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Chancelloriid sclerites from the L ower C ambrian ( M eishucunian) of eastern Y unnan, C hina, and the early history of the group

2014; Wiley; Volume: 57; Issue: 4 Linguagem: Inglês

10.1111/pala.12090

1475-4983

John L. Moore, Guoxiang Li, Susannah M. Porter,

Geological formations and processes

Abstract Chancelloriids are a group of enigmatic sessile animals that are covered with sclerites shaped as rosettes of spines, producing an appearance like that of a barrel cactus. They are known only from C ambrian rocks. Isolated sclerites of chancelloriids are widespread in small shelly faunas, but they have proven difficult to treat taxonomically due to the variation within and between individual animals. We report on large samples of chancelloriid sclerites from the M eishucunian (pre‐trilobitic L ower C ambrian) of eastern Y unnan P rovince, C hina, including material from the D ahai M ember of the Z hujiaqing F ormation (the Z hujiaqing section, D ahai, H uize C ounty) and the S hiyantou F ormation (the X iaotan section, Y ongshan C ounty). The material from the D ahai M ember appears to come from a single species, C hancelloriella irregularis . Statistical analysis of morphotype co‐occurrences in different samples suggests that several species are represented in the material from the S hiyantou F ormation, which we herein tentatively place in four species, A llonnia erromenosa , A llonnia tetrathallis , A rchiasterella charma sp. nov. and A rchiasterella cf. pentactina . We suggest therefore that careful study of large collections of isolated chancelloriid sclerites permits the identification of different species, opening the possibility of their use in evolutionary or stratigraphical studies. In particular, in S outh C hina, it seems that C hancelloriella irregularis is characteristic of rocks of the P aragloborilus subglobosus – P urella squamulosa A ssemblage Z one and the W atsonella crosbyi Assemblage Z one, whilst A rchiasterella and A llonnia first appear in the overlying S inosachites flabelliformis – T annuolina zhangwentangi A ssemblage Z one. C . irregularis is thus amongst the oldest well‐characterized chancelloriids, and the irregularity and poor organization of its sclerites relative to those of younger forms support the hypothesis that chancelloriid sclerites are compound structures arising from the fusion of originally separate elements, perhaps homologous with sclerites of halkieriids and other coeloscleritophorans. The similarity between the arrangement of rays in a chancelloriid sclerite and the arrangement of bubbles in small bubble clusters suggests that in many cases, aspects of the form of chancelloriid sclerites are the result of simple physical processes.