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Archaeocyathids (Greek: "ancient cup"), members of the unranked clade Archaeocyatha, are a group of ancient sponges which existed for a geologically brief period of time but had a huge impact during their tenure. Emerging in the Lower Cambrian, about 530 million years ago, archaeocyathids disappeared just 10 million years later, about 520 million years ago. Archaeocyathid fossil species are divided into two classes, six orders, 12 suborders, 120 families and nearly 300 genera.
Like other sponges, archaeocyathids were stationary filter feeders that made up an important part of the marine fauna. However, archaeocyathids are so unusual that it took decades for scientists to generally agree that they are sponges, and there is still some controversy over their affinity. Occasionally, Archaeocyatha is improperly referred to as its own phylum. Instead of actively pumping water through themselves to extract nutrients, like sponges, archaeocyathids probably utilized passive flow.
As their name suggests, archaeocyathids were shaped roughly like cups, sometimes with just a single porous wall, but more often with two concentric porous walls. The characteristics of these walls are used to classify archaeocyathids and tell them apart. Like other sponges, archaeocyathids fastened themselves to the seabed with a holdfast. A few archaeocyathid fossils have been found which suggest that some species were flat, like pancakes.
Archaeocyathid fossils can be found in large numbers in Lower Cambrian strata, making up one of the most common fossils from the period. In a few million years, they diversified into hundreds of species of fascinating cup-like shapes, creating the planet's first reefs. These Cambrian reefs were the first of three major reef-building pulses in the history of life on Earth. Archaeocyathid reefs would have helped encourage evolution among the earliest Cambrian ecosystems, by providing places to hide from predators and creating a complex habitat.
Archaeocyathids generally lived in shallow tropical waters, in the photic zone where they could be assured of ample light. Archaeocyathid fossils often co-occur with fossils of cyanobacteria, which they would have been symbiotic with. By providing a relatively safe place for cyanobacterial colonies to live and reproduce, archaeocyathids could exact a tax on the bacteria by digesting a portion of them regularly.
It is unknown why archaeocyathids went extinct. It may have been due to competition from other filter feeders, the evolution of hyper-successful predators, or for some other reason. Archaeocyathids are the only major group of sponges with no living representatives.