A group of fossil invertebrates. They lived in the Cambrian period- on the beginning of the Paleozoic era. They are important as index fossil.

At the very beginning Archeocyatha were classified as reletad to sponges (type Spongiae). Secondly most scientist considered that differences between those organisms were very big and excluded direct relations, so they had no fixed place in taxonomy. But recently, as last scientific researches had shown, they have been recognized as a distinct group of sponges - lacking spicules.

They build calcium cup-shaped skeleton with thick wall. Probably skeleton delimit body of archeocyaths from outside as well as from inside.

Skeleton’s wall probably had a porous structure. Outer and inner skeleton walls were connected by radial dividing walls. Space between walls were sometimes filled additionally by bladders, rods, horizontal dividing walls. See drawing on the left.

Archeocyaths lived in seas and conducted sessile mode of life. There lived in aggregations creating reef-like structures.