[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.