Archaeocyatha
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The Archeocyatha, also called Archaeocyathids, were sessile, reef-building marine organisms that lived during the Lower Cambrian period (500-600 million years ago). All species being extinct, they are known only from fossils.
Archeocyatha resemble hollow horn corals. Each had a conical or vase-shaped skeleton of calcite similar to that of a sponge. The structure is something like a pair of perforated, nested ice cream cones. 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.
Archeocyata inhabited areas of shallow seas that were near the shoreline. Their widespread distribution over almost the entire world, as well as the diversity of the species, can be explained, among other things, by the fact that they were like plankton during their larval stage.
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 Archeocyatha. Archaeocyatha were important reef builders in their time. Flow tank experiments suggest that their morphology allowed them to exploit flow gradients to passively pump water through the skeleton, as in modern sponges.
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