Two abnormal pygidia of the trilobite Toxochasmops from the Upper Ordovician of the Oslo Region, Norway

Two pygidia of the trilobite Toxochasmops from the Upper Ordovician (Katian) of the Oslo Region, Norway, display different types of abnormalities. A juvenile pygidium, treated in open nomenclature as Toxochasmops sp. A, has a partially developed axial ring restricted to the right side of the axis which is interpreted as a teratology. The other pygidium, treated in open nomenclature as Toxochasmops sp. B, shows a local fusion of two pleural ribs with a poorly developed furrow crossing the fused area; this may either represent a teratology or regeneration after an injury. In the latter case, it could represent a rare example of metaplasia in trilobites with replacement of a pleural furrow by an interpleural furrow during regeneration. The posterior position of the partially developed axial rings, without corresponding pleural ribs, indicates that the posterior part of the axis possibly represents non-functional somites, and abnormalities in this part probably did not significantly affect the vital organs of the trilobite.