Part 14

Members of this order secrete tiny chambered shells which are very useful microfossils. The forams are predominantly marine organisms and have shells composed of chitin, silica, or calcium carbonate. In addition, some forms construct a shell of sand grains or some other material which is cemented together by a sticky substance that is secreted by the animal.

Forams are very abundant in the rocks of Texas and particularly so in rocks of Mesozoic and Cenozoic age. The most numerous and easily observed Paleozoic foraminiferans are the fusulinids (fig. 9a), and their small spindle-shaped remains are very abundant in many of the Pennsylvanian limestones of north-central and Trans-Pecos Texas. Some typical Texas forams are illustrated in figure 9.

Order Radiolaria.—

The radiolarians (fig. 10) have delicate spine-covered shells composed of silica, and their remains are very abundant in certain recent marine sediments. They may also be found as fossils and have been reported from Devonian and Permian rocks in Trans-Pecos Texas, and probable radiolarians have been reported from still younger beds.

Phylum Porifera

These are sponges and are the simplest of the many-celled animals. Living sponges secrete a skeleton which may be composed of chitin, silica, or calcium carbonate. These substances are commonly found in the form of spicules—tiny hard parts that are used to help support the soft tissues of the animal. These spicules take on a variety of shapes (Pl. 14) and are occasionally found as microfossils in some marine sediments.

Although sponges are not particularly common fossils, their remains occur in some parts of the State. Sponges have been collected from Paleozoic and Mesozoic formations of north and Trans-Pecos Texas, and their spicules have been reported from well cuttings.

Phylum Coelenterata

The coelenterates are multicelled animals which, though more complex than the sponges, are rather primitive animals. The living animal is characterized by a sac-like body cavity, a definite mouth, and tentacles which bear stinging cells. Some forms, for example, the jellyfishes, have an umbrella-shaped body and are single free-moving organisms. Others, like the colonial corals, are composed of many individuals living together in a colony.

Most zoologists and paleontologists recognize three classes of coelenterates: (1) the Hydrozoa, containing the small animals known as hydroids, (2) the Scyphozoa, which includes the jellyfish, and (3) the Anthozoa, which includes the corals and sea anemones. Because of their extreme fragility and lack of hard parts, hydrozoans and scyphozoans are not commonly found as fossils. They do, however, have a long geologic history and may be preserved when unusual conditions of fossilization occur. The anthozoans, especially the corals, are by far the most important class geologically, and these forms have left a very good paleontological record.

CLASS ANTHOZOA.—

This class is composed of a group of exclusively marine organisms and includes the corals and sea anemones. The coral animal, or polyp, secretes a cup-shaped calcareous (limy) exoskeleton. This skeleton, called a corallite, is usually divided by radial partitions called septa. The polyp lives in the calyx, which is the central bowl-shaped depression in the top of the corallite (fig. 11a).

Solitary corals form an individual corallite for each polyp, and because of their shape these may be given such names as “horn corals” (Lophophyllidium, Pl. 15) or “button corals” (Micrabacia, Pl. 16). Colonial or compound corals (Pl. 15) live together in colonies, which are formed of many individual skeletons attached to each other (fig. 11b), and the compound mass of coral skeletons formed in this manner is called a corallum. Fossil corals commonly occur in many marine limestones and in places constitute a large portion of the rock.

a Columella Septum Corallite b Calyx Septum Corallum

The class Anthozoa has been divided into several subclasses, but only one, the Zoantharia, is of paleontological importance.

Subclass Zoantharia.—

Most corals and all sea anemones belong to this subclass. Zoantharians are either colonial or solitary and, because most of them possess a hard preservable exoskeleton, they are the most important group of anthozoans geologically. The various orders of the subclass Zoantharia are discussed below.

Order Rugosa.—

These are corals in which the septa are arranged in cycles of four. Both solitary and colonial forms occur, and they are found only in rocks of Paleozoic age. Rugose corals are abundant in many of the Paleozoic formations of Texas, and two of the more typical forms (Lophophyllidium and Caninia) are illustrated in Plate 15. Members of this order have been placed in the subclass Tetracoralla of older classifications.

Order Scleractinia.—

The scleractinians are solitary or colonial corals in which the septa grow in multiples of six, and they are the most important and abundant of the modern corals. These corals were the dominant reef builders of Mesozoic and Cenozoic seas, and their remains are common in many of the marine formations of the State. Plate 16 illustrates some typical scleractinian corals from the rocks of Texas. This order has also been referred to as subclass Hexacoralla, and its members have been called hexacorals.

Order Tabulata.—

These are corals that are now extinct but are known from fossils in both Paleozoic and Mesozoic rocks. Tabulate corals are characterized by horizontal partitions called tabulae, and septa are absent or poorly developed. The tabulates were the most abundant reef-building corals during Paleozoic time and are well known as fossils. Because of certain similarities with other anthozoans, some paleontologists have treated the Tabulata as a distinct subclass rather than as an order of the Zoantharia.

Tabulate corals are not uncommon in many of the Paleozoic rocks of Texas, and two of these (Cladochonus and Striatopora) are illustrated in Plate 15.

Phylum Bryozoa

Bryozoans are colonial animals that are often referred to as “sea mats.” They have been called this because they are commonly found matted on shells, rocks, fossils, and other objects. The living animal is quite small, has a tentacle-bearing ridge surrounding the mouth, and secretes a tiny cup-like exoskeleton composed of calcareous or chitinous material. These little chambers, known as zooecia (or autopores), are seen as small pits on the surface of the bryozoan colony (Rhombopora, Pl. 17). The zooecia grow together to form the bryozoan colony, and some fossil colonies grow to be as much as 2 feet across. Such colonies may be spiral (fig. 12b), branching, or lace-like (fig. 12a), and the latter two types are very common in many of the fossiliferous strata of Texas. Undoubted bryozoan fossils have been recorded in rocks of Lower Ordovician age, but questionable Cambrian forms have also been reported. Bryozoans are abundant in the seas of today, but only a few forms inhabit fresh waters.

MEANDROSTIA × 1 HELIOSPONGIA × 1 ASTRAEOSPONGIUM × ½ ASTYLOSPONGIA × ½ RECEPTACULITES × ½ GIRTYOCOELIA × 2

CLADOCHONUS × 1 STRIATOPORA × 1 LOPHOPHYLLIDIUM PROLIFERUM × 1 MICHELINIA × 1 CANINIA × 1 LOPHOPHYLLIDIUM RADICOSUM × 1