Soft and luxurious, many sponges have a basket-work skeleton of tiny struts and spikes. Adult sponges can change shape to fit the base to which they are attached.

Sponges are very common both alive today and as fossils, as their intricate hard skeletons fossilise well. They have existed on Earth for about 430 million years and are among the most primitive multicellular organisms.

The shapes of fossil sponges range from conical to mushroom-like to tulip-like to flannel-like but they usually have the same rough, porous surface.

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