Fool’s gold fossils
Beautiful coiled ammonite fossils are often found in the Lias limestone, preserved in the mineral fool’s gold (the mineral iron pyrites), so called for its resemblance to gold. This tells us a lot about the conditions on the seabed where the ammonite died. Pyrite forms when there is a lot of organic carbon and not much oxygen in vicinity. The reason for this is that although bacteria in the sediment usually respire aerobically (using oxygen) but if there is no oxygen, they manage to respire without oxygen (anaerobic), typically using sulphate, a substance common in the sediment. High quantities of organic carbon in the sediment form a barrier to oxygen in the water, also encouraging anaerobic respiration. Anaerobic respiration using sulphate releases hydrogen sulphide, which is one of the major components used to make pyrite. So, the fool’s gold ammonites must have died on sediment with low quantities of oxygen and high quantities of organic carbon. Pyrite may even help preserve fossils because it often forms nodules, protective lumps, around the fossil.
One particularly intriguing characteristic of many fossil skeletons is that whilst some parts of the skeleton are well preserved, with all the bones articulated in their natural relationships, other parts in the same skeleton are often less well preserved, with the bones scattered. This is particularly common in the skeletons of extinct fish-shaped reptiles known as ichthyosaurs. The explanation for this inconsistent preservation in single animals is that the seabed was soupy – a watery mixture of fine particles.
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