Summary 1. Living echinoderms are characterized by an extensive water vascular system developed from the larval left hydrocoel, a complex, multi‐plated endoskeleton with stereom structure, and pentamery. Fossil evidence shows that stereom evolved before pentamery, but both were acquired during the Lower Cambrian. 2. Cladistic analysis of Lower Cambrian genera reveals very few characters in common between carpoids and true echinoderms, and that the split between them was the first fundamental evolutionary dichotomy within the Dexiothetica. 3. Helicoplacoids are stem group echinoderms with spiral plating and three ambulacra arranged radially around a lateral mouth. They are the most primitive echinoderms and the first to show a radial arrangement of the water vascular and ambulacral systems. Unlike later echinoderms, their skeleton shows no dorsal/ventral (aboral/oral) differentiation. They were probably sedentary suspension feeders. 4. Camptostroma is the most primitive known pentaradiate echinoderm and, in our view, possibly a common ancestor of all living groups. It had a short conical dorsal (aboral) surface with imbricate plating, a ridged lateral wall and a slightly domed ventral (oral) surface with five curved ambulacra in a 2‐1‐2 arrangement inherited from the triradiate pattern of the helicoplacoids. Interambulacral areas bore epispires and the CD interambulacrum contained the anus, hydropore and/or gonopore. All parts of the theca had plates in at least two layers. 5. All other echinoderms belong to one of two monophyletic subphyla, the Pelmatozoa and the Eleutherozoa. 6. Stromatocystites is the earliest known eleutherozoan and differs from Camptostroma in having a test with only one layer of plates and having lost the dorsal elongation. In Stromatocystites the dorsal surface is flat and the plating tesselate. Stromatocystites was an unattached, low‐level suspension feeder. 7. The lepidocystoids are the earliest known pelmatozoans. They differ from Camptostroma in having an attached dorsal stalk which retained the primitive imbricate plating, and by developing erect feeding structures along the ambulacra. In Kinzercystis, the ambulacra are confined to the thecal surface and erect, biserial brachioles arise alternately on either side. Lepidocystis has a similar arrangement except that, the distal part of each ambulacrum extends beyond the edge of the theca as a free arm. 8. Pelmatozoans diverged more or less immediately into crinoids, with multiple free arms composed of uniserial plates, and cystoids sensu lato, which retained brachioles. Gogia (Lower to Middle Cambrian) is the most primitive known cystoid and differs from Kinzercystis principally in having all plating tesselate, while Echmatocrinus (Middle Cambrian) is the most primitive known crinoid and differs from Lepidocystis in lacking brachioles and in having more than five free arms with uniserial plates. 9. Post Lower Cambrian differentiation of pelmatozoan groups proceeded rapidly, exploiting the primitive suspension‐feeding mode of life. Maximu...

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The carbon isotope (613C) stratigraphy of the late Lower Cenomanian to early Lower Turonian is presented for three sections (Folkestone in the Anglo-Paris Basin, Wfinstorf in the Lower Saxony Basin and Speeton in the Cleveland Basin). The similarity between these isotope curves suggests that they were controlled by synchronous, global processes and can be used for high resolution correlation. Furthermore, sequence stratigraphic analysis of this interval reveals that each isotope excursion is associated with a sequence boundary and/or onlap surface. This is also demonstrated for the whole of the Cenomanian for the section at Speeton. We show that most (if not all) 6~3C excursions are synchronous within the limits of current stratigraphic resolution.We interpret the increase in background 613C values as representing an increase in the area of ocean floor (specifically continental shelf) available for burial of marine organic carbon caused by the mid-Cretaceous rise in eustatic sea-level. Thus background 613C values may provide an independent method for estimating past eustatic sea-levels. We interpret the sharp 613C excursions as reflecting more rapid changes in the carbon cycle particularly the rate of burial of organic carbon within sediments, and/or of storage in deep and intermediate water masses, at times of rapidly changing sea-level. Carbon excursions may be useful in locating sequence boundaries when other criteria are obscure or lacking.

Almost all known cyclocystoid specimens have been examined and the class revised. Cyclocystoids are very distinctive and consist of a complexly plated disc surrounded by a marginal ring of stout, perforate, cupule-bearing ossicles, in turn encircled by a narrow, flexible, plated peripheral skirt. The disc consists of: (1) a ventral surface with branched rays bearing irregular, immovable cover plates over narrow channels and unbranched interrays (except in Actinodiscus nov.); both uniserial and composed of imbricate plates; and (2) a dorsal surface usually covered by polygonal, annular plates. Sutural pores occur between radial and interradial plates ventrally and align exactly with the central perforations of the dorsal annular plates. A ventral mouth and dorsal anus occur subcentrally in the disc. The marginal ring consists of large ossicles each with a ventral crest and one to seven cupules. Marginals articulated with each other laterally and were perforated by two types of canal. The peripheral skirt consists of larger frontal plates, one per cupule, and smaller imbricating roofing plates. It was flexible and could cover the entire cupule zone of the marginal ring. The cyclocystoid test resembled a tambourine, not a drum. Space for internal anatomy was extremely limited. The gut was straight and very short. Microphagous organic feeding was the only plausible method. We believe that food was gathered in the cupules, passed through the radial ducts of the marginals into covered radial channels of the disc and thence to the mouth. Locomotory tube feet were housed in the ventral sutural pores. We believe that cyclocystoids lived with the oral surface below, were mobile and gathered food from the sediment surface. The test grew in a complex fashion. Disc elements were added peripherally throughout growth. Marginal ossicles were added very quickly early in growth and the number remained fairly stable thereafter, but cupules and radial ducts continued to be added so that the number per ossicle increased. Cyclocystoids appeared in the Middle Ordovician of North America and are last recorded from the Middle Devonian of Europe. They were originally relatively rare but much more diverse than previously suspected. Analysis of gaps suggests that the fossil record of cyclocystoids at generic level is at least 36% incomplete. The class consists of a single family, the Cyclocystoididae, characterized by cupulebearing, perforate marginals and branched radii. Species with imperforate marginals (including the Middle Cambrian ‘ C .’ primotica Henderson & Shergold) are removed from the class. As restricted, the family includes the following genera: Cyclocystoides Salter & Billings, 1858; Narrawayella Foerste, 1920; Actinodiscus nov.; Apycnodiscus nov.; Diastocycloides nov.; Polytryphocycloides nov.; Sievertsia nov. and Zygocycloides nov. Genera are distinguished on the marginal ossicles (which may be in contact or separated dorsally and may or may not have cupular tubercles) and on the presence or absence of interradii or dorsal interseptal plates. We accept 25 species, plus five left under open nomenclature. New species are Cyclocystoides latus, C. scammaphoris, C. tholicos, Diastocycloides stauromorphos, Polytryphocycloides grandis, Sievertsia , Zygocycloides marstoni (Salter ms.) and Z. variabilis .

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New interpretations of morphology, new occurrences and new taxa of British Silurian cystoids are summarized. Two ambulacral patterns occur in the Callocystitidae. In one the first two brachiole facets branch to the left in ambulacra B and D, but only the first one in ambulacra A, C and E (the B þ D arrangement). In the alternative pattern all ambulacra are the same with only the first brachiole branching to the left. The B þ D pattern is plesiomorphic. Among callocystitids with four ambulacra the type species of Tetracystis has the B þ D arrangement as does 'Apiocystites' elegans; both are referred to Tetracystis. The British species 'T.' oblongus has all ambulacra the same and becomes type species of the new genus Troosticystis. New records include Homocystites brenchleyi sp. nov. from the Rhuddanian of South Wales, which demonstrates that the family Cheirocrinidae did not become extinct at the end of the Ordovician as previously suspected. The British Silurian cystoid Prunocystites fletcheri has been recorded from Norway, the only species known to occur outside Britain. Schizocystis armata (Forbes, 1848) occurs at a new locality near Buildwas, Shropshire. Glansicystis glans sp. nov. is described for 'the common form' of G. baccata. It differs from G. baccata in having a larger oral area, weaker thecal ornament, shallower plate sutures and a more cylindrical theca.