Cemented shells of marine organisms form a type of sedimentary rock called limestone.

Here's why:

* Composition: Limestone is primarily composed of calcium carbonate (CaCO3).

* Shell Formation: The shells of many marine organisms, such as clams, snails, corals, and foraminifera, are made of calcium carbonate.

* Cementation: Over time, the shells accumulate on the ocean floor. As more sediment builds up, the pressure and chemical reactions within the sediment cause the shells to become cemented together, forming solid rock.

While limestone can also form from other sources like chemical precipitation, the presence of fossilized shells is a strong indicator of a biogenic (organism-derived) limestone formation.