Crinoids (Phylum Echinodermata, Class Crinoidea)
Crinoids are exclusively marine suspension feeding echinoderms that typically have many arms that radiate from a cup-like body (calyx) that may or may not have a thin, columnar stalk. They have an endoskeleton composed of many individual elements (ossicles) composed of calcium carbonate and connected by ligamentary tissue. Stalked crinoids, commonly known as sea lilies, have a relatively long, stem-like stalk composed of many stacked disk-shaped ossicles. The stalk attaches the organism to the sea floor and raises the body off of the bottom and into the current where it can feed. Comatulid crinoids, commonly known as feather stars, have secondarily lost their stalk and may attach themselves to surfaces using grasping appendages at the base of their calyx known as cirri. Crinoids, like other echinoderms, have a water vascular system with pod-like extensions known as tube feet. The feather-like arms of crinoids have many smaller lateral extensions known as pinnules. The water vascular system extends into the arms and pinnules where the protruding tube feet assist in capturing suspended food particles and transporting these along grooves in the arms towards the mouth. Stalked crinoids have limited mobility, although they have been observed using their arms to “crawl” along the bottom. Comatulids, however, are more mobile, and some can use their feather-like arms to swim gracefully from one spot to another. Although today there are only about 80 species of stalked crinoids and about 460 species of unstalked comatulids, the group has a very rich fossil record dating back to the Ordovician Period, and over 5,000 fossil species (mostly stalked forms) have been described. The stalked crinoids were particularly diverse and abundant during the Paleozoic Era and were widespread in relatively shallow marine environments. Crinoids also contributed significantly to the accumulation of carbonate (limestone) deposits. The disarticulated ossicles of crinoids are common sedimentary particles and components of many limestones. Stalked crinoids nearly went extinct during the mass extinction at the end of the Paleozoic Era (~250 mya), and although they survived, they were largely replaced in shallow water settings by the unstalked comatulids, which appeared during the Mesozoic Era. Today stalked forms are mostly restricted to relatively deep water (>200 meters), whereas the comatulids are most common in waters less than 200 meters deep and are quite successful in many shallow reef environments.
Blastoids (Phylum Echinodermata, Class Blastoidea)
Blastoids are an extinct class of suspension feeding echinoderms that, like many crinoids (see above), had a body (theca) attached to a stem-like stalk that raised the body off of the bottom. The theca is marked by five major food grooves from which many thin, delicate arm-like structures known as brachioles extended, trapping suspended food particles and transporting them to the mouth. Like all echinoderms, blastoids were exclusively marine. Blastoids appeared during the Ordovician Period and went extinct during the mass extinction at the end of the Permian Period (~250 mya) that marked the end of the Paleozoic Era. They were most common in relatively shallow marine environments and were especially abundant and diverse during the Lower Carboniferous (Mississippian) Period (~ 323-359 mya).