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An Archeocyathid was a simple, reef-building, marine organism that lived during the Cambrian period (500-600 million years ago). They are known only from fossils. Archeocyathids had a conical or vase-shaped skeleton of calcite similar to that of a sponge. Their skeletons consist of either a single porous wall (Monocyathida), or more commonly as two concentric porous walls, an inner and outer wall separated by a space. Inside the inner wall was a cavity (like the inside of an empty ice cream cone). At the base, they were held to substrate with holdfast.
Though they have a long history of phylogenetic uncertainty and changing interpretations, consesus now has it that they were indeed a kind of sponge. Still, some authorities have placed them in the extinct phylum Archeocyathida.
Fossils
