What Is a Crinoid? The Animal People Mistake for a Plant
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Sea lilies look like flowers on stalks and are animals related to starfish and sea urchins. They once carpeted shallow seabeds so densely that entire limestone formations are made of their broken stems, and the survivors mostly live in deep water and have given up the stalk.
What the animal is
Crinoids are echinoderms, sharing with starfish and urchins a five-part radial symmetry, a skeleton of calcite plates and a water vascular system of fluid-filled canals operating tube feet. The body plan is inverted relative to a starfish, with the mouth facing upward and the arms held out to filter food from the water rather than pressed against the seabed. The stalk, present in many species, is built of stacked disc-shaped segments that articulate slightly and can be shed, and it attaches to the substrate by a root-like holdfast. The arms branch repeatedly and carry fine side branches lined with tube feet coated in mucus, which capture particles drifting past and pass them down grooves to the mouth. The whole structure is a passive filter oriented into the current, which is why crinoids position themselves on raised ground and why their arms form a fan rather than a circle in flowing water. Stalkless forms, called feather stars, can detach, crawl and swim, which most people are surprised to learn.
Why the fossils are everywhere
They are among the most abundant fossils in Palaeozoic marine rocks for reasons that follow from their construction:
- •The skeleton is made of many separate calcite plates and stem segments, so a single animal disarticulates into hundreds or thousands of pieces on death
- •Those pieces are durable, so they survive transport and burial that would destroy a delicate whole animal
- •Entire limestone beds consist substantially of crinoid debris, called crinoidal limestone, which is quarried as building stone in several regions
- •Individual stem segments, small circular discs with a central hole, are found in such numbers that they have folk names in several countries and were strung as beads
- •Complete specimens with arms and calyx intact are rare and valuable, requiring rapid burial by storm deposits or mudflows
- •The abundance makes them useful for dating and for reconstructing ancient environments, since different forms occupied different depths and current conditions
The gardens of the Palaeozoic
During the Carboniferous in particular, crinoids formed dense communities on shallow seabeds that are frequently described as meadows or gardens, with many species at different heights on stalks of differing length, partitioning the water column in the way a forest partitions light. That arrangement supported a whole associated community, including animals that lived on the stalks, parasites that infested the calyx and predators that fed on the arms. Their decline is tied to the end-Permian extinction, which eliminated the great majority of lineages, and to the subsequent rise of durophagous predators, meaning fish and crustaceans capable of crushing hard skeletons, which appear to have made life on a stalk in shallow water untenable. The response is visible in the fossil record: stalked forms retreated to deep water where such predators are scarce, and the shallow-water survivors are overwhelmingly the stalkless feather stars, which can move and hide. That is a clear case of predation pressure reshaping where a whole group lives.
The living ones
Around six hundred species survive, and they are considerably more interesting than the popular image of a living fossil suggests. Feather stars are abundant on coral reefs, are frequently brightly coloured, come out at night, can crawl using their arms and can swim by coordinated arm movements, which looks remarkable on film. Stalked forms live mainly below two hundred metres and have been filmed moving, dragging themselves across the seabed by their arms at speeds of several metres an hour, which overturned the assumption that they were fixed. They regenerate arms readily after damage, and the proportion of regenerating arms in a population is used as an index of predation pressure. Their larvae are free-swimming before settling. Deep-sea surveys continue to find new species. The group is a reminder that a lineage with a long fossil record and an ancient body plan is not therefore unchanged, since the living species are different from their Palaeozoic relatives in habitat, in mobility and in the predators they avoid.
The takeaway
These are echinoderms related to starfish, inverted so the mouth faces up and the arms filter passing water, with a stalk built of stacked discs. A skeleton of many separate plates disarticulates into thousands of durable pieces, which is why whole limestone formations are made of their debris. Crushing predators appear to have driven stalked forms into deep water, leaving mobile feather stars in the shallows.