Article: The Wenlock Cyrtograptus species of the Builth Wells district, central Wales
Mark Williams and Jan Zalasiewicz
The Wenlock sequence of the Builth Wells district, central Wales is dominated by long-ranging Monograptus, Pristiograptus and Monoclimacis species. Cyrtograptus species, which underpin the graptolite biozonation, represent only about 10 per cent of recovered individuals. Ten Cyrtograptus species are present, most being zonal index species for the Builth district or other Wenlock sequences. Redescription of these Cyrtograptus species and comparison with all pertinent type material indicates that: the early Wenlock Cyrtograptus insectus, C. centrifugus and C. murchisoni are best distinguished by means of first whorl diameter, sicula length and differences of cladial development; mid Wenlock Cyrtograptus rigidus may bear a secondary cladium, but at Builth there are no stratigraphically discrete subspecies; Cyrtograptus linnarssoni is known only from its type locality; the long, gracile proximal part of the rhabdosome of C. ellesae facilitates its distinction from C. perneri, with which it has been confused, and indicates similarities to C. ramosus and C. lundgreni. Differences in the ranges of key Cyrtograptus species present problems for correlation between the lundgreni Biozone of the Builth district and the perneri-ramosus and lundgreni biozones of central Europe. The low diversity and abundance of the cyrtograptid fauna of the Builth district, compared to those of Arctic Canada and the Czech Republic, suggests relatively inhospitable conditions for graptolites.