Understanding Palaeozoic stromatoporoid growth

Abstract

Stromatoporoids were abundant components of reefs, reef complexes and associated facies for ca. 100 million years between Middle Ordovician and end-Devonian time. A lot of environmental information stored in their skeletons may be used to develop: a) understanding of stromatoporoid growth controls; and b) interpretations of sedimentary environments in which they lived. General patterns of stromatoporoid distribution are well known, but knowledge of detailed interactions between stromatoporoids and their environments is poorly developed and under-used, potentially of great value in analysis of facies and palaeoecology. From a pool of several thousand specimens examined over four decades, this study identifies four key attributes of stromatoporoids that may be applied to enhance broad-scale knowledge of these fossils and their environments: 1) Substrates: Most stromatoporoids grew directly on wackestone to packstone substrates comprising micrite, clay and bioclasts. Evidence from the relationship between stromatoporoids and sediments demonstrates they were able to grow on soft substrates, but also leads to interpretation of partial lithification of the sea floor (and/or stabilisation by microbial filaments that may not be preserved) in mid-Palaeozoic carbonate facies, with potential implications for models of oceanic carbonate cycling. 2) Growth interruption: Almost all stromatoporoids examined show growth interruption, mostly caused by sedimentation and movement. Stromatoporoids normally recovered quickly and completely from interruption, thus were resilient to interruption events; 3) Associated organisms: Stromatoporoids have abundant associated organisms in two groups: (i) epibiotic encrusters and borers; and (ii) endobiotic organisms embedded in their structure, alive as the stromatoporoids grew. Epibionts used stromatoporoid surfaces that are presumed dead in almost all cases; some are associated with interruption events, but in most cases those were overgrown by successive stromatoporoid growth. Endobionts (mostly corals, plus spirorbids and others) are common to abundant in many stromatoporoid taxa. Stromatoporoid growth was little affected by presence of endobionts but in many cases (commonly restricted to certain stromatoporoid taxa) there was a complex biological interaction valuable in understanding controls on stromatoporoid development. 4) Growth form and taxonomy: Stromatoporoid assemblages are low diversity in almost all cases, regardless of age and facies, with two or three taxa much more abundant than the others. Some stromatoporoid taxa are limited to certain growth forms, thus taxonomic information is very important for facies analysis and palaeobiological interpretations. Stromatoporoids occur commonly with rugose and tabulate corals, both of which could also live on unconsolidated fine-grained substrates, therefore sediment-tolerance cannot be the only reason for stromatoporoid ability to outgrow corals in reefs. Arguments in the literature for photosymbiosis in both stromatoporoids and Palaeozoic corals are currently inconclusive for both fossil groups. Nevertheless, the sum of evidence indicates stromatoporoids were fast-growing, resilient and flexible benthic organisms, in a range of water depths, capable of dealing with soft substrates and largely unaffected by presence of endobionts. These characteristics are interpreted to have made stromatoporoids successful during mid-Palaeozoic time and valuable as tools in facies analysis. Recognition of the four key attributes which encompass all aspects of stromatoporoid growth controls makes palaeobiological study of stromatoporoids, in palaeoenvironmental reconstructions where they occur, readily accessible to researchers.