Sponges are one of the simplest animals. Most sponges found as fossils lived in the sea but modern sponges can also be found in fresh water. Sponges filter food from the water around them. Like sea urchins they are particularly common in the Cretaceous chalk.

 Sometimes hollow flint nodules can contain a sponge. The flint itself was from the silica spicules that make up a sponges skeleton. They formed a silica ooze under the mud of the sea floor. This ooze sometimes enclosed the remains of sea creatures or filled their empty shells. This is how flint casts of sea urchins and shells were formed.

	Fossil sponge encased in flint
	This sea urchin has been encased in flint. The flint has preserved the impression of the sea urchins shell long after the original shell has dissolved away. The flint is formed from the remains of sponges
	Cretaceous sponge

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