Crinoids are marine invertebrates belonging to the phylum Echinodermata, making them relatives of modern-day starfish, sea urchins, and sand dollars. Often nicknamed “sea lilies” due to their plant-like appearance, many crinoids attach to the seafloor using a specialized stalk, or stem. This stem anchored the animal to the substrate and elevated the main body, or crown, into the water column to capture food. The skeletal remains of this stem are now among the most frequently encountered and recognizable fossils worldwide.
Anatomy and Biological Role of the Stem
The crinoid stem is a segmented, flexible column built from hundreds of interlocking, disc-shaped plates called columnals. These columnals are composed of porous calcium carbonate, a structure common to all echinoderm skeletons. Strong but flexible organic ligaments held these plates together in a continuous column when the animal was alive.
A central, hollow passage, known as the axial canal or lumen, runs the entire length of the stem. This canal housed a nerve and a fluid-filled sac, suggesting a role in nervous control and the water vascular system. The stem acted as a tether, positioning the crinoid’s feathery arms high above the substrate. This height allowed the crinoid to efficiently filter detrital material and plankton from passing ocean currents, a feeding strategy known as suspension feeding.
How Crinoid Stems Become Fossils
The prevalence of crinoid stem fragments in the fossil record results from how they decompose. When a crinoid dies, the soft organic ligaments and tissues holding the columnals together rapidly decay. This decay causes the stem to immediately fragment, scattering the individual columnals across the seafloor.
Because each columnal is a solid, calcite plate, it is resistant to decay and erosion. This durability ensures that the individual discs are preserved even when the more complex crown and arm structures are lost. Isolated columnals are thus vastly more common than complete, articulated crinoid skeletons.
In many locations, these fragments accumulated in such numbers that they became the primary constituent of sedimentary rock formations. These rocks are known as crinoidal limestones or encrinites, representing ancient marine environments where “crinoid thickets” thrived. The abundance of these fossilized stem pieces provides a detailed record of marine life from the Paleozoic Era onward.
Identifying Crinoid Columnals
Identifying an isolated crinoid columnal relies on recognizing its shape and structural characteristics. The most common fragments are small, coin- or gear-shaped discs, often measuring only a few millimeters in diameter. While many are circular, some species produced columnals that are pentagonal, elliptical, or star-shaped, reflecting the five-fold symmetry of all echinoderms.
The definitive identification feature is the small, centrally located hole, the fossilized opening of the axial canal. This central lumen is typically round, but in star-shaped columnals, the hole may also exhibit a five-pointed star shape. The flat surfaces where columnals connect, called the articulation facets, often display fine, radiating ridges that helped the stem pieces interlock tightly. These unique characteristics often led to public misidentification, with columnals historically being mistaken for ancient coins or given local folklore names such as “St. Cuthbert’s beads” or “fairy money”.
Variations in Crinoid Attachment Structures
While the stalked “sea lily” form is prevalent in the fossil record, not all crinoids possess a permanent stem. Comatulids, or feather stars, lose their juvenile stalk and are free-swimming as adults, using appendages called cirri to temporarily grasp the substrate. Crinoids that retain a stem connect to the seafloor via a specialized anchoring structure called a holdfast.
The morphology of the holdfast varies widely, reflecting the crinoid’s ecological niche. Some species developed simple, flattened discs that attached the animal onto a hard surface, like a rock or a shell. Others, living on soft or muddy substrates, utilized complex, root-like structures called radices. These radices branched out into the sediment to provide stability.