Palaeobiology of Palaeozoic medusiform stem group echinoderms.
The morphological details of both external and internal anatomy of a group of Palaeozoic fossil medusiform animals are described with the aid of text-figures and plates with explanatory drawings. This fossil group had a worldwide distribution with a stratigraphic range from the Lower Cambrian to the Upper Devonian and includes the following taxa:- Eldonia ludwigi Walcott 1911, E. eumorphus sp. nov., Rotadiscus grandis Sun and Hou 1987, Discophylluni peltatum Hall 1847, D. mirabile Chapman 1926, D. cryptophya (Clarke) 1900. Newly recognised anatomical structures for E. ludwigi include c.30, internal, radially-arranged, bifurcating lobes, a coelomic sac surrounding the alimentary canal, internal structures assumed to represent gonads and c.4 oral tentacles. E. eumorphus has c.44 internal bifurcating lobes associated with rows of pores on the ventral surface, which form a possible respiratory system. R. granclis has a possibly mineralised dorsal surface, rows of pores on the ventral surface and a tentacular appendage with arm-like extensions. Discophylluni 1s characterised by an ornamented dorsal surface with rows of elaborate pores. The nomenclature 1s revised, anatomical reconstructions are presented and modes of life in terms of feeding and benthic versus pelagic existence are discussed. It is concluded that this group, the Discophylla (equivalent in status to a new class), lies within the stem group Echinodermata. As a consequence, current understanding of the early evolution of the Echinodermata, especially with respect to internal anatomy, is questioned . A number of medusiforrn fossils, not studied in detail as part of this work, are discussed and tentatively assigned to the Discophylla.
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