Palaeozoic stromatoporoid futures: A discussion of their taxonomy, mineralogy and applications in palaeoecology and palaeoenvironmental analysis

Abstract

Palaeozoic stromatoporoids are calcified sponges common between Middle Ordovician and Late Devonian times in reefs and related facies. Taxonomic work is well known, but controversial because of conflict between classification schemes based on the calcareous skeleton versus spicules (which are almost completely lacking in Palaeozoic stromatoporoids); however, lower-level taxonomy (at genus-level) of the calcareous skeleton is considered reliable to be applied in palaeobiological study. Knowledge of stromatoporoid ecology is poorly developed, such that comprehensive information is available for only a few case studies, in some Silurian and Devonian examples. Thus an overall understanding of stromatoporoid responses to environmental conditions has not yet been achieved, although stromatoporoids were likely able to deal with fine-grained sediment where they mostly occur. Many stromatoporoid genera have only certain growth forms, so future focus on the use of low-level taxonomy in ecological studies, by comprehensive sampling in high-resolution studies, may establish the relationships between stromatoporoids and their environments. Intergrown organisms and growth banding in stromatoporoids are aspects that have great potential in such work. Mineralogy of stromatoporoids remains poorly understood. Regardless of their apparent state of preservation (ranging from apparently well-preserved to complete loss of calcareous skeleton features) all stromatoporoids are in fact substantially recrystallized. They underwent a peculiar diagenesis, whereby the calcareous skeleton and gallery cements of all stromatoporoids are overprinted by irregular elongated calcite crystals arranged normal to the growth laminations, most clearly visible in cross-polarized light. Stromatoporoids co-occur with mollusc shells that are always either fully recrystallized or dissolved (present as internal and external moulds); this difference means that while molluscs are likely under-represented in the fossil record, stromatoproids are not, providing confidence for palaeoecological work on their assemblages. Stromatoporoids lack characters which would readily classify them as being originally aragonite or low-Mg calcite; they may have been high-Mg calcite but the evidence is circumstantial. Their peculiar diagenetic fabric also has implications for the debate about the relationship between stromatoporoids and the concept of aragonite/calcite seas, which requires more work.