Mummies, they’re called, these strange shapes that form one of the largest structures that ever existed on Earth.
Stretching some 2,900 kilometers from Spain to Romania, the long, sinuous curve of millions of mummies—once-living, vase-shaped animals—is a fossil reef. In its heyday in the Jurassic, the reef dwarfed today’s Great Barrier Reef on Australia’s northeast coast. Now visible only in outcrops dotted across a vast area of central and southern Spain, southwest Germany, central Poland, southeastern France, Switzerland, and eastern Romania near the Black Sea, the ancient reef was made up not of corals, but of deep-sea sponges called hexactinellids.
Hexactinellid, or glass, sponges use silica dissolved in seawater to manufacture a skeleton of four- or six-pointed siliceous spicules. Individual glass sponges, such as the beautiful Venus’ flower basket sponge (Euplectella aspergillum), are still found in the deep sea, but they are different species than the Jurassic reef-builders.
Reef-building glass sponges became extinct 100 million years ago, leaving evidence of their presence only in fossilized remains across Europe. The reef-builders were likely driven out by competition from newly arrived diatoms. Single-celled algae, diatoms also use the silica in seawater to build cell walls.
Diatoms, however, need the light of the sea’s euphotic zone and so don’t live in the deepest parts of the ocean. These nether regions were an open niche for reef-building glass sponges. But none was able to colonize the deep.
Or so it was thought.
“Nature had a few tricks up her sleeve,” said Sally Leys, a glass sponge biologist at the University of Alberta, Canada, “tricks that none of us could have imagined.”