Crinoid

Crinoids are fascinating marine invertebrates that belong to the class Crinoidea. They are part of the larger group called echinoderms, which also includes starfish, brittle stars, sea urchins, and sea cucumbers. Crinoids can be found living in both shallow waters and very deep oceans, even over 9,000 meters deep!

There are two main types of crinoids. Those that stay attached to the sea floor by a stalk throughout their lives are called sea lilies. The other type, called feather stars or comatulids, lose their stalk when they grow up and can move around freely. Feather stars are actually the largest group of crinoids today.

Even though crinoids look complicated, they have a simple way of eating. Their mouths sit on the top surface of their bodies, surrounded by special arms covered in feathery structures. These arms catch tiny particles of food floating in the water, which the crinoids then eat. Though they often start life attached to the ocean floor, many crinoids eventually become free-moving as adults.

Today, we know of about 700 different kinds of crinoids living in the oceans. However, in the past, they were much more common and diverse. Some thick layers of limestone from millions of years ago are made almost entirely from the remains of ancient crinoids.

Etymology

The name "Crinoidea" comes from the Ancient Greek word κρίνον (krínon), meaning "a lily", with the suffix –oid meaning "like". This name reflects the flower-like appearance of many crinoids.

Morphology

Crinoids are interesting sea animals with a special body shape. They have a stem (in some types), a cup-like central body called the theca, and five arms that are usually branched and look feathery. The mouth and anus are on the top side of the theca, which is different from other sea animals like starfish and sea urchins. Their bodies are made mostly of small, bony plates with just a little soft tissue.

Sea lilies have a stem made of porous pieces connected by tissue, and they attach to the ocean floor with a holdfast or root-like structures. Feather stars lose their stem as they grow but keep a few root-like pieces. Most living crinoids can move freely and have only a tiny stalk. Their arms can curl up and have tiny branches called pinnules that give them a feather-like look. These arms help them catch food.

Locomotion

Most modern crinoids, called feather stars, can move around on their own. They crawl using their feather-like arms and can even swim for short bursts. When they swim, they move their arms in a coordinated way to travel upward and then horizontally, moving about 7 cm per second.

In 2005, scientists observed a stalked crinoid called Neocrinus decorus moving much faster than previously thought—up to 4 to 5 cm per second. This was a surprising discovery, showing that even stalked crinoids can move quickly along the sea floor.

Evolution

See also: List of echinodermata orders

Reconstructions of various extinct crinoids show many different shapes and forms. The earliest known crinoids appeared about 480 million years ago during a time called the Ordovician. Scientists are not sure exactly where crinoids came from, but they may have evolved from other similar sea creatures called blastozoans or from edrioasteroids.

Crinoids were very common in ancient seas and changed a lot over time. After a big event that killed many sea creatures at the end of the Permian period, crinoids looked quite different and lived in new ways. Later, around 230 million years ago, crinoids changed again, developing flexible arms to help them move and avoid being eaten. Some crinoids lived attached to floating wood, which sometimes sank and became fossils on the ocean floor. Today, we can find many crinoid fossils in certain types of rock.

Taxonomy

Crinoidea has been recognized as a distinct group of echinoderms since 1821. It includes many extinct orders and four living orders: Comatulida, Cyrtocrinida, Hyocrinida, and Isocrinida. These living orders are part of a subgroup called Articulata. Today, there are about 540 species of living crinoids.

The class Crinoidea includes several subgroups and orders, both living and extinct. Some of the main groups are Pentacrinoidea, which contains many extinct orders, and Articulata, which includes the living orders. The classification of crinoids is complex and continues to be studied by scientists.

In culture

Fossilized pieces of crinoids have been used for beautiful decorations for many years. In the Middle Ages, people in places like Lindisfarne made necklaces and rosaries from these fossils, calling them St. Cuthbert's beads. In the Midwestern United States, similar fossils are sometimes called Indian beads. One type of crinoid, Eperisocrinus missouriensis, is even the official state fossil of Missouri. Also, the design of the aliens in the movie Alien was inspired by crinoids.

Fossil crinoid gallery

Here are some interesting fossils of crinoids from different times and places. We see crushed stems from the Jurassic period in Iran, and fossils from Germany showing stems, calyxes, and arms. There are also 330 million year old crinoid fossils from Iowa. Other fossils include crinoid holdfasts and bryozoans from the Upper Ordovician in northern Kentucky, and Seirocrinus subangularis from the Early Jurassic Posidonia Shale in Holzmaden, Germany. We also find crinoid columnals (Isocrinus nicoleti) from the Middle Jurassic Carmel Formation in Utah, and root-like holdfasts from the Upper Ordovician in southern Ohio. There are also internal molds of crinoid stems from the Lower Carboniferous in Ohio, and fossils of Seirocrinus subsingularis from the Jurassic Holzmaden Black Shale Formation in Germany. Some extinct crinoids, like Monstrocrinus from the Early to Middle Devonian, had complex spines attached to their calyx plates.