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SPONGES What are sponges? Sponges are ancient animals that mostly live in the sea. They have a very simple body structure made up of groups, or colonies, of cells. You can find fossil sponges on beaches where most of the pebbles are made of flint. Look out for flint pebbles with patterns of holes. Quite often you can see the whole shape of the colony. The image above shows a piece of flint broken open to reveal the fossil sponge Ventriculites inside. Although sponges have soft bodies, they fossilise very well because they are made up of tiny pieces of calcium or silica joined together. When you pick up a piece of flint you are touching the recycled remains of the silica that the sponge was built from. Good places to find fossil sponges: pebbly beaches, southern and eastern England Age of rocks: 145 to 65 million years old (Cretaceous Period) http://www.nhm.ac.uk/nature-online/earth/fossils/fossil-folklore/themes/decoration06.htm http://www.nhm.ac.uk/kids-only/earth-space/fantastic-fossils/ Glossary - what does that word mean? Calcium - the substance that makes our bones and teeth strong. Other animals also use it to make their hard parts. Flint - a rock that sometimes contains fossils of animals or plants that lived in the sea. Flint is usually dark grey or brown but it often occurs in round lumps with a thin, white outer layer. Silica - a very hard substance. It's what sand is usually made of, and some living things use it in their hard parts.