archaeocyath

An archaeocyath (plural archaeocyaths) is an extinct, calcified, cup- or cone-shaped marine organism that lived during the Early to Middle Cambrian. Archaeocyaths are among the earliest reef-building animals, commonly forming extensive skeletal frameworks in shallow seas. They are known primarily from their two-part, porous calcareous skeletons, which consist of an outer and an inner wall that together enclose a central cavity. The walls are perforated by a network of pores and canals that allowed water to circulate through the skeleton, a system analogous to sponge-like filter feeding, though exact affinities are debated.

Systematic placement has varied; archaeocyaths are traditionally regarded as sponge-like metazoans and are sometimes described as a separate, extinct phylum Archaeocyatha. However, their precise evolutionary relationships to modern sponges (Porifera) and other early animals remain unsettled, and some schemes treat archaeocyaths as a stem group to higher metazoans. The organisms were sessile and anchored to the seafloor, building reef-like structures in warm, shallow Cambrian seas.

Fossil evidence of archaeocyaths is widespread in Cambrian sequences worldwide, and their rapid diversification and subsequent decline help mark the early evolution of skeletal metazoans. Their study provides important clues about early Cambrian ecology, the origin of calcified skeletons, and the development of reef ecosystems during the Cambrian explosion.