Archaeocyatha or
archaeocyathids ("ancient cups") is a taxon of extinct,
sessile,
reef-building
marine 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 is 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.