Article: A new Ordovician anthropod, Soomaspis, and the agnostid problem

A new uncalcified arthropod, Soomaspis, is described from the latest Ordovician Soom Shale of South Africa. It is included with other primitive and uncalcified trilobites in the family Naraoiidae, the composition and evolution of which are re-evaluated. Naraoiids are considered to have arisen by the heterochronic process of hypermorphosis, whereas Agnostida, to which they bear a superficial resemblance, are progenetic. The place of Soomaspis and naraoiids within the trilobite-like arthropods is analysed cladistically; they comprise a sister group of the rest of the clade. The problem of classifying Agnostida is discussed in this context. Their relationships within, or outside Trilobita depend upon the methods of character analysis, and the emphasis placed on particular characters. In the analysis presented here, Agnostina comprise the sister group of Eodiscina. The inclusion of all these groups within the Trilobita is favoured.