Late Ordovician sponge spicules from the Yangtze Platform, South China: Biostratigraphical and palaeobiogeographical significancen

A diverse and abundant isolated sponge spicule assemblage of Late Ordovician age is first reported from the western margin of the Yangtze Platform in South China. The spicule assemblage was recovered from argillaceous limestone sampled for conodonts. It includes 14 types which can be split into two categories: hexactinellid spicules and demosponge spicules. The assemblage primarily comprises hexactines and their derivatives, including not only some distinctive types such as Kometia cruciformis, but also large numbers of morphotypes that are quite common and diagnostic for hexactinellids, e.g. swollen hexactines, curved hexactines, and stauractines. Demosponge spicule types are represented by tetraxons, and various desmas typical for lithistids also occur. A highly modified hexactine, Kometia cruciformis, is recovered from China for the first time. This record represents currently the only recovery of such spicules from strata outside of Australia, and is coeval to the assemblage known from the Malongulli Formation. The presence of such unique modified hexactines (Kometia) in both time-equivalent assemblages suggests that certain spicules may have potential importance in biostratigraphic correlations and palaeobiogeographical interpretations, demonstrating the dispersal of taxa and their migration along the peri-Gondwana shelf areas during the Ordovician, driven by ocean current systems. The Late Ordovician sponge spicule assemblage from the Yangtze Platform provides new perspectives for investigating phylogeny, biostratigraphy, and palaeobiogeography of Palaeozoic sponges, and promotes some understanding of macroevolution in siliceous sponges.