Crinoidal limestone

Crinoids are an ancient fossil group that first appeared in the seas of the Middle Cambrian, about 300 million years before dinosaurs. They flourished in the Palaeozoic and Mesozoic, and some survive to the present day. Although sometimes different in appearance from their fossil ancestors, living forms provide clues about how fossil crinoids must have lived.

Crinoids are marine animals belonging to the phylum Echinodermata and the class Crinoidea. An array of branching arms (brachials) is arranged around the top of a globe-shaped, cup-like structure (calyx) containing the main body of the animal. In many fossil forms the calyx was attached to a flexible stem that was anchored to the sea bed.

Anatomy and feeding psoition of a stemmed crinoid
'Star stones' columnals from crinoid stems

Crinoids are sometimes referred to as sea lilies because of their resemblance to a plant or flower. In parts of England, the columnals forming the stem are called fairy money, and star-shaped examples of these were associated with the sun by ancient peoples, and given religious significance.

Polished slabs of crinoidal limestone make attractive ornamental stone. In Derbyshire the limestone sometimes contains internal moulds of crinoid stem fragments, which have a distinctive screwlike thread pattern and have been called screwstones.
Polished slab of crinoidal limestone