Why are echinoderm fossils often easy to recognize from their skeletal fragments?

Echinoderm fossils are often easy to recognize from their skeletal fragments because no other animal group possesses the same type of skeleton.1

The echinoderm skeleton is composed of individual units called ossicles, which consist of a complex, three-dimensional lattice known as a stereom. This structure is made of mineral calcite—a form of calcium carbonate—derived from living tissue. In fossil strata, the original living tissue (stroma) within these ossicles is often replaced by secondary calcite, which is laid down in continuity with the original skeletal material. This unique mineral composition and the distinct latticed structure of the ossicles allow paleontologists to identify echinoderm fragments with high confidence, as they bear no resemblance to the internal structure of bone.12