What Is a Sea Lily? An Animal on a Stalk That Looks Like a Plant

A crinoid consists of a stalk anchoring it to the seafloor, a cup containing the body, and a set of arms radiating from the cup and bearing fine side branches. The arms carry food grooves lined with tube feet, which catch particles from passing water and pass them down to the mouth at the centre of the cup, so the animal feeds by holding a filtering surface into a current. Everything is built from calcite plates, with the stalk made of stacked discs held by connective tissue, and those discs separate readily after death, which is why the stalk pieces are so abundant as fossils while complete specimens are rare. The mouth and anus both open on the upper surface, which is an arrangement that makes sense given that everything else faces the water.

The group survives in two forms with very different habits:

• •Stalked crinoids, which retain the ancestral arrangement and are now confined almost entirely to deep water
• •Feather stars, which have a stalk as juveniles, detach as adults and grip the substrate with a ring of clawed appendages
• •Feather stars can swim by beating their arms and can relocate, which the stalked forms cannot
• •Feather stars are abundant on shallow reefs and are the forms most people encounter
• •The retreat of stalked forms to deep water is attributed to predation pressure that increased through the Mesozoic
• •Both groups feed the same way and differ mainly in whether they stay put

The group was enormously abundant in the Palaeozoic and its remains built substantial quantities of rock. Crinoidal limestone, composed largely of the disarticulated stalk discs and other fragments, occurs widely in rocks of Carboniferous and comparable age and is quarried as building stone, so the fossils are visible in polished floors and facades in many cities. Dense meadows of stalked crinoids covered large areas of the shallow sea floor, and the animals reached the highest diversity of their history in that period. The individual stalk discs, roughly circular with a hole in the centre, are among the commonest fossils an amateur collector finds, and their resemblance to beads gave them folk names and led to them being strung as rosaries in some regions, which is where one traditional name comes from.

Identifying the remains in the field is straightforward once the shapes are known. Stalk discs are small circular or five-sided plates with a central hole, frequently found loose in weathered limestone and looking like beads. Short lengths of stalk survive where the connective tissue held, giving a stack of discs resembling a screw thread. Cup plates are angular and fit together in a recognisable pattern. Arm fragments are slender and segmented. Rock composed largely of these pieces has a distinctive glittery appearance in fresh surfaces, because each fragment is a single crystal of calcite and reflects light as one unit, which is a reliable field indicator. Complete specimens with cup and arms intact require rapid burial and are valuable enough that the best come from a small number of famous localities.

The shift of stalked forms from shallow water to the deep sea is a documented pattern with a proposed explanation. Through the Mesozoic, predators capable of attacking such animals became more numerous and more effective, including fish and echinoids with crushing jaws, which is part of a wider reorganisation of marine ecosystems sometimes called the Mesozoic marine revolution. A stalked animal fixed in shallow water cannot escape and is conspicuous, and the fossil record shows stalked forms declining in shallow settings over the same interval that the mobile feather stars appear and expand. Evidence of predation on fossil crinoids exists directly, including damaged and regenerated arms. The pattern fits the explanation well without proving it, and comparable retreats to deep water by other vulnerable groups over the same period support the general account.

A stalk of stacked calcite discs, a cup and arms bearing tube feet that filter passing water, which makes it an echinoderm rather than anything botanical. The discs separate after death, so they are abundant as fossils and build entire limestone beds visible in polished building stone. Stalked forms retreated to deep water as predators capable of attacking them increased.