Deep shelf trilobite biofacies from the upper Katian (Upper Ordovician) of the Grangegeeth Terrane, eastern Ireland

Abstract

The trilobite fauna of the upper Katian (mid-Ashgill in terms of Anglo-Welsh chronostratigraphy) Oriel Brook Formation in the Grangegeeth Terrane, eastern Ireland comprises 16 species. It is dominated numerically by mesopelagic cyclopygids (Cyclopyge cf. marginata Hawle and Corda and much rarer Symphysops sp.), which comprise 63% of the 156 identifiable sclerites known unequivocally from the formation. Other elements of the trilobite fauna include species of Staurocephalus, Phillipsinella, Nankinolithus and the agnostids Trinodus and Sphaeragnostus. The abundance of cyclopygids in a terrane interpreted as lying within the Iapetus Ocean closer to Laurentia than the Anglo-Welsh sector of Avalonia is consistent with the Late Katian spread of cyclopygids onto low latitude shelves seen in Quebec and Maine in eastern North America, Girvan in SE Scotland and Co. Clare in the Irish Central Terrane. With the exception of the agnostid trilobites, whose mode of life is strongly debated, associated benthic trilobites lacking eyes are fairly rare and the bottom-dwelling fauna does not conform to the atheloptic assemblages that typified many mid-to high latitude Ordovician outer shelf and upper slope settings. The benthic assemblage can be considered in terms of a bathymetrically arranged spectrum of faunas and is closest to that of the deep shelf Phillipsinella parabolaStaurocephalus clavifrons fauna, known from the Upper Ordovician of Avalonia and Baltica and to some extent marginal Gondwana and South China. This is the first record of such a fauna from Laurentia or its marginal terranes and reflects the Late Ordovician breakdown of provincialism in the Iapetus Ocean. The deep-water setting is compatible with that of the associated Foliomena brachiopod fauna.

The only known cyclopygid–‘atheloptic’ trilobite fauna from North America: the upper Ordovician fauna of the Pyle Mountain Argillite and its palaeoenvironmental significance

“…2009, fig. 1), abundant and diverse cyclopygid faunas appeared both on the Laurentian margin – as evinced by the Pyle Mountain Argillite fauna documented herein – and in the outboard terranes represented in the Whitehouse Subgroup at Girvan, Scotland (Ingham in Thomas, Owens & Rushton, 1984) and the Ballyvorgal Group and Oriel Brook Formation in Ireland (Weir, 1959; Owen & Romano, 2011). The benthic trilobite faunas associated with the last of these is most similar to the deep shelf Phillipsinella parabola–Staurocephalus clavifrons fauna seen in the upper Katian of Avalonia and Baltica, whereas the Whitehouse and Ballyvorgal Group faunas include a much more substantial component of trilobites lacking eyes.…”

“…Bartholomew & Tillman, 1977; Lespérance et al . 1987; Fortey & Owens, 1987, p. 107) and its outboard terranes now preserved in Scotland and Ireland (Owen & Romano, 2011).…”

“…46, figs 11, 12) assigned material from the Ashgill of Sardinia to S. clavifrons and reviewed the upper Ordovician species of Staurocephalus . They considered that differences in the surface sculpture of specimens described by Ingham (1977) from the uppermost Rawtheyan of the Cautley district of northern England compared to Kielan's Polish material were not significant and hence they reassigned this and other Ashgill material from northern England, Wales and eastern Ireland to Angelin's species unequivocally (see also Whittington, 1965 a ; Dean, 1971; Price, 1980; Owen & Romano, 2011). In addition to the European occurrences of S. clavifrons , Hammann & Leone (2007) supported the assignment of material from the Rawtheyan of Inner Mongolia by Zhou & Zhou (1982) to Angelin's species.…”

Early Palaeozoic ecosystems, environments and evolution: a synopsis

“…Pat and his colleagues did much to advance our knowledge and understanding of the poorly‐exposed Ordovician successions of eastern Ireland. Owen and Romano () return to the Grangegeeth area, and describe a new and diverse, deep‐water, mid‐Ashgill trilobite fauna from the Oriel Brook Formation, north of the Iapetus Suture. The assemblage of 16 species is dominated by mesopelagic cyclopygids, typical of the deep shelf.…”