Which unicellular organism forms calcium carbonate shells and is widely used as a proxy in paleontology?

• A. Angiosperm
• B. Blade
• C. Foraminifera
• D. Chlorophyta

Answer: (C)

Foraminifera are single-celled protists that build shells of calcium carbonate. These tiny calcareous tests preserve well in ocean sediments, creating rich fossil records. Because the species present and the chemical signatures in their shells (like oxygen isotopes) reflect past water temperature, depth, and productivity, they’re extensively used to reconstruct ancient ocean conditions in paleontology and paleoceanography. The other options don’t form widespread calcareous shells used as paleoproxies: angiosperms are flowering plants, blades are parts of larger algae, and green algae (Chlorophyta) are typically not known for calcite shells used in these reconstructions.