Part 18

The members of this subphylum are extinct arthropods which were most abundant during early Paleozoic time.

CLASS TRILOBITA.—

The trilobites are a group of exclusively marine arthropods which derive their name from the typical three-lobed appearance of their bodies (fig. 21a). The trilobite body is divided into a central or axial lobe and two lateral lobes. The body of the animal was encased in a chitinous exoskeleton. The top part of this exterior covering, the carapace, is very thick, and it is this part of the trilobite that is usually preserved.

FOSSIL INSECT × 1 FOSSIL CRUSTACEANS ENOPLOCLYTIA × 1⅓ ASTACODES × ¾ NOTOPOCORYSTES × 2 OSTRACODES × 40

Cephalon Thorax Pygidium Axial lobe Lateral lobes

The body is also divided into three parts from front to back. Beginning at the front of the animal these divisions are the cephalon or head, the thorax or abdomen, and the pygidium or tail (fig. 21a). The body segments of the thorax were arranged in such a manner as to permit the animal to roll up into a ball, and many trilobites are found in this position (fig. 21b).

Trilobites first appeared in the Cambrian and were extinct by the end of the Permian. They occur sparingly in certain of the Paleozoic rocks of Texas and when found are likely to be fragmental and in a poor state of preservation.

Subphylum Crustacea

The crustaceans are the crabs, shrimp, crayfish, and ostracodes. Although not abundant, fossil crabs have been described from certain Cretaceous and Tertiary formations of the State (Notopocorystes, Pl. 34). However, the most useful and abundant crustacean fossils are the members of the class Ostracoda.

CLASS OSTRACODA.—

The ostracodes are small, bivalved, aquatic crustaceans which have the external appearance of small clams (Pl. 34). The remains of these tiny animals are so small that they are best studied under a low-power microscope, and because of their small size they are particularly useful to the micropaleontologist.

Fossil ostracodes range from Ordovician to Recent in age and have been recorded in the Paleozoic, Mesozoic, and Cenozoic rocks of Texas. Their remains are particularly abundant in certain of the Cretaceous and Tertiary marine formations of the State.

Phylum Echinodermata

The echinoderms are a large group of exclusively marine animals, most of which exhibit a marked five-fold radial symmetry (Pls. 35, 36). Living echinoderms have well-developed nervous and digestive systems, a distinct body cavity, and are a relatively complex group of organisms.

The typical echinoderm has a skeleton composed of numerous calcareous plates which are intricately fitted together and covered by a leathery outer skin (the integument). The echinoderm body often exhibits a typical star-shaped form, but some types may be heart-shaped, biscuit-shaped, or cucumber-shaped.

Members of this phylum range from Cambrian to Recent in age and are abundant as fossils in many of the marine formations of Texas.

The phylum Echinodermata has been divided into two subphyla, the Pelmatozoa (those forms that were attached to sea floor by a stem or a stalk) and the Eleutherozoa (the stemless unattached echinoderms).

Subphylum Pelmatozoa

These are echinoderms which are more or less permanently attached to the bottom of the sea by means of a stalk which is composed of slightly movable, calcareous, disk-like segments (fig. 23).

Pelmatozoans range from Cambrian to Recent in age, and their fossilized remains are particularly abundant in Paleozoic rocks. The subphylum has been divided into several classes, but only three of these, the Cystoidea, Blastoidea, and Crinoidea, are discussed here. With the exception of the Crinoidea, all of the attached echinoderms are extinct.

CLASS CYSTOIDEA.—

These are primitive attached echinoderms which were relatively common in early Paleozoic time. The typical cystoid has a somewhat globular or sac-like calyx (the main body skeleton) composed of numerous, irregularly arranged, calcareous plates (fig. 22b). The plates composing the calyx are usually perforated by pores or slits which were probably used in excretion or respiration. The calyx was attached to the sea bottom by a short stem.

Cystoids range from Cambrian to Devonian in age and were especially abundant during Ordovician and Silurian time. Their remains are rare or absent in the rocks of Texas.

CLASS BLASTOIDEA.—

The blastoids are extinct short-stemmed echinoderms with a small, symmetrical, bud-like calyx. The blastoid calyx is composed of 13 calcareous plates arranged in a typical five-sided pattern (fig. 22a). The mouth is located in the center of the calyx and is surrounded by five openings called spiracles. Five distinct ambulacral or food grooves radiate outward from the mouth.

Blastoids range from Ordovician to Permian in age and were especially abundant during the Mississippian period. No blastoids have been reported from any of the rocks of Texas.

CLASS CRINOIDEA.—

The crinoids are commonly called sea-lilies because of their flower-like appearance. The calyx is composed of symmetrically arranged calcareous plates, and most crinoids have a long stem. Other crinoids are free-swimming in the adult stage and are attached only during the earlier phases of their development.