THE MIDDLE CAMBRIAN EODISCOID TRILOBITE DAWSONIA OELANDICA (WESTERGÅRD, 1936)

Eodiscoid trilobites are important for correlations of upper lower Cambrian and lower middle Cambrian strata. Five species of eodiscoids are known from the middle Cambrian of Scandinavia: Eodiscus scanicus (Linnarsson, 1883), E. borealis Westergård, 1946, E. punctatus (Salter, 1864), Dawsonia oelandica (Westergård, 1936), and Opsidiscus bilobatus (Westergård, 1946). Dawsonia oelandica is redescribed and reconstructions of the dorsal surface are presented. The study is based on specimens collected by Anton H. Westergård from two localities on the island of Öland, supplemented with new material from Jämtland, central Sweden. The Öland specimens (four cephala and one pygidium) are preserved in a greenish grey, arenaceous shale, and were collected from the Mossberga drill core and a locality near Mörbylånga. The Jämtland material is from a single block of a dark grey limestone collected by Tomas Weidner at Mon, south of Östersund. The Jämtland material includes five well preserved cephala and two nearly complete pygidia, providing additional information on the species. All the available material is from the Ptychagnostus praecurrens Zone (formerly the Eccaparadoxides pinus Zone) of the lower middle Cambrian, which approximately correlates with the Eccaparadoxides benetti Zone of western Avalonia, the “Paradoxides” harknessi Zone of eastern Avalonia, and the Kymataspis arenosa Zone of Morocco. The external exoskeletal surface of D. oelandica is distinctly granulose, particularly on the cheeks, the anterior border, the posterior part of the occipital spine, and on the pleural fields of the pygidium. The stoutly-based occipital spine is about as long as the cephalon, and rises at approximately 45 degrees from the horizontal. The third and fourth pygidial axial rings probably had prominent, vertically directed spines. The material also shows the presence of a spine on the posterior thoracic axial ring. The intraspecific variability, the ontogeny, and the possession of probable sensory organs are discussed.