Archaeocyathi

a type of extinct invertebrate that lived in the seas of the early Cambrian period at a depth of up to 100 m. The Archaeocyathi had a calciferous, bowl- or horn-shaped, porous shell averaging 5–10 cm in length and 1.5 to 3 cm in diameter (but occasionally reaching 50 cm). More than 1,000 species are known; they are found in all parts of the globe. In the USSR they are found in the Urals, Siberia, and the Far East. They usually occur singly, more rarely in colonies. They led a sessile life, sometimes forming reeflike masses. Archaeocyathi are the predominant fossils in Lower Cambrian sedimentation.