Crinoids are bottom-dwelling marine animals that first appeared in the Cambrian Period and have survived until today. They lived in shallow water and flourished, especially, during the Palaeozoic and Mesozoic eras, when a large number of species evolved. They belong to the phylum of Echinodermata, which also includes sea urchins (echinoids), and they take two main forms: some (the sea lilies) have stalks, and others (the feather stars) root directly onto the seabed.
The stalked form is more common in the fossil record, and is often represented by small sections of stem or the individual ossicles that make up the stalk. These take different shapes: some are circular in cross-section, others elliptical, square, hexagonal or star-shaped (stellate). More interesting, however, are fossils of the crown, which is composed of a number of arms (usually five) bearing feathery pinnules. The arms emerge from the stem in a cup-shaped feature called the calyx or aboral cup. At the base, the stem is fixed to the surface by a set of root-like limbs, known as the holdfast. These features are shown diagrammatically below.
All these features may vary from species to species, so formal descriptions of species often refer to the shape or number of these features. Since good fossils of the crown and holdfast are rare, however, identification often relies on the shape and structure of the columnals from the stem and, more occasionally, the arms. As the diagram, below right shows, the columnar is made up of three conjoined ossicles: a nodal (i.e. central) ossicle, and two internal ossicles. The structure of each of these varies in a diagnostic way. On the upper (proximal) surface of the nodal ossicle the lobes (areola) are concave; on the lower (distal) surface they are convex. On the internodal ossicles, the pattern is reversed. The two thus fit neatly into each other. Also of importance for identification is the detailed shape of the stem, and of the projection (interradius) and the cavity (radius) made by each of the lobes. Based on these various features, a first level of identification may be possible. More detailed identification, however, may require microscopic analysis of the ossicles, or goos specimens of the crown.
For more information on crinoids, a useful source is the British Geological Survey site. Another valuable source is: Hess, H. et al. 1999 Fossil crinoids. Cambridge: Cambridge University Press.