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.