SUMMARY Miocidaris keyserlingi is found in the Upper Permian Ford Formation, Zechstein Cycle 1, of the Sunderland district. It is confined to reef core and fore-reef talus slope facies and is absent from higher energy reef-crest and reef-flat habitats. Well preserved material permits the description of stereom microstructure and tooth construction. M. keyserlingi has teeth identical in structure to those of Recent cidarids and this contrasts with tooth structure in both the Carboniferous archaeocidarids and the early Jurassic euechinoids. Hemipyramid morphology and perignathic girdle structure also demonstrate the close relationship between M. keyserlingi and Recent cidaroids. Euechinoids and cidaroids must have diverged prior to the Upper Permian and at least two separate lineages of echinoids passed across the Permo-Triassic boundary.

ABSTRACT. An exceptionally well-preserved fauna from a new exposure in the Peterborough Member (Oxford Clay Formation) of southern England, equivalent to the famous and now inaccessible Lagersta Ètte at Christian Malford, Wiltshire, is described. It comes from a single bed and includes coleoid cephalopods with phosphatized soft tissues, and fully articulated ®sh. The level is unusual in that it lacks permanent benthos and is dominated by`rain-out' from the upper water column. It was deposited on a sea¯oor that experienced prolonged periods of anoxia and which was overlain by a water column that was at least intermittently strati®ed. It is postulated that the coleoids formed large shoals that were killed en masse, together with other elements of the associated fauna, in one or more catastrophic mass mortality events that affected a signi®cant area of the Peterborough Member sea. During the event(s), many of the coleoids preyed upon moribund ®sh and other coleoids, sometimes of the same species, before becoming overcome themselves. Crucially for the phosphatization of their soft tissues, the substrate surface was`soupy' and allowed them to sink to a zone of rapid diagenesis where levels of dissolved phosphorus were greatly augmented by the large number of associated decaying carcasses.

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Septarian concretions occur at several horizons within the Oxford Clay Formation, a marine mudstone containing pristine aragonite and immature biomarker molecules. They record the passage of at least four generations of pore fluids, the first of marine origin and the last still present in cavities. Concretion bodies formed, cracked, and calcite and pyrite precipitated in and around the cracks within the sulphate reduction zone, as demonstrated by C, O, S and Sr isotopic composition (Pore fluid 1). Before major compaction, sandstone dykes were intruded locally, and baryte precipitated, followed by coarse calcite cements with isotopically light oxygen and radiogenic strontium, indicating the introduction of meteoric‐derived water (Pore fluid 2). Later, coarse celestine within concretions has distinct sulphur‐isotopic composition and requires a further, geographically restricted, water source (Pore fluid 3). Celestine‐bearing concretions contain water in tight cavities whose isotopic composition is close to that of modern precipitation. Its chemistry shows that it is equilibrating with pre‐existing minerals implying a relatively recent origin (Pore fluid 4). The mineralogy of the Oxford Clay concretions shows that complex results can follow from a simple burial and uplift history, and that multiple generations of pore fluids can pass through a low‐permeability clay.

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