Stromatoporoid growth form and taxonomy in a Silurian biostrome, Gotland

A biostrome at Kuppen, in the middle Ludlow Hemse Beds (Silurian) on Gotland, Sweden, is dominated by densely packed laminar, domical and bulbous stromatoporoids to the exclusion of almost all other organisms. Within the biostrome two contrasting growth forms developed, growing together in the same environment. Clathrodictyon mohicanum Nestor forms laminar to low domical shapes, while Stromatopora bekkeri Nestor and Plectostroma scaniense Mori developed higher domical and bulbous shapes. The interactions between environmental factors and the genotype brought about the development of characteristic shapes (phenotype) for each species. P. scaniense and C. mohicanum show these and other morphotypes elsewhere on Gotland, while S. bekkeri forms only higher domical shapes within the limits of its known occurrence on Gotland. P. scaniense has the greatest potential genotypic plasticity for shape variability and shows a sensitivity to the environment not shared by the other two species. Variations in environmental conditions and changes in selection pressures and functional requirements of stromatoporoids appear to be responsible for shape variations. Identifying genetic components of shape, and showing how they relate to associated faunas and environments should make interpretations of stromatoporoid morphology more meaningful.