Crinoid

Crinoids are fascinating marine invertebrates that belong to the class Crinoidea. They are part of the 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 as deep as 9,000 meters!

There are two main types of crinoids. Those that stay attached to the sea floor by a stalk are called sea lilies. The other type, called feather stars or comatulids, do not have a stalk when they grow up. Feather stars are actually the largest group of crinoids today.

Crinoids have a special way of eating. Their mouths are on the top surface of their bodies, surrounded by feeding arms. These arms have tiny branches called pinnules that spread out to catch tiny particles floating in the water, known as plankton. Even though they often look simple, crinoids have a five-fold symmetry, meaning their bodies are arranged in fives.

Today, there are only about 700 kinds of crinoids living in the ocean. However, in the past, they were much more common and varied. Some thick limestone layers formed during the middle of the Paleozoic era through the Jurassic period are made almost entirely from pieces of ancient crinoids.

Etymology

The name "Crinoidea" comes from an ancient Greek word meaning "a lily", with a suffix that means "like". This is why some crinoids are called sea lilies.

Morphology

Crinoids are interesting sea animals with a special body shape. They have a stem (like a tail) and a crown made of a cup-shaped main body and five arms that often branch out and look feathery. The mouth and anus are on the top side of the cup, which is different from other sea animals like starfish and sea urchins. Their bodies are mostly made of small, hard plates, with only a little soft tissue.

The stem of some crinoids, called sea lilies, helps them stick to the ocean floor. Other types, called feather stars, lose their stem when they grow up and can move around. Their arms are lined with tiny parts called pinnules that help them catch food. These arms can curl up and have tiny feet that help grab bits of food from the water.

Locomotion

Most modern crinoids, called feather stars, can move around freely. They usually crawl using their feather-like arms to lift their bodies. They might move to find better spots for feeding or to avoid other crinoids.

Crinoids can also swim by moving their arms in a special way. They swim upwards at first, then move sideways, traveling about 7 centimeters every second. This swimming usually happens for short periods, like half a minute, and can be a way to escape from something that might hurt them. In 2005, scientists saw a stalked crinoid move much faster than before, sliding across the sea floor at up to 5 centimeters per second.

Main article: Florometra serratissima
Main article: Neocrinus decorus
Main article: Grand Bahama Island

Evolution

See also: List of echinodermata orders

Reconstructions of various extinct crinoids show how these sea animals looked long ago. Crinoids first appeared about 480 million years ago during a time called the Ordovician. Scientists are not sure exactly where they came from, but they believe they may have evolved from other similar sea creatures.

Crinoids were very common in the ancient seas and changed a lot over time. They had two big bursts of growth, one during the Ordovician and another during the Triassic period about 230 million years ago. During these times, crinoids developed new ways to move and protect themselves. Some attached to floating wood, which later sank to the ocean floor, taking the crinoids with it. Today, we can find many crinoid fossils in rocks, showing how these interesting animals lived long ago.

Taxonomy

Crinoidea has been recognized as a unique group of echinoderms since 1821. It includes many ancient orders as well as four living orders (Comatulida, Cyrtocrinida, Hyocrinida, and Isocrinida), which belong to the subgroup Articulata. Today, there are about 540 known species of living articulates.

• Class Crinoidea
  • †Protocrinoidea (incertae sedis)
  • Subclass †Camerata
    • Order †Diplobathrida
    • Order †Monobathrida
  • Subclass Pentacrinoidea
    • Parvclass †Disparida
      • Order †Eustenocrinida
      • Order †Maennilicrinida
      • Order †Tetragonocrinida
      • Order †Calceocrinida
    • Parvclass Cladida
      • Superorder †Porocrinoidea
        • Order †Hybocrinida
        • Order †Porocrinida
      • Superorder †Flexibilia
        • Order †Sagenocrinida
        • Order †Taxocrinida
      • Magnorder Eucladida
        • †Ampelocrinida (incertae sedis)
        • Superorder †Cyathoformes
        • Superorder Articulata
          • Order †Encrinida
          • Order †Holocrinida
          • Order †Millericrinida
          • Order †Roveacrinida
          • Order †Uintacrinida
          • Order Comatulida
          • Order Cyrtocrinida
          • Order Hyocrinida
          • Order Isocrinida

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. Interestingly, the design of the alien creatures in the Alien movie series was based on crinoids.

Fossil crinoid gallery

Here are some interesting fossils of crinoids from different times and places:

• Crushed crinoid stems from Shamshak Formation, Jurassic, Iran
• Fossil from Germany showing the stem, calyx, and arms with pinnules
• 330 million year old crinoid fossils from Iowa
• Crinoid holdfasts and bryozoans on an Upper Ordovician cobble from northern Kentucky
• Seirocrinus subangularis from the Early Jurassic Posidonia Shale at Holzmaden, Germany
• Crinoid columnals (Isocrinus nicoleti) from the Middle Jurassic Carmel Formation at Mount Carmel Junction, Utah
• Root-like crinoid holdfast from the Upper Ordovician, southern Ohio
• Internal mold of crinoid stem lumen (and external mold of stem) from Lower Carboniferous, Ohio
• Fossils of Seirocrinus subsingularis from the Jurassic Holzmaden Black Shale Formation, Germany
• Crinoid
• Some extinct crinoids, like Monstrocrinus from the Early to Middle Devonian, could have complex spines attached to their calyx plates