Dawsonia Nicholson : linguliform brachiopods, crustacean tail-pieces and a problematicum rather than graptolite ovarian vesicles

Though little is known of the graptoloid reproductive mechanism, graptolites with putatively sac-like appendages, supposedly ovarian vesicles, have been known from the Moffat Shales Group, Southern Uplands, Scotland, for over 150 years. Locally, these co-occur with isolated, two-dimensional, discoidal or ovatotriangular fossils. In the 1870s, Nicholson interpreted these isolated fossils as being graptoloid ‘egg-sacs’, detached from their parent and existing as free-swimming bodies. He assigned them to the genus “Dawsonia”, though the name was preoccupied by a trilobite, and named four species: “D.” campanulata, “D.” acuminata, “D.” rotunda (sic.) and “D.” tenuistriata. A reassessment of Nicholson’s type material from the Silurian of Moffatdale, Scotland, and the Ordovician Lévis Formation of Quebec, Canada, shows that Dawsonia Nicholson comprises the inarticulate brachiopods Acrosaccus? rotundus, Paterula? tenuistriata and Discotreta cf. levisensis, the tail-piece of the crustacean Caryocaris acuminata and the problematic fossil “D.” campanulata. Though “D.” campanulata resembles sac-like graptolite appendages, morphometric analysis reveals the similarity to be superficial and the systematic position of this taxon remains uncertain. There is no definite evidence of either “D.” campanulata or sac-like graptoloid appendages having had a reproductive function.