Which classification includes organisms with shells made of calcium carbonate?

Foraminiferans are a group of protists classified under the supergroup Rhizaria, and they are well-known for their intricate shells, which are primarily composed of calcium carbonate. These shells, or tests, are highly diverse in form and structure, which can be important for identifying species and studying their fossil record. The calcium carbonate in their shells contributes to the composition of marine sediments, having significant ecological and geological implications.

This characteristic makes foraminiferans a key group in paleoclimatology and biostratigraphy, as their presence in sediment layers can provide insights into past environmental conditions. The calcium carbonate structure is not found in the other groups listed; for example, Alveolates include a variety of organisms such as ciliates and dinoflagellates, which do not typically possess calcium carbonate shells. Cyanobacteria are photosynthetic bacteria without shells, and Excavata encompasses a diverse group of protists that are often characterized by their unique cell structures, none of which involve calcium carbonate shells.