Crinoids: The Fossils That Inspired 'Alien'

Crinoids  are commonly known as sea lilies due to their "flower-like" appearance , though they are animals, not plants. 

Crinoids are part of a large group of marine invertebrate animals called echinoderms. Other echinoderms are starfish, brittle stars, sand dollars, sea urchins, and sea cucumbers. All living echinoderms have what is called pentameral symmetry, which means their bodies are organized in patterns of five; for example, the five arms of the common starfish. Crinoids may have as few as five arms, but usually they have arms in multiples of five.

Crinoids derived in the Cambrian Period from pelmatozoan ancestors.

All echinoderms also have calcite plates (ossicles) embedded in their skin, which form their skeleton. That is why living starfish feel scratchy when you touch them. The skeletons of fossil crinoids are very representative of what the animals looked like a-live because only the outer skin layer is missing.

Crinoids: The Fossils That Inspired 'Alien'
Crinoids: The Fossils That Inspired 'Alien'. Agaricocrinus americanus, crinoïde, carbonifère, Provenance: Indiana.

 The skeleton is usually divided into four basic parts:

  • Holdfast, a disc-like sucker, which anchors the crinoid to the ocean bottom;
  • Stem, filled with muscles, which raises the calyx above the substrate;
  • Calyx, a cup-shaped central structure , which contains the internal organs; and
  • Arms - from five to as many as 200 feeding arms (in multiples of five).

Crinoids: The Fossils That Inspired 'Alien'
Fossils of Seirocrinus subsingularis from the Jurassic Holzmaden Black Shale Formation, Germany

The largest Sea Lily has a large calyx which with its arms gives it a diametrer of 1.5 metres. The largest Feather star has an armspan of 35 cm. The smallest Crinoids are around 3 cm in diameter.

Most species are nocturnal filter feeders consuming plankton and decaying organic matter.

To feed they spread their feeding arms to sieve the passing sea water for microscopic organisms and detritus. Mucus, on the tube feet traps their food which is passed down the arms into the mouth by beating cilia. They have a U-shaped digestive system with the anus next to the mouth.

Not much is known about what eats them although fish and other Echinoderms (especially Sea urchins) are known predators. Sea lilies have been observed crawling away from Sea urchins.

Crinoids are either male or female with fertilization taking place in the water. The eggs hatch to form free-swimming larva which do not feed and settle on the bottom after a few days after which they metamorphise into an adult in 8 to 12 months. Some hatch as miniature adults, while some females even hold the eggs in their arms until they hatch.

 The first true Crinoids appeared during the Lower Ordovician. Following the global mass extinction at the Silurian boundary, they and underwent several major radiations at the early Devonian, Missisippian (peak) and Pennsylvanian. They almost became extinct at the end of Paleozoic Era in the Permian, but recovered to flourish again during the Mesozoic, in the Triassic and Jurassic (Lias, Dogger, Malm). Decreasing numbers in the Cretaceaous, fossil record of crinoidsis rare in the Tertiary. More than 6,000 fossil species, belonging to more than 800 genera, have been described. 

Today, approximately 600 living species are known; most free-living feather stars or comatulids living in the shallow seas.  About 80 species of stalked sea lilies are restricted to the deeper water of today`s ocean. 

Crinoids were very successful in the Paleozoic. They were most abundant and diverse in the Mississippian when several continents were covered with shallow seas. Thick layers of limestone stratigraphy throughout North America and Europe are full of crinoid stems.

See also:
The Largest Insect Ever Existed Was a Giant 'Dragonfly'
The Largest Ammonite Ever Found?
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