Fig. 23. Typical modern crinoid, or “sea lily,” showing principal parts.
[Calyx] Arm Plate Stem [Columnal] [Cirri] Holdfast (root)
The crinoid stem is attached to the base of the [calyx] and serves for purposes of support and attachment. This stem consists of a relatively long flexible stalk composed of numerous [calcareous] disk-shaped segments called columnals ([fig. 23]; [Pl. 35]), each of which contains a round or star-shaped opening in its center. Many crinoids have very long stalks (some are as much as 50 feet in length), and when the animal dies the columnals become separated and are scattered about on the ocean floor. Many [Paleozoic] limestones contain such great numbers of crinoid columnals that they are referred to as crinoidal limestones ([fig. 8]). Crinoidal limestones occur in some of the [Mississippian] and [Pennsylvanian] formations of central Texas and in the Pennsylvanian of north-central and Trans-Pecos Texas.
The stalk is attached to the sea floor or some other object by means of a root [system] called the holdfast ([fig. 23]). This structure commonly branches out into the surrounding sediments, and in this manner the crinoid animal is firmly anchored to the bottom of the sea.
Crinoids, like most echinoderms, are gregarious animals—that is, they commonly live together in large numbers, and for this reason great numbers of crinoid remains are commonly found concentrated in relatively small local areas. Most [fossil] crinoids are found as stem fragments because the more fragile [calyx] and root [system] are less likely to be preserved.
The earliest known crinoids have been found in rocks of [Ordovician] age, and their remains are particularly abundant in [Paleozoic] rocks. Crinoids are living today but most of them are stemless free-swimming forms called “feather stars,” much less abundant than their Paleozoic ancestors.
Subphylum Eleutherozoa
The eleutherozoans are free-swimming, bottom-dwelling, echinoderms which have been divided into two classes. The class Asterozoa (star-shaped echinoderms) contains the subclasses Asteroidea (the starfishes) and the Ophiuroidea (the brittle stars). Although they are known as fossils, neither of these groups is of paleontological importance. The class Echinozoa (echinoderms without laterally directed arm-like extensions) contains the subclasses Echinoidea (the sea urchins and sand dollars) and Holothuroidea (the sea cucumbers). Of these two subclasses, only the Echinoidea are useful fossils.