An addition to the graptolite fauna from the Superstes Mudstone Formation, Girvan District
An addition to the graptolite fauna from the Superstes Mudstone Formation, Girvan District (in Advances in Scottish graptolite biostratigraphy, Philip Stone, Susan Rigby and James D. Floyd)
Scottish Journal of Geology (2003) 39 (1): 87-88
In a recent paper I figured some examples of the graptolite Dicellograptus alabamensis Ruedemann, 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 irregularis Hadding, 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 . . .