Synopsis

A 1.89-m section excavated from the Stimulograptus sedgwickii Biozone (Silurian: Llandovery, Aeronian) at Dob’s Linn, Southern Uplands, Scotland, representing some 0.2 Ma of deposition, shows pronounced, quantitatively established patterns of graptolite abundance and diversity. The most striking signature comprises several marked peaks in abundance of the distinctive taxon Neolagarograptus tenuis (Portlock) within this interval. Such ‘bloom events’, if recognized further afield, may provide very fine-scale stratigraphic resolution, seemingly akin to the ammonite biohorizons of the Jurassic. Taxonomic richness as a whole decreases upwards and appears associated with bentonite layers, suggesting that volcanic ash falls might have led to local extinctions of graptolite populations with little or no subsequent recovery. Repetition of the analysis on a duplicate section along-strike at Dob’s Linn showed comparable patterns, demonstrating the robustness of the methodology.

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