[1] These were the earliest of brachiopods, ranging from the Cambrian into the Holocene.

They rapidly diversified during the Cambrian into the Ordovician, but most families became extinct by the end of the Devonian.

The pedicle is a hollow extension of the ventral body wall.

Posterior body wall separates dorsal and ventral mantles.

The shells are usually made up of apatite (calcium phosphate), however rare cases have calcite or aragonite shells.