FOSSIL INVERTEBRATES/Corals and Other Cnidaria 327 that Furthermore, the earliest scleractinians suggest several already divergent stocks, which became mineralized at about the same time, the origins of which recent DNA work on living descendants suggests can be traced back to the Carboniferous (Figure 5) Following severe extinctions at the end of the Triassic, scleractinians steadily diversified through the Mesozoic, with a peak of reef-building activity in the Late Jurassic and less significant losses at the end of that period The end-Cretaceous extinction event severely affected coral stocks, but from the survivors evolved the highly diverse and ecologically successful scleractinian faunas of the present day No other group of zoantharian corals is recognized after the end of the Palaeozoic Coral Structure and Taxonomy Figure Representative Cambrian and minor group corals (A C) Moorowipora chamberensis; Lower Cambrian, Moorowie For mation, Flinders Ranges, South Australia, Â5: (A, B) cross sections and (C) longitudinal section; photographs courtesy of Margaret Fuller (D) Cothonion sympomatum; early Middle Cam brian, Coonigan Formation, New South Wales, Australia, Â4.5; photograph courtesy of John Jell and reproduced by permission of the Australasian Association of Palaeontologists; figure in Jell PA and Jell JS (1976) Early Middle Cambrian corals from western New South Wales Alcheringa 1: 181 195 (E) Kilbucho phyllia discoidea, mould of a juvenile specimen showing clear cyclic septal insertion; Ordovician, Caradoc Series, Kirkcolm Formation, Southern Uplands, Scotland, UK, Â3.5 (F) Heterophyl lia grandis, cross section showing distinctive heterocoral septal pattern; Carboniferous, Dinantian, Northumberland, England, UK; Â5 (G) Heterophyllia ornata, scattered specimens on a bedding plane; note the spines on the edges of the corallite top right; Carboniferous, Dinantian, Northumberland, England, UK; Â1 conclusion: the lack of Early Triassic corals; a change in skeletal mineralogy from calcite (Rugosa) to aragonite (Scleractinia); contrasting patterns of septal insertion between the two orders; and proof of the antiquity of the scleractinian septal-insertion pattern with the discovery of Kilbuchophyllia in the Ordovician This implies that the ancestral anemone stock survived from the Palaeozoic, and the Early Triassic trace fossil Dolopichnus may be proof of Coral taxonomy and classification is based principally on the skeletal characteristics of the corallum, the skeletal microstructure, and growth structure and form Coral skeletons are often highly variable in internal structure and growth form In addition, similar structures often reoccur, and it can be difficult to separate genetic relationships from convergent homoeomorphy Given that coral structure is relatively simple to start with, the result is that considerable problems exist with both taxonomy and classification within the major orders Many species were erected in the past with little or no regard to intraspecific variation Only in recent years have more serious attempts been made to take variation into account, with the result that many species and genera are now being placed in synonymy Rather more stability is apparent in the grouping of genera at the family level, at least in Palaeozoic corals Here the Treatise on Invertebrate Paleontology and its revision have been very influential, although now much modified by more recent work However, evolutionary relationships within the Rugosa particularly are rather poorly understood The Treatise has also been a major influence in Scleractinian classification, but here major revisions are being made as a result of more recent studies of better-preserved material, particularly from faunas in which the original aragonitic mineralogy has been preserved, allowing septal microstructure to contribute to unravelling relationships Microstructure has yet to play a very significant role in classifying Palaeozoic corals Their calcitic skeletons contain up to mol% MgCO3, and there is great controversy over which textures are original and which are secondary as a result of recrystallization The Kilbuchophyllida are solitary corals, whilst the Heterocorallia, the Rugosa and the Scleractinia