Crinoids are marine animals belonging to the phylum Echinodermata and the class Crinoidea. They are an ancient fossil group that first appeared in the seas of the mid Cambrian, about 300 million years before dinosaurs. They flourished in the Palaeozoic and Mesozoic eras 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.
The animal
An array of branching arms (brachia) 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.
The skeleton is made of the mineral calcite and consists of hundreds of individual plates of different shapes and sizes. Decay of the soft tissue that held many of these plates together means that complete specimens are rare, but parts of the stem are common fossils.
The geologists’ tool
Fossil crinoids indicate that the rocks containing their remains were formed in a marine environment and, where abundant in Palaeozoic rocks, they suggest the former existence of shallow water conditions. In the early Carboniferous, their rich remains (particularly stem fragments) were solidified into rock called crinoidal limestone. Rare occurrences of complete fossilised crinoids indicate rapid burial in quiet, possibly poorly oxygenated waters.
Occasionally, crinoids can be a useful guide to the age of the rocks in which they occur. This is the case in the strata the late Cretaceous the Chalk Group, which form the famous White Cliffs of Dover. Species of Uintacrinus, Marsupites, and Applinocrinus are so abundant over four narrow intervals in the Chalk that they have been used to define biozones and subbiozones.
Crinoids through time
Crinoids are common fossils in the Silurian rocks of Shropshire, the early Carboniferous rocks of Derbyshire and Yorkshire and the Jurassic rocks of the Dorset and Yorkshire coasts.
