The role of Archaeocyatha in Cambrian biostratigraphy and biogeography
The Archaeocyatha is a group of Cambrian fossils successively considered as cnidarians or sponges or as an independent phylum convergent with many groups lacking clear affinities. Comparisons with Recent calcified sponges discovered in submarine caves have demonstrated that the sponge model is consistent with their structural organization. Thus their systematic position is now agreed as a class within the phylum Porifera, permitting realistic investigations of their comparative physiology and life strategies. Archaeocyatha is an important part of research programmes on the Cambrian System, initiated by different commissions of the IUGS since 1970. Archaeocyathan biozones are available in some key regions. Faunal and palaeocommunities distribution, especially of the reefs they helped build in epeiric seas, and migration pathways constrain Cambrian palaeogeographical reconstructions. A database, using recent compilations of the group, is now online. This free access data source offers specialists a tool, easy to use not only as an identification key but also to establish faunal, geographical and stratigraphical distributions of archaeocyathan genera, and a rapid first step towards Cambrian palaeogeographical reconstructions.
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The Early Palaeozoic was a critical interval in the evolution of marine life on our planet. Through a window of some 120 million years, the Cambrian Explosion, Great Ordovician Biodiversification Event, End Ordovician Extinction and the subsequent Silurian Recovery established a steep trajectory of increasing marine biodiversity that started in the Late Proterozoic and continued into the Devonian. Biogeography is a key property of virtually all organisms; their distributional ranges, mapped out on a mosaic of changing palaeogeography, have played important roles in modulating the diversity and evolution of marine life. This Memoir first introduces the content, some of the concepts involved in describing and interpreting palaeobiogeography, and the changing Early Palaeozoic geography is illustrated through a series of time slices. The subsequent 26 chapters, compiled by some 130 authors from over 20 countries, describe and analyse distributional and in many cases diversity data for all the major biotic groups plotted on current palaeogeographic maps. Nearly a quarter of a century after the publication of the ‘Green Book’ (Geological Society, London, Memoir 12, edited by McKerrow and Scotese), improved stratigraphic and taxonomic data together with more accurate, digitized palaeogeographic maps, have confirmed the central role of palaeobiogeography in understanding the evolution of Early Palaeozoic ecosystems and their biotas.