Issues

Graptolites were colonial organisms that first appeared in the Cambrian. In early Ordovician times they migrated from the benthos into the plankton, becoming some of the most significant zooplankton of the Lower Palaeozoic. Planktonic graptolites radiated quickly, reached an acme of variation in the late Ordovician and Silurian, then declined into extinction by the mid-Devonian. The benthic graptolites followed them to extinction in the Carboniferous.

Planktonic graptolites underwent rapid evolution, recorded in their widespread and relatively durable, colonial skeletons. Their evolutionary variation through time affords important biostratigraphical data, but there was also latitudinal or palaeogeographical control on the distribution of different assemblages. In terms of British Caledonian geology, the Ordovician and Silurian periods saw the growth and closure of the Iapetus Ocean with contrasting graptolite faunas on its northern, tropical margin (Scotland) and southern, temperate margin (England and Wales). As the Ocean narrowed during the early Silurian, faunas became more cosmopolitan, but major differences are apparent in the Ordovician, where graptolite biostratigraphy has long been bedevilled by problems of correlation. Despite these difficulties, for the Lower Palaeozoic in any one region, graptolite faunas allow a remarkably detailed level of biostratigraphical resolution.

Biostratigraphy depends empirically on observing similar successions of faunal assemblages in various places. It can be effective only if based on exact identifications, continuous taxonomic refinement and nomenclatural consistency. Systematic, detailed fossil collecting from a wide range of localities is an essential prerequisite. Graptolites provide a particular challenge taxonomically as the preserved remains are of a colony, not of . . .

In a recent paper I figured some examples of the graptolite Dicellograptus alabamensisRuedemann, 1908, from the Superstes Mudstone Formation in the Girvan cover-sequence, and used that species to support the inference that the formation lies in the lower part of the gracilis graptolite zone (Rushton 2001, p. 49, fig. 2b–g). However, one figured specimen (Rushton 2001, fig. 2b) appears to have been incorrectly identified, and should rather be referred to Dicranograptus irregularisHadding, 1913.

The outline of the specimen in question, British Geological Survey GSE 8213 (Fig. la), shows what appears to be a short biserial portion with two pairs of thecae. Unlike the best preserved of the graptolites from the Superstes Mudstone, it is strongly flattened and has lost much of the periderm, so that the critical median part of the proximal end is preserved only as an external mould; as the specimen was associated with several specimens of Dicellograptus alabamensis, and has similar dimensions and thecal form, I made the assumption that the specimen was a D. alabamensis in which the spiral form of the rhabdosome had allowed the proximal parts of the stipes to become flattened with one on top of the other. I had briefly considered the possibility that it was a Dicranograptus, and compared it with the smallest species known to me, D. irregularis Hadding, as revised by Hughes (1989, text-fig. 20a and b), but rejected identification with that species because the specimen from the Superstes Mudstone seemed significantly smaller.

Further consideration has . . .