Calathium

Calathium is an extinct genus of organism found in marine beds of Ordovician age. Its classification is enigmatic: It has long been placed among the receptaculites,^[1][2] but it has also been described as a quasi-sponge,^[3] possibly akin to the archeocyathids^[4] or other hypercalcified sponge.^[5] The chief difference from archaeocyathids is that their walls were connected by rods rather than septae.^[4]

Calathium
Calathium formosum
Scientific classification
Kingdom:	Animalia
Phylum:	Porifera (?)
Genus:	†Calathium Billings 1865

The organisms were important reef-forming organisms during the Ordovician, forming communities with lithistid sponges that gradually displaced the earlier microbial mounds.^[4]