Hexactinellids, or glass sponges, use silica dissolved in seawater to manufacture a skeleton of four- or six-pointed siliceous spicules. The ancient reef was made up not of corals but of deep-sea sponges called hexactinellids. Now it is visible only in rock outcrops dotted across a vast area of central and southern Spain, southwestern Germany, central Poland, southeastern France, Switzerland, and eastern Romania near the Black Sea. In its heyday in the Jurassic, the reef dwarfed today's Great Barrier Reef off Australia's northeastern coast. Stretching some 2900 kilometers from Spain to Romania, the long, sinuous curve of millions of mummies-once-living, vase-shaped animals-is a fossil reef.
Mummies, they're called, these strange shapes that form one of the largest structures ever to exist on Earth.
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