ABSTRACT. The Burrellian trilobite faunas of the Leinster terrane comprise at least 31 species from a spectrum of faunal associations. Strong faunal links with the Anglo-Welsh area confirm the likely close proximity of these parts of Avalonia during the mid Caradoc but together with existing magmatic and palaeomagnetic data, the Leinster faunas also indicate that the existing simple structural models of the relationship between these areas need to be reappraised. Of the 22 identifiable trilobite genera, eight are not known from equivalent or older horizons in the Anglo-Welsh area and indicate a significant link with Scoto-Appalachian faunas on the margins of Laurentia. This colonization by trilobites of Scoto-Appalachian origin may have been achieved by 'volcanic island hopping' across the shrinking Iapetus Ocean. The Leinster trilobites are therefore critical in documenting the breakdown of faunal provincialism and in fingerprinting the faunas and likely palaeogeographical setting of terranes now caught up in the Iapetus suture zone to the north. Eighteen species are described or discussed in detail including a new species of Ampyxina, A. hibernica sp. nov., a probable new species of Calyptaulax and revision of the M'Coy species Trinodus agnostiformis, Autoloxolichas laxatus, Flexicalymene forcipata and Remopleurides platyceps.
The Irish Caledomde terranes occupied a variety of sites marginal to the cratons of Laurentia, Baltica and Gondwana. Palaeontological data provide age constraints on the timing of sedimentary linkage and the age of overlap sequences. The Northwestern terrane is characterized by the development of Toquima–Table Head faunas in the early Ordovician in contrast to the Celtic faunas of the Bellewstown and Rosslare terranes. During the middle Ordovician the Northwestern terrane and the Scottish equivalent of the Central terrane were occupied by the Scoto–Appalachian fauna. The Leinster terrane was dominated by Anglo-Welsh elements with varying proportions of the Scoto-Appalachian and Baltic faunas across the terrane. In the Iapetus suture zone in the east of Ireland the Grangegeeth terrane is characterized by a predominantly Scoto-Appalachian fauna.
A newly discovered brachiopod fauna from the Hilltown Formation (lower Llanvirn), Bellewstown Terrane, eastern Ireland, contains elements more typical of sites within the Iapetus Ocean than those associated with the bordering platform provinces. The association of Paralenorthis, "Ahtiella," and Jaanussonites suggests a comparison with the more diverse fauna of the Summerford Group on New World Island, central Newfoundland, and correlation of the Bellewstown Terrane of the Irish Caledonides with parts of the Dunnage and, possibly, Gander terranes of the northern Appalachians.
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