Sea urchin fossil. Fossilised sea urchin (class: Echinoidea) embedded in rock. The sea urchin's spines, which had a protective function, can be clearly made out. Sea urchins are marine invertebrates which live on the sea bed where they feed off plants and smaller animals. The first sea urchins appeared in the Ordovician era (505-400 million years ago). This fossil was found in lias rock which consists of clays, sandstones and limestones laid down in the early Jurassic period (195-172 million years ago). It is part of the Martin Rigby collection.