Distinctive Scleritome with Marginal Tubercles of a New Palaeoscolecid Worm from the Shipai Fauna(Cambrian Epoch 2) at Three Gorges, South China

摘要

SEM analysis of trunk ornamentation on a compressed palaeoscolecid fossil from the Shipai fauna, Cambrian Series 2 and Stage 4 at Three Gorges, South China, allows a reassignment of the material to a new taxon, Sanxiascolex papillogyrus gen. et sp. nov. The preserved body is entirely annulated. Each annulus is ornamented by four rows of irregularly alternating plates and each plate bears numerous tubercles at the marginal region and a variable number(zero to six) of nodes at its central part. These features are unique among known palaeoscolecids. Comparative analyses with other compressed palaeoscolecid macrofossils and phosphatized material on the pattern of trunk ornamentation indicates that a detailed examination of the cuticle ornament weighs heavily in analyses of their taxonomic distinctions. A similar sclerite pattern to that of S. papillogyrus has been found in some phosphatized palaeoscolecid cuticle fragments as well as in isolated sclerites. The upper series of nodes and tubercles of these sclerites suggest an accretional mode of sclerite growth. The considerable variation in node number observed across the plates of S. papillogyrus demonstrates that phosphatic palaeoscolecid taxonomy based on relatively minor variations in the number of sclerite nodes should perhaps be reconsidered. As the origins of the phosphatized specimens are quite diverse, a large variety of isolated sclerites from the Early Palaeozoic may not reflect actual biodiversity.