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

Abstract

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 ovato-triangular 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 his genus Dawsonia, though the name was pre-occupied by a trilobite, and named four species: D. campanulata, D. acuminata, D. rotunda and D. tenuistriata. A reassessment of Nicholson’s type material from the Silurian of Moffatdale, Scotland, and from 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.

The classification of the Pterobranchia (Cephalodiscida and Graptolithina)

“…Gürich (1928) recognized the similarity of Dawsonia to Peltocaris. Page et al (2009) discussed the problematical origin and interpretation of the genus Dawsonia.…”

Polymorphic organization in a planktonic graptoloid (Hemichordata: Pterobranchia) colony of Late Ordovician age

“…Comparable structures would be difficult to see in flattened specimens: the delicate layer of cortical tissue that surrounds the site of infestation is much thinner than the fusellar and cortical material that compose the rhabdosome wall, and thin films of cortical fabric are prone to decay and compactional distortion (see discussions of Hallograptus scopulae and the Climacograptus wilsoni vesicle in Page et al . 2009).…”

Soft-part preservation in a bivalved arthropod from the Late Ordovician of Wales

“…Berdan, 1984;Vannier, Wang & Coen, 2001), contrary to that inferred for the Cardigan arthropods (see Section 7). Similarly, the Cardigan specimens show no resemblance to the elongate and spinose Ordovician bivalved arthropod Caryocaris (see Vannier et al 2003), which though common in graptolitic mudrocks, is of an organophosphatic composition (Churkin, 1966;Page et al 2009).…”

“…Ruedemann, 1934), arthropods (see Vannier, 2007) and problematica (e.g. Page et al 2009) have been reported. These provide evidence of the existence of a more diverse biota in addition to the graptolites and rare shelly fossils that are typically preserved.…”