Archaeocyathid
| Archaeocyatha Temporal range: Early — Late Cambrian |
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| Scientific classification | |
| Kingdom: | Animalia |
| Subkingdom: | Parazoa |
| Phylum: | Porifera? (sponges) |
| (unranked): |
Archaeocyatha Vologdin, 1937 |
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Archaeocyatha (or archaeocyathids ancient cups" /ˈɑːrkioʊsaɪəθə/) is a taxon of extinct, sessile, reef-buildingmarine organisms of warm tropical and subtropical waters that lived during the early (lower) Cambrian Period. It is believed that the centre of the Archaeocyatha origin are now located in East Siberia, where they are first known from the beginning of the Tommotian Age of the Cambrian, 525 million years ago (mya). In other regions of the world, they appeared much later, during the Atdabanian, and quickly diversified into over a hundred families. They became the planet's very first reef-building animals and are an index fossil for the Lower Cambrian worldwide.
The remains of Archaeocyatha are mostly preserved as carbonate structures in a limestone matrix. This means that the fossils cannot be chemically or mechanically isolated, save for some specimens that have already eroded out of their matrices, and their morphology has to be determined from thin cuts of the stone in which they were preserved.
Today, the archaeocyathan families are recognizable by small but consistent differences in their fossilized structures: Some archaeocyathans were built like nested bowls, while others were as long as 300mm. Some archaeocyaths were solitary organisms, while others formed colonies. In the beginning of the Toyonian Age around 516 mya, the archaeocyaths went into a sharp decline. Almost all species became extinct by the Middle Cambrian, with the final-known species, Antarcticocyathus webberi, disappearing just prior to the end of the Cambrian period. Their rapid decline and disappearance coincided with a rapid diversification of the Demosponges.
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