Skeletal variation in the early clathrodictyid stromatoporoids of Upper Ordovician and its paleoecological and phylogenetic implications

This study reports four species belonging to two genera of clathrodictyid stromatoporoids from the Sanqushan Formation (Katian, Upper Ordovician), Changshan County, Zhejiang Province, East China: Clathrodictyon cf. mammillatum (Schmidt), C. plicatum Webby and Banks, C. cf. microundulatum Nestor and Labyrinthodictyon cascum (Webby and Morris). Of these, three species (C. cf. mammillatum, C. plicatum and L. cascum) show a wide range of skeletal variation, representing the variation from the normal skeletal phase to abnormal Ecclimadictyon-like crumpled skeletal phase, especially in the axial part of the columnar growth forms. It is uncertain whether the skeletal variation in early clathrodictyids was the result of environmental stress (such as abrupt sediment influxes that often cause growth interruptions of the stromatoporoids) or genetic inheritance. These characteristics suggest that this group had greater environmental tolerance than the earlier labechiids and increased ecological complexity during Late Ordovician by providing a stable, solid and elevated substrate for the other benthic organisms. This may imply that the labechiids and clathrodictyids are quite different groups, probably independently derived during the early evolution of stromatoporoids.