Brachiopoda -

Lack complex hinges, held together by muscles.

Possess a toothed hinge for opening shells, making them the most common fossils.

Composed of calcium carbonate (articulates) or calcium phosphate and chitin (inarticulates). brachiopoda

A stalk extending from the ventral valve used for attaching to substrate.

While considered a "minor" phylum today, they were dominant in Paleozoic marine ecosystems and are invaluable as index fossils . Lack complex hinges, held together by muscles

The Treatise on Invertebrate Paleontology (Part H) is the primary reference. If you'd like to refine this further, let me know:

Unlike bivalve mollusks (clams), brachiopod shells are symmetrical across a vertical plane (between the valves) rather than a horizontal plane. Lack complex hinges

Survived all five mass extinctions.