Fossil Crinoids

About Crinoids

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

Crinoids are echinoderms related to starfish, sea urchins, and brittle stars. Like other members of their phylum they are spinny skinned, have a five-sided or pentaradial symmetry as adults and a calcium carbonate endoskeleton.

Although crinoids are the least understood of living echinoderms, their skeletal remains are among the most abundant and important of fossils. Crinoids were major carbonate producing organisms during the Paleozoic and Mesozoic. In fact, in many Paleozoic and Mesozoic settings entire carbonate shelves were composed predominantly of crinoidal remains. In such examples, the enormous volume of skeletal material controlled the sedimentary environment.

Evolution of Crinoidea

Crinoids derived in the Cambrian Period from pelmatozoan ancestors.

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. Watch deep sea crinoids attached to sponge at 1200 feet off the coast of Roatán, © Stanley:

broadhead and waters (1980)_Evolutionary history of crinoid genera in geologic time

Morphology of Crinoidea

Crinoids can very basically be described as upside-down starfish with a stems. The stem of a crinoid extends down from what would be the top of a starfish, leaving the mouth of the organism opening skyward, with the arms splayed out. However, crinoid arms look articulated and feathery. The stalk extends down from the aboral surface of the calyx. The stalk column has holdfasts which attach the animal to substrate.

The three main sections of a crinoid give it the lily-like appearance. These sections are the segmented column or stem, the calyx where the body cavity and digestion occurs, and the arms which filter food from the environment. Most crinoids live attached to substrate, though there are free swimming species in the fossil record.

Learn more about the various types of crinoids watching this video by © Prof. Messing from the from Nova Southeastern University's Oceanographic Center.

The calyx is a cup-shaped body part which contains the u-shaped digestive system and the organs for reproduction.

Five sided, calcareous plates create the lower cup of the calyx. The plates are in rows of five and radiate around the calyx. The mouth is atop the dorsal cup, and the anus is beside it. The tegmen is the skyward surface of the calyx.

The tegmen has five ambulacral areas which includes a deep groove in each area. Arms extend from the grooved areas. The typically five arms, or brachials, can splay open for filter feeding. Smaller ossicles form the structure of the arms. The arms and calyx are called the crown. Cirri are special appendages, emerging from the bottom of the calyx or along the stem. Tey can act as a hook, or wrap around an object.

Classification of Crinoidea

Five major morphological groups of Crinoidea were recognized in the "Treatise".

The fundamental division of Crinoidea has been based upon a combination of characters such as the number of plate circlets in the cup, the rigidity with which they are sutured together, the presence and position of any additional plates within these circlets, the structure of the oral surface and the structure of the arms.

Four distinct groups occured only during the Paleozoic:

Camerata:
- tegmen chamber above the cup
- rigidly sutured cup,
- heaviliy plated, rigid tegmen,
- subtegminal mouth

Disparida:
- unequal size of cup plates
- arms free above the cup
- two-circlet (monocyclic) cup
- cone/barrel-shaped cup

Cladida:
- richly branched arms structure
- arms free above the cup
- three-circlet (dicyclic) cup
- prominent anal sac
- few or no oral plates

Flexibilia:
- weakly united calyx plates
- three-circlet (dicycylic) cup
- reduced infrabasals
- reduced anal plates
- no rigid tegmen, exposed mouth
- uniserial arms without pinnulae
- brachials with flanges

A fifth group, the Articulata, were present after the Palaeozoic. They all have pinnulae arms and a cup lacking any anal plates. They are characterized by the predominantly biserial arrangement of the arms.