Ordovician stromatoporoids from Tasmania

Stromatoporoids are important and conspicuous constituents of the Ordovician limestones of the Gordon Group in Tasmania. The fossils occur through much of the succession, from the Cashions Creek Limestone and equivalents upwards, and from many different localities through west-central and northern Tasmania. A total of 16 species (6 new) of the Order Labechiida are described and illustrated from the sequences. They include the new species Stylostroma ugbrookense, S. bubsense, Aulacera denensis, A. gunnensis, Thamnobeatricea gouldi and T.? vesiculosa. The fauna also comprises representatives of Labechia, Stromatocerium, Labechiella, Pachystylostroma, Rosenella, Pseudostylodictyon, Cystostroma and Alleynodictyon. A number of cyanobacterial associations are also described, including occurrences of Labechiella variabilis with its normal pillars colonized by Cliefdenia like clusters, and Cystostroma involved in sheet-like intergrowths with an unnamed cyanobacterium resembling Epiphyton. In general the stromatoporoid occurrences are in deposits interpreted as of shallow subtidal to possibly intertidal origin. Stromatocerium bigsbyi assemblages characterize the lower part (Cashions Creek Limestone and equivalents) of the Gordon Group, Thamnobeatricea to Stylostroma associations the middle part (Lower Limestone Member of the Benjamin Limestone and correlatives), and Pachystylostroma, Aulacera and clathrodictyid-dominated assemblages the top (uppermost part of the Benjamin Limestone and equivalents). The early assemblages have closest biogeographic relationships with North America but higher in the sequence affinities seem to be mainly Asian but also with some North American links. Species such as Rosenella woyuensis, Labechiella regularis and L. variabilis have been recorded previously from central New South Wales, North China (all three species), Malaysia (only the last two), and Kazakhstan (only the first two). In contrast, the cylindrical Aulacera and Thamnobeatricea are more typical of the North American fauna.