We thank Dr Maletz for his comments on the graptolite fauna from the North Ballaird Borehole. In identifying the horizon as Yapeenian, we relied partly on the identification of Yutagraptusl v-deflexus. Williams & Stevens thought that their X. declinatus was, like Y.? v-deflexus, a Yapeenian fossil; they, like us, laid more weight on the presence of a supposed index for that unit than on the absence of other such local indices as Oncograptus. We agree that, if one accepts that Xiphograptus declinatus is the same as Yutagraptus? v-defiexus and that both occur in the immediately underlying Castlemainian 4 interval, our case for the presence of a Yapeenian fauna is weakened.
We are grateful to him for drawing our attention to Pseudisograptus initialis and accept the suggestion that the specimens we identified as Arienigraptus gracilis (our fig. 4) are better referred to his species. We also accept that the supposed stipe of Tylograptus (our fig. 10a) may be better referred to Meandrograptus, a possibility we had not duly considered on account of its difference from the type species, M. schmalenseei.
We are less inclined, however, to accept Dr Maletz's suggestion that our specimens of Isograptus caduceus australis (our fig. 7) should be referred to Parisograptus imitatus (or P. caduceus imitatus). He is correct to point out that the notch between the sicula and theca l1 is wider in our specimens than is shown in Cooper's figures (1973, fig. 16), and in this regard are more like imitatus, but in most other respects it falls nearer the range of variation shown by australis than imitatus. We measured ten attributes on about 30 specimens of Isograptus from the North Ballaird Borehole, and, of these, we eventually assigned nearly 20 to australis. The latter we plotted onto a copy of Cooper's (1973) figure 15: none fell within the range of variation of imitatus and all were scattered in (and beyond) the field of australis - even two specimens that we had considered referring to imitatus (our fig. 7c and BGS 6E 6530; Fig. 1 herein). Comparison of the sicular length and its supradorsal height and of the stipe widths at th3 and thlO with the statistics given by Cooper (1973, p.l 13), show that our specimens are closer in all those features to australis than imitatus, though we recognize that in our specimens the average supradorsal height (1.4 mm) exceeds even the maximum given by Cooper for australis (1.2 mm). If the wide ventral notch in our material is considered to exclude the taxon from australis, it should remain in open nomenclature, perhaps under the label “P. cf. caduceus australis", and can be compared with part of Williams & Stevens's material that they assigned to australis.
Parisograptus cf. caduceus australis (Cooper), with larger ventral notch than the type series. North Ballaird Borehole, Ballantrae, Scotland. BGS specimen no. 6E 6530, magnification x 5. Formerly provisionally (in MS) identified as Isograptus caduceus imitatus Harris, but in Stone & Rushton (2003) was placed in I. c. australis.
We sympathize with Dr Maletz's doubts about / gibberulus, the type species of Isograptus, and for that reason had illustrated a specimen (our fig. 8g) that is less deformed than the lectotype. Dr Maletz follows Jenkins (1982) in regarding it as occurring in the hirundo Zone but we accept the revisions by Fortey et al. (1990, p. 128) and Cooper et al. (1995, p. 190 191), in which the hirundo Zone is recognized as occurring above the range of I. gibberulus, which is confined to the gibberulus Zone. A more detailed account of this will appear in the BGS Memoir on the Skiddaw Group, English Lake District (Cooper et al. 2004).
If a Castlemainian 4 age is preferred for the borehole graptolite fauna, and if-the graptolitic sequence is accepted as syn-obduction, then two outcomes follow. Firstly, the temporal range of the Ballantrae Complex is slightly reduced, and secondly, a regional tectonic event linked to the Grampian Orogeny becomes a little earlier than we had previously thought. Both of these changes are well within the range of possibilities allowed by current dating accuracy, but may become more significant as other age constraints are refined.
- © 2004 Scottish Journal of Geology
