The modern ocean is full of scary, disgusting, bizarre, awesome, and adorable organisms (multiply that by several thousand times, and you can cover prehistoric oceans too). While crinoids might not strike terror into your heart, they are pretty strange animals, which are often mistaken for plants at first glance (the name crinoid means “sea lily”). I personally find them somewhat adorable (living, swimming feather dusters? I mean, come on).
Crinoids are a class of echinoderms, the group which includes creatures like sea stars, urchins, sand dollars, and sea cucumbers. Echinoderms are characterized by having some form of five-sided symmetry, and a system of thousands of tiny appendages called tube feet, which allow the organisms to move and feed. Many echinoderms have five or more arms which are lined with tube feet, often improving their ability to catch food. Another characteristic of echinoderms is that they have a skeleton formed in the middle layer of the skin, just like vertebrates, which actually makes echinoderms our closest invertebrate relatives. Most other invertebrates have a skeleton which forms in the outer layer of skin, like the exoskeleton of an insect.
Crinoids are composed of three or four main sections: the holdfast, stalk (sometimes), calyx, and arms. At the base of the animal is a kind of root system, which is used for attachment onto a surface or substrate. This is called the holdfast (get it? It holds the crinoid fast/tight to something. I love it when terminology makes sense). Some crinoids then have a stalk, which leads to the head, including the mouth and anus, which is called a calyx.
Why the feather duster analogy for crinoids? Coming from the calyx, crinoids have a lot of very thin, long arms, with secondary arms (called pinnules) that look like the main barb and secondary filaments on a feather. The pinnules on a crinoid’s arm are covered in long tube feet, which act like a net to catch food particles out of the water. The tube feet then move food down the arm to the mouth, which is located on the calyx. The overall effect is that crinoid arms look like a bunch of feathers. Hence: feather duster of the sea.
As I mentioned, modern crinoids can, indeed, swim, flicking their arms to paddle through the water. They do get tired fairly easily, so swimming isn’t usually sustained for more than a minute or so. Some crinoids can also walk or drag themselves along the ocean floor. This is especially advantageous if they get knocked over, covered in sediment, or perhaps need to flee a predator. Check out this YouTube video of a swimming crinoid in action:
While most modern crinoids just have the holdfast, calyx, and arms, the majority of fossil crinoids also had a stalk. The stalk gives them the appearance more of a pinwheel than a feather duster. The stalk is comprised of a series of disks (called columnals) which are stacked on top of one another, and elevate the calyx high above the ocean floor, much like the trunk of a tree. Imagine a series of poker chips (the cheap plastic ones with the ridges along the edge) stacked on top of one another, with a hole in the middle and a string (liagment) passing through and holding them together. That is basically the stalk of a fossil crinoid. However, not that long ago, modern, stalked deep sea crinoids were discovered, leading to the group being termed “living fossils”.
Given the nature of a crinoid’s skeleton, which is composed of many tiny pieces, just like the bones of vertebrates, we usually don’t see intact crinoids in the fossil record. Instead, we more commonly find crinoid columnals, and other disarticulated pieces, scattered among our other fossils. Sometimes, you can find the calyx still intact, but they are often still very fragile. There are some beautiful examples of completely intact crinoids, but these are somewhat rare, and require exceptional preservation, just like the conditions needed to find intact vertebrate skeletons.
Crinoids may seem like an obscure group now, but they were one of the dominant ocean invertebrate groups until they were badly decimated by the end-Permian mass extinction, which wiped out about 85 – 95% of all living things, and is the worst mass extinction event ever recorded on Earth. Luckily, crinoids did manage to recover a bit, and you can now enjoy pictures and videos of the ocean’s very own feather dusters and pinwheels.
As always, if you have questions, comments, or requests for blog topics, please let me know!
For more information on crinoids, check out these resources: