GEOLOGY | FOSSILS | key fossil groups
Crinoids (cry-noyd) are marine organisms of the phylum Echinodermata and the class Crinoidea. They are an ancient group that first appeared in the seas of the mid Cambrian. They were abundant and diverse in the Palaeozoic and Mesozoic eras. Only 600 species of crinoid exist today. Occasionally distinct in appearance to their fossil ancestors, living forms nonetheless provide clues of ancient crinoidal modes of life.
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 interconnecting soft tissues, following the death of the animal, means that complete specimens are rare, but parts of the stem (the ossicles) are commonly found as fossils.
The calyx is made of polygonal plates, arranged differently in different groups of crinoids. It contained the mouth, to which food was conveyed via grooves in the brachia. In some fossil crinoids the top of the calyx was a flexible membrane, but in others it is preserved as a rigid dome, and may have an elongated anal tube for the disposal of waste products away from the mouth and the water vascular system intake, both of which are found in the lower cup of the calyx.
The stem typically consisted of disc-like plates ossicles stacked on top of each other. Ossicles were rounded, oval, square, five-sided or star-shaped, and some were decorated with petal-like designs. The different shapes of crinoid stem plates are useful for classification, but some fossil crinoids, like many modern forms, lack stems.
Crinoids abounded in shallow water, particularly in the late Silurian and early Carboniferous. Stemmed forms could bend towards water currents and use their brachia as a net to trap food particles. Side branches to the brachia (called pinnules) improved this ability in some groups, and very long stemmed forms may have exploited the best food supply from a range of water depths. Crinoid stems with movable appendages (cirri), or possibly a prehensile capability, allowed temporary anchorage where food was plentiful.