A number of graptolites with thickened periderm, both dendroids and graptoloids, have been examined with the SEM. The thickening is by the deposition of layers of ectocortical tissue, clearly as bandages in the graptoloids. In Colonograptus colonus and some ‘diplograptids’, the thickening is most pronounced in the proximal regions of the rhabdosome, and can lead to the partial or complete occlusion of the aperture of the sicula and proximal thecae, as well as thickening the virgella. This thickening is interpreted as an adaptation connected with an extra gerontic production of cortex, which might have served as a reinforcement of the rhabdosome, or as an adaptation to modify or retain, the centre of gravity in the water column, or both. In Amphigraptus sp. the thickening appears to be common, and is concentrated on the dorsal side of the stipes. In Dendrograptus sp. thickening is also common: the main branches become excessively thickened, uniformly around the stipes, the side branches less thickened. In Cyrtograptus, thickening is rare, and appears to be gerontic, with the exception of a new form tentatively identified as Cyrtograptus in which thickening is uniform throughout the rhabdosome, and may have been formed initially. In Colonograptus and several ‘diplograptids’, the thickened proximal part shows it is much heavier than the distal part, suggesting a downwards vertical direction of the proximal end. In Amphigraptus the asymmetrical thickening suggests an orientation with the ventral side uppermost.

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