Corals and other reef-builders

Abstract

Reefs and reef-building organisms have been a key part of marine ecosystems throughout most of the Phanerozoic. The present issue of Lethaia includes several papers on corals and other reef-building organisms, which together demonstrate the value of these groups in palaeoecology and particularly in understanding the influence that environmental and other factors had on colonial organisms at a wide range of scales.As part of our Lethaia Review series, Seilacher & Thomas (2012) discuss the range of adaptations to living on soft substrates shown by favositid corals. In a very clearly illustrated article, they argue that various growth habits were achieved by self-organization of the colonies. This includes the secretion of 'suicidal lids' by polyps at the margins of colonies, entombing themselves but producing a secondary epitheca that benefitted the colony as a whole as it subsided into the sediment. The paper draws comparisons with growth patterns in other tabulates and some members of the Rugosa, and thus provides an extremely useful 'way in' to the literature on soft substrate adaptations in Palaeozoic corals.There was a major and long-lasting switch from microbially dominated to skeletally dominated reefs at the end of the Middle Ordovician as part of the Great Ordovician Biodiversification Event. However, metazoan framework reefs did occur much earlier in the Ordovician and Adachi et al. (2012) describe the construction of the earliest known of these, the Lower Ordovician (upper Tremadocian) bryozoan reefs of the Three Gorges area of South China. They show that lithistid sponges were an essential component of these reefs, providing the substrate for the attachment of the bryozoans. The bryozoan colonies grew outwards and, surprisingly, downwards from this 'scaffolding' and subsequently encrusted each other to form a rigid three-dimensional skeletal framework. This mechanism for the formation of reefs by skeletal organisms was short-lived and the authors speculate that it was only possible because ecological pressures were relatively weak at this early stage of the Ordovician Biodiversification.From an anthropomorphic point of view, the description of 'lucky' rugose corals by Berkowski & Klug (2012) provides something of a contrast to the self-sacrificing coral polyps documented by Seilacher & Thomas. The 'lucky' corals in question are from the Devonian of Morocco and were epizoans that grew on the stems of crinoids. Berkowski & Klug show that this encrustation certainly occurred during the life of the crinoids, as there is clear evidence that the latter grew around the corals as they themselves grew. The advantages of living elevated above the seafloor are discussed and the authors pose the question as to whether this attachment to living crinoids occurred by chance, or whether it was an ecological strategy adopted by the corals.In contrast to corals acting as epibionts, Kołodziej et al. (2012) describe an Early Cretaceous example of microborings that were made in living corals. This is the first well-document...