Australia: The Land Where Time Began

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Palaeozoic Stromatoporoids - Their Taxonomy, Mineralogy and Applications in Palaeoecology and Palaeoenvironmental Analysis

Stromatoporoids are calcified sponges that were common from the Middle Ordovician to the Late Devonian in reefs and related facies. As a result of conflict between schemes of classification that are based on the skeleton verses the spicules, the spicules being almost completely lacking on the stromatoporoids from the Palaeozoic, the taxonomic work is well known, though it is controversial. At the genus level, a lower level taxonomy, it is considered to be reliable enough to be applied to stromatoporoids in palaeobiological work. Comprehensive information is available on the ecology of stromatoporoids from only a few case studies of some examples from the Silurian and Devonian. Therefore an understanding of the response of stromatoporoids to environmental conditions is yet to be achieved, though the author1 suggests stromatoporoids were likely to be capable of dealing with the fine-grained sediments found in the situation where they mostly lived. The use of low-level taxonomy in future ecological studies in high resolution studies involving comprehensive sampling, may, according to the author1, establish the relationships between stromatoporoids and the environment, as many genera of stromatoporoids have only certain growth forms. The author1 suggests aspects of stromatoporoids that have great potential for such work are the intergrowth of these organisms and growth banding. All stromatoporoids are substantially recrystallised, whatever their apparent preservation state, that ranges from apparently well-preserved to a complete loss of calcareous skeletal features. The stromatoporoids underwent a peculiar diagenesis in which calcareous skeleton and gallery cements of all stromatoporoids are overprinted by calcite crystals, that are elongated and irregular, arranged normal to the growth laminations, that are seen most clearly in cross-polarised light. Stromatoporoids co-occur with mollusc shells that are invariably fully recrystallised or dissolved, in which case they are preserved as external and internal moulds. As a result of this difference molluscs are likely to be underrepresented in the fossil record, but stromatoporoids are not, which provides confidence for palaeoecological work on their assemblages. Stromatoporoids may have been high-magnesium calcite, though the evidence for this is circumstantial, and they lack characteristics that would allow them to be readily classified as being originally aragonite or low-magnesium calcite. The peculiar diagenetic fabric of stromatoporoids also has implications for the debate about the relationship between stromatoporoids and the concept of aragonite/calcite seas which the author1 suggests needs more work.


Sources & Further reading

  1. Kershaw, Steve. "Palaeozoic Stromatoporoid Futures: A Discussion of Their Taxonomy, Mineralogy and Applications in Palaeoecology and Palaeoenvironmental Analysis." Journal of Palaeogeography 2, no. 2 (2013): 163-82.


Author: M. H. Monroe
Last updated 31/05/2013

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