Earth and Environmental Science Transactions of the Royal Society of Edinburgh

Research Article

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

Alex Pagea1a2, Philip R. Wilbya3, Claire J. T. Mellisha4, Mark Williamsa1 and Jan A. Zalasiewicza1

a1 Department of Geology, University of Leicester, Leicester, LE1 7RH, UK

a2 Present address: Department of Earth Sciences, University of Cambridge, Cambridge, CB2 3EQ Email: aap30@esc.cam.ac.uk; mri@le.ac.uk; jaz1@le.ac.uk

a3 British Geological Survey, Keyworth, Nottingham, NG12 5GG, UK Email: prwi@bgs.ac.uk

a4 Palaeontology Department, Natural History Museum, London, SW7 5BD, UK Email: c.mellish@nhm.ac.uk

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.

Key Words:

  • Arenig;
  • Llandovery;
  • Canada;
  • graptolites;
  • Henry Nicholson;
  • reproduction;
  • Scotland;
  • Silurian;
  • taxonomy