Exceptional Preservation in the Upper Carboniferous Coseley Lagerstätte
The Late Carboniferous Coseley Lagerstätte of the West Midlands, UK, contains exceptionally preserved plant and animal fossils that occur as hard parts and mineralised soft tissues sealed within siderite nodules, which vary in size from 15–250 mm. The nodules are recovered from Westphalian B siltstones and mudstones that lie above the Thick Coal of the Coal Measure Group, and were abundant enough in these horizons to form commercial ironstone beds in the nineteenth century.
The Coseley fossils have been examined using various techniques, including scanning electron microscopy and electron microprobe analysis, that have revealed several distinctive phases of preservation. Soft tissues have been replicated by kaolinite and pyrite, voids have been extensively filled with sulphide minerals, and all of these phases are encased in siderite. The growth of clay minerals on the surface of decaying soft tissues could have been controlled either by purely inorganic processes or through bacterially mediated biofilms. This growth was accompanied by early framboidal pyrite formation and closely followed by void-filling sphalerite, galena and pyrite precipitation. Siderite formation occurred both during and shortly after soft tissue preservation, producing a nodule that inhibited compaction of the tissues.