Part 19

The crinoid calyx is typically cup-shaped (fig. 23) and five grooves radiate out from its center. These grooves continue outward along the complexly segmented arms and are used as channels to convey food to the mouth.

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.

CLASS ASTEROZOA.—

These are typical star-shaped free-moving echinoderms in which the body is divided into a central disk and radiating arms.

Subclass Asteroidea.—

This class contains the starfishes which, although not common fossils, illustrate well the typical echinoderm characteristics (Pl. 35). Fossil starfishes have been found sparingly in certain formations in Texas, but well-preserved specimens are quite rare. However, excellently preserved starfishes have been found in slabs of Cretaceous limestones from central and north-central Texas.

Subclass Ophiuroidea.—

The ophiuroids are echinoderms with a well-defined central disk and five long, slender, whip-like arms. They have been called brittle stars because of their ability to shed their arms when they are disturbed. Their long, slender, snake-like arms have also resulted in their being called serpent stars. Ophiuroids range from Ordovician to Recent in age, but because of the delicate nature of their bodies they are seldom found as fossils. Ophiuroid remains have been found in certain Mesozoic and Cenozoic rocks of Texas, but they consist largely of small segments of the arms or body fragments.

CLASS ECHINOZOA.—

The echinozoans are a group of unattached echinoderms whose bodies consist of numerous calcareous plates and spines, but they do not possess the radiating arm-like extensions which characterize the asterozoans.

Subclass Echinoidea.—

Echinoids are free-moving echinoderms with disk-shaped, heart-shaped, biscuit-shaped, or globular exoskeletons (Pl. 36). Modern representatives of this group include the familiar sea urchins, heart urchins, and the sand dollars.

CRINOIDS CRINOID CALYX × ½ CRINOID COLUMNALS × 1 HOLOTHURIAN SCLERITES (GREATLY ENLARGED) CRETACEOUS FOSSIL STARFISHES PENTAGONASTER × 1 PENTACEROS × 1

SALENIA × 1 ECHINOID SPINES × 2 ECHINOID PLATE × 2 HEMIASTER × 1 HOLASTER × 1 HOLECTYPUS × 1

The echinoid test (exoskeleton) is composed of many intricately fitting calcareous plates (Pl. 36) which enclose the animal’s soft parts. The exterior of the test is typically covered with large numbers of movable spines (Pl. 36) which vary greatly in size. These spines are of some aid in locomotion, support the skeleton of the animal, and provide a measure of protection from enemies.

The oldest known echinoids have been recorded from rocks of Ordovician age, but it was not until the Mesozoic that the group began to flourish. They were especially abundant during the Cretaceous and have been abundant and varied from that time until the present.

Echinoids are particularly numerous in many of the Lower Cretaceous formations of Texas where they are commonly found in an excellent state of preservation. Heart urchins and biscuit urchins may be found in large numbers in many areas of the State, and especially in areas where there are good exposures of fossiliferous Lower Cretaceous rocks.

Subclass Holothuroidea.—

Members of this class, commonly called sea cucumbers, have a rather elongate, sac-like, cucumber-shaped body and bear little resemblance to other members of the phylum Echinodermata. The sea cucumbers do not have a well-defined skeleton; rather the body is supported by many small, disconnected, calcareous plates or rods called ossicles or sclerites (Pl. 35). These minute structures are embedded in the leathery skin which covers the body of the sea cucumber and may be preserved as fossils. Such remains are locally abundant in certain formations in Texas, but because of their small size, scattered occurrence, and problems in classification, this group is of little use to most paleontologists.

Holothuroid body impressions have been reported from the Middle Cambrian, and sclerites from rocks as old as Mississippian.

Phylum Chordata

The chordates are the most advanced of all animals and are characterized by the presence of a well-developed nervous system and a body supported by a bony or cartilaginous notochord and/or spinal column. In the higher chordates (the vertebrates) the notochord is normally replaced by bone, but in the lower chordates (for example, the graptolites) it remains in a cartilaginous condition.