Crinoids

Periechocrinus, a Silurian crinoid

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: the animal

Anatomy and feeding position of a stemmed crinoid. The current flow is left to right.

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.

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.

Antedon mediterranea (Lamarck, 1816)
Calyx of Amphoracrinus portlocki Wright, 1995 (Carboniferous).

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 brachials. 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.

The stem typically consisted of disc-like plates (columnals) stacked on top of each other. Individual columnals were rounded, elliptical, square, pentagonal or stellate, and some plates 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.

Crotalocrinites (Silurian)
Pentacrinites (Jurassic)
Bourgueticrinus (Cretaceous)

Their environment

Some Palaeozoic crinoids may have been preyed on by sharks with specialised crushing teeth, but most fossil crinoids probably lacked predators, perhaps detered by toxic secretions. However, fossil remains show that parasitic worms infested the food supply in the brachials, and that the Carboniferous gastropod Platyceras was nourished by the stream of excreted material.

Life on the reef
Pentacrinites briareus (Miller, 1821) (Lower Jurassic). A rare example of complete preservation of a crinoid skeleton.

Fossil crinoids abounded in shallow water, particularly in the Upper Silurian and Lower Carboniferous. Stemmed forms could bend towards water currents and use their brachials as a net to trap food particles. Side branches to the brachials (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.

As with living animals, fossil crinoids are classified into species and genera whose scientific names are Latin words or words that have been latinised. The full scientific name of a particular crinoid comprises the name of the species, preceded by the name of the genus to which it belongs, and followed by the name of the first person to describe it, and the date of that description.

Probable living position of Uintacrinus and Marsupites.

In the Mesozoic Era, crinoids evolved highly flexible brachials allowing them to discard their stems and search the sea floor for better feeding conditions. The small, stemless Saccocoma (Jurassic-Cretaceous) was free-swimming, but the much larger stemless Uintacrinus and Marsupites (Cretaceous) probably rested on the sea bed, their brachials outstretched as a food collecting bowl.

Crinoids: the geologists' tool

The calyx of Applinocrinus cretaceus (Bather, 1924)(Upper Cretaceous) — greatly magnified
Pentacrinites briareus - crinoidal limestone

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 Lower 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, e.g. Pentacrinites briareus.

Occasionally, crinoids can be a useful guide to the age of the rocks in which they occur. This is the case in the strata of Late Cretaceous age, known as 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 Sub-biozones.

Complete specimen of Uintacrinus socialis
Uintacrinus anglicus Rasmussen, 1961
Uintacrinus socialis Grinnell, 1876

Calyx of Marsupites testudinarius (Schlortheim, 1820) (Upper Cretaceous)

Work by R M Brydone (1873—1943) showed that the different sizes and ornaments of the calyx plates of Marsupites were also useful for biostratigraphy, and that species of the crinoid Bourgueticrinus could be used for correlation in the Chalk.

Bourgueticrinus palliformis Griffith & Brydone, 1911
Bourgueticrinus ellipicus (Miller, 1821)

Bourgueticrinus fritillus Griffith & Brydone, 1911
Bourgueticrinus elegans Griffith & Brydone, 1911

Different sizes and ornamentations of Marsupites testudinarius calyx plates.

Crinoids through time

Crinoids are common fossils in the Silurian rocks of Shropshire, in the Lower Carboniferous rocks of Derbyshire and Yorkshire, and in the Jurassic rocks of the Dorset coast and Yorkshire (Robin Hoods Bay).

Liliocrinus? prattii (Gray, 1828) (Jurassic)
Periechocrinus? awthornenis Wright, 1955 (Carboniferous)
Sagenocrinites expansus (Phillips in Murchison, 1839) (Silurian)

Crinoids: folklore and fact

Star stones

Crinoids are sometimes referred to as Sea Lillies 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. Robert Plot (1640—1696) named these stellate forms star stones.

Polished slab of crinoidal limestone, preserving a long crinoid stem fragment.

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 screw-like thread pattern and have been called screwstones.

Screwstone

Most living crinoids lack stems, but those that still have them inhabit much deeper water(up to 5000 metres) than their fossil ancestors.

Most fossil crinoid stems were probably less than a metre long, but some giants reached nearly 30 metres. The columnals forming the stem can sometimes be threaded into a necklace. The name St, Cuthbert's beads refers to the saint associated with the legend of making them into rosaries.

Seriocrinus attached to driftwood (Lower Jurassic, Germany); the actual size is 50 times greater.

The giant Seirocrinus, from the Jurassic of Germany, is highly unusual in that it is thought to have lived upside down, suspended in the water by its long stem that was attached to driftwood or a similar floating object.

3D model

BGS GSM7615 – Holotype
Load 3D model (BGS GSM7615 – Holotype)

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