New age constraints for the Ordovician Tyrone Volcanic Group, Northern Ireland

Journal of the Geological Society, Jan 2008 by Cooper, M R, Crowley, Q G, Rushton, A W A

Assessment of the graptolite fauna

Stratigraphically the most significant find is Isograptus victoriae lunatus Harris (Fig. 2h?, i? and j). A juvenile proximal end from a cherty layer is flattened but is relatively undeformed (Fig. 2j). A fragmentary stipe on the same block that shows proximal curvature (Fig. 2h and i) is referred to the same species, but with some reserve. The sicula is 2.9 mm long, of which the supradorsal part is 0.6 mm wide and 1.4 mm long; sicular aperture 0.55 mm wide; ventral notch wide, 0.6 mm deep; nema preserved for nearly 1 mm. Stipe width at th2 is 1.4-1.5 mm, and this width is maintained on the longer stipe fragment which has a two-theca repeat distance of 1.4 mm (equivalent to about seven thecae in 5 mm). There are two pairs of pendent thecae. These specimens therefore agree with I. victoriae lunatus (Cooper 1973, p. 59, fig. 8), and most of the dimensions lie close to the mean for the subspecies (Cooper 1973, table, p. 112). The small number of pendent thecae is not matched by other I. victoriae subspecies.

In Australasia I. victoriae lunatus is confined to Ca1 of the Castlemainian, (VandenBerg & Cooper 1992, p. 62), and it is used as a zonal fossil for the correlative level in western Newfoundland (Williams & Stevens 1988). This lies within the much longer reported ranges of the other species now recorded from the Slieve Gallion site. The top of the victoriae lunatus graptolite zone, the lowest of four zones of the Castlemainian (VandenBerg & Cooper 1992, p. 42), is correlated approximately with the top of the Whitlandian Stage of the British succession, and the base of the zone is used to recognize the base of the Middle Ordovician in graptolite-bearing successions.

Dichograptus octobrachiatus (J. Hall) (Fig. 2a and b?)

Preparation of the specimen in the collections of Trinity College, Dublin, number TCD.28759a-d (counterparts) shows that it has five stipes and it is now identified as a pentad form of Dichograptus octobrachiatus. This species most commonly has eight stipes, but Elles & Wood (1902, p. 78, plate 10b, d and e) recorded specimens with 7, 6 or 5 stipes. The specimen in Figure 2a agrees best with D. octobrachiatus, examples of which were described by Williams & Stevens (1988); their figure 14B shows a specimen with comparably close initial branching. Other robust species such as D. octonarius (Hall 1865, plate 10, figs 1 and 2) and D. solidus Harris & Thomas (Cooper & Fortey 1982, fig. 13) have the thecae more widely spaced. The stratigraphical range of D. octobrachiatus is long: VandenBerg & Cooper (1992, p. 59) recorded its range in Australasia from the base of the Bendigonian to the middle of the Darriwilian (basal Arenig to mid-Llanvirn).

Didymograptellus nitidus (J. Hall) (Figure 2d-f, g? and k?)

Maletz (1994) transferred this species from the genus Expansograptus or subgenus Didymograptus (Expansograptus), where it was formerly placed, to his revised concept of Didymograptellus Cooper & Fortey (1982, p. 220). In the new collection are two slender horizontal specimens of Didymograptellus (Fig. 2d and e) that can be referred to D. nitidus, and a more doubtful robust specimen (Fig. 2k) that is poorly preserved proximally. The stratigraphical ranges recorded for D. nitidus in different regions are not consistent, possibly reflecting uncertainties surrounding the identification of some of the recorded material. According to Williams & Stevens (1988, fig. 3) D. nitidus ranges through the Bendigonian and Chewtonian, not reaching the lunatus Zone at the base of the Castlemainian. VandenBerg & Cooper (1992, p. 60) gave its range through the Chewtonian and into the lower half of the Castlemainian. In contrast, the British material referred to nitidus by Elles & Wood (1901) occurs in strata of equivalent age to the mid- to late Castlemainian (Ca2-3) and early Yapeenian (YaI). D. nitidus appears, therefore, to indicate only an undifferentiated Arenig age.