CLASSIFICATION AND CONTROLS OF INTERNAL BANDING IN PALAEOZOIC STROMATOPOROIDS AND COLONIAL CORALS

Abstract:  Palaeozoic corals and stromatoporoids exhibit a variety of internal banding phenomena, many of which have been commonly interpreted as annual growth bands. We evaluate bands through analysis of colonial corals and stromatoporoids from three stratigraphic intervals: Upper Ordovician of Manitoba Canada, and Llandovery–Wenlock and Ludlow of Gotland, Sweden. Banding features are divided into four categories: (1) absence of banding; (2) density banding formed by variation in density or form of elements; (3) growth-interruption banding indicating growth cessation and regeneration; and (4) post-mortem banding caused by compaction or diagenesis. For discrimination of band types, it is essential to examine internal structures and skeletal margins in thin sections or acetate peels. Species vary considerably in degree and type of banding; each has a distinct pattern of variation. We propose criteria to determine if banding is consistent with seasonally induced growth variation: (1) consistency in band character and thickness; (2) continuity of skeletal growth; (3) marginal features; and (4) evidence of diagenetic alteration. Density bands in tabulate and rugose corals probably represent annual growth variations, but results for stromatoporoids are more ambiguous; although stromatoporoids commonly show banding, unequivocal density banding is poorly developed and growth interruption generated most stromatoporoid banding. Cerioid rugose and tabulate corals possess the thickest density bands; the thinnest bands are in stromatoporoids and heliolitid tabulates.     

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.