Paleozoic seas were dominated by crinoid echinoderms. The Permian extinction, 244 million years ago, devastated the marine biota. Tabulate and rugose corals, blastoid echinoderms, graptolites, and most crinoids died out, as did the last of the trilobites. Articulate brachiopods and one lineage of crinoids survived, but never again dominated the marine environment. All modern crinoids have evolved from this lineage.
Crinoid bodies consist of three main parts: A calyx or aboral cup, the arms and the stem.
The calyx contains the vital organs of the animal. It is small when compared to the total mass, most of which is devoted to food collection. A simple digestive system is located within the upper body.
The arms are composed of an articulated series of ossicles that are used in suspension feeding and respiration. Reproductive organs are also located in the arms as fertilisation takes place in open water during mass spawnings.
The stem supports the animal and together with the roots and cirri serve as a means of attachment to the sea bed.
However, the function of these skeletal parts may vary for different species of crinoids. For example, some fossil crinoids have been found anchored to logs that presumably floated as the animal dangled in the water.
Crinoids possess an endoskeleton composed of calcareous plates and covered by a thin epidermis. Living, shallow water forms are extensively pigmented. Each plate is a single, very porous calcite crystal. Unfused plates are held together with ligaments or muscles.
Most modern crinoids have more flexible arms than the fossil species and do not have stalks (at least as adults), but are free to swim or crawl over the sea floor. These types of crinoids are