Worm track fossils. Fossilised tracks (traces) of an ancient worm, preserved in rock from the Kimmeridgian age (157 to 152 million years ago). A hammer gives approximate scale. A trace is a feature caused by an organism whilst it is alive and is the result of behaviours such as moving or feeding. Traces can be preserved in rocks, as fossils, in the same way as organisms. The study of trace fossils is known as palaeoichnology. It provides useful information about the conditions which prevailed in ancient environments, including oxygen levels and salinity. Photographed in Portugal.
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