Here's why:
* Biogenic means "originating from living organisms."
* Limestone is primarily composed of calcium carbonate (CaCO3), which is often formed from the skeletal remains of marine organisms like corals, shellfish, and algae.
* The presence of fossils directly indicates that the limestone was formed from the accumulation and cementation of these organic remains.
While limestone can also be classified as a chemical sedimentary rock (due to the precipitation of calcium carbonate from solution), the presence of fossils makes it primarily a biogenic rock.