Crinoid morphology

Crinoids originated in the early Ordovician and persist to the present day. Their maximum abundance was in the Palaeozoic. Most fossil crinoids were attached to the substrate by a stalk and occupied shallow water environments (Fig. 7.2). Modern species are more widely distributed, living in habitats ranging from tropical reefs to cold, deep waters at polar latitudes. Reef-dwelling crinoids are stemless, are able to crawl

Calyx: connects the stem and the arms and contains the vital organs. The calyx is formed of a series of plates called radials, basals, and infrabasals

Monocyclic crinoids: have one series of plates between the stem and the radials, the basals

Dicyclic crinoids: have an additional series, the infrabasals, between the basals and the stem

In some crinoids the lower parts of the arms are incorporated into the calyx. These plates are known as fixed brachials. Interbrachials fill in the space between the brachials

Holdfasts: some crinoids were cemented to hard substrate, others were anchored by root-like structures. . In this example the distal part of the stem coils around a fixed bryozoan

Pinnules: unbranched extensions that increase the surface area and efficiency of the filter fan

Ordovician Crinoids

Fig. 7.2 Ordovician crinoid Pycnocrinus.

Arms: crinoid arms radiate from the calyx. Five arms are attached to the radial plates of the calyx. Usually arms branch at least once, at the base. Each arm consists of a series of brachial plates

Stem: formed from a series of ossicles called columnals. Columnals are joined by elastic ligaments and the stem is flexible. On death these ligaments quickly decay and the stem disintegrates into individual ossicles that are a common component of limestones. Some columnals are smooth but others interlock and are adapted to resist stem torsion and swim, and live in dense aggregations. Less abundant, deeper water forms have a stem and resemble fossil crinoids.

All living crinoids are passive suspension feeders. The arms and pinnules are arranged into a filtration fan with the mouth facing downcurrent (Fig. 7.3). Fine food particles are trapped by the tube feet, which form a fine net across the fan, and are passed to the mouth. The analysis of arm branching patterns of coexisting living crinoid species shows that different branching styles gather different-sized particles. Early crinoids with simple, unpinnnulated arms were probably unable to form filtration fans. Most Palaeozoic crinoids were less flexible than modern forms and were, therefore, probably unable to alter the orientation of their fan.

Stems raise the filtration fan away from slower moving water close to the substrate, upwards into a zone where water moves faster. Some modern stemless crinoids feed in the faster currents by attaching to coral heads or prominent rocks.

Current

Anal tube

Current

Pycnocrinus

Anal tube

Fig. 7.3 Reconstruction of crinoid feeding.

Fig. 7.2 Ordovician crinoid Pycnocrinus.

Fig. 7.3 Reconstruction of crinoid feeding.

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  • Sinit
    What did fossilised crinoid heads look like?
    8 years ago

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