna described from Ireland

The Portrane Limestone, which crops out in a small inlier about 17 km north of Dublin, is one of the few significant carbonate units in the Ordovician of Ireland. Samples from this formation, which has long been famous for its diverse silicified shelly fauna, have produced a biostratigraphically diagnostic conodont fauna of more than 2200 specimens representing a total of 26 taxa. The Amorphognathus superbus and A. ordovicicus zonal conodont species have been documented. Our study, the first one illustrating diverse assemblages of Ordovician conodonts from Ireland, shows that this fauna, which represents the Hamarodus brevirameus-Dapsilodus mutatus-Scabbardella altipes biofacies, is strikingly similar to middle Katian (Stage Slice Ka3) faunas from Wales, England, and continental Europe. This age dating is consistent with that advocated based on shelly fossils. The Portrane Limestone was deposited in a volcanic arc environment off the Avalonian microcontinent and is now located southeast of the Iapetus suture across Britain and Ireland. The fact that the Portrane area was placed on the southeastern side of the Iapetus and relatively close to Wales and northern England but probably widely separated from North America, may at least partly explain why the Portrane conodont fauna differs in important respects from broadly coeval faunas from eastern North America and has much closer affinities with European faunas.