Part 15

CLADOPHYLLIA × 1 PARASMILIA × 1 PLEUROCORA × 1

ENDOPACHYS × 1 ASTRHELIA × 1 FLABELLUM × 1 MICRABACIA × 2 TROCHOSMILIA × 1

In Texas one may expect to find bryozoan remains in the Pennsylvanian rocks of north-central and Trans-Pecos Texas where they are abundant in certain of the marine shales and limestones. Bryozoans may also be collected from some Cretaceous and Tertiary beds, but their remains are small and fragmental and they are easily overlooked. Bryozoans have also been found matted on the shells of fossil mollusks and other invertebrates.

Phylum Brachiopoda

The brachiopods are a large group of exclusively marine organisms with shells composed of two pieces called valves (fig. 13). These valves are usually composed of calcareous or phosphatic material and enclose and protect the soft parts of the brachiopod animal. The soft parts are composed of muscles, the mantle (which secretes the valves), digestive, respiration, reproductive, and excretory organs, and the tentacle-bearing lophophore.

In adult life the brachiopod is attached to the sea bottom by means of a fleshy stalk called the pedicle (fig. 14), and this is usually extruded through a hole (the pedicle foramen) which is located in the ventral or pedicle valve. The upturned area which is usually present on the pedicle valve is called the beak. The other valve, known as the dorsal or brachial valve, is usually the smaller of the two (fig. 13b). The two valves are opened by means of muscles, and since death results in relaxation of these muscles, fossil brachiopods are typically found with valves closed.

Brachiopods vary greatly in size and shape and exhibit a wide variety of ornamentation, such as spines, ribs, nodes, and other structures. They are abundant fossils in many of the Paleozoic rocks of Texas but are relatively rare in Mesozoic and Cenozoic formations.

The phylum has been divided into two subclasses, the Inarticulata and the Articulata. This classification is based upon the nature of the hinge-line—the edge of the shell where the two valves articulate.

a Pedicle foramen Hinge line b Pedicle valve Beak Brachial valve

CLASS INARTICULATA.—

The members of this class are rather primitive and have a long geologic history. These brachiopods have valves which are not provided with hinge teeth, the valves being held together by muscles, and a hinge-line is lacking (fig. 14). Most inarticulate brachiopods are circular or tongue-like in shape and commonly composed of chitinous and phosphatic material. Inarticulate brachiopods range from Lower Cambrian to Recent in age but were never as common as the articulate brachiopods, which are described below. Brachiopods belonging to this class have been recorded from several Paleozoic formations in Texas (Pl. 17, Lingula, Apsotreta, Angulotreta).

FISTULIPORA × 6 POLYPORA × 5 RHOMBOPORA × 8

APSOTRETA × 10 LINGULA × 4 ANGULOTRETA × 10

RHIPODOMELLA × 1 DICTYOCLOSTUS × 1 CAMAROTOECHIA × 1

Pedicle Valve

CLASS ARTICULATA.—

Articulate brachiopods have a well-defined hinge-line (fig. 13a). One valve has well-developed teeth which articulate with sockets in the opposing valve, and there is a well-developed muscle system which aids in opening and closing the shell. Articulate brachiopods are characterized by calcareous shells which are typically of unequal size and a wide variety of shapes (Pls. 18, 19). The class has been divided into several orders which have been established primarily on the nature of the pedicle foramen and the nature of shell growth.

Articulate brachiopods range from Lower Cambrian to Recent in age and are particularly abundant in certain Pennsylvanian formations of north-central and Trans-Pecos Texas. They are also present in certain other fossiliferous strata of Paleozoic age but are less abundant and not as well preserved. The only Cretaceous brachiopod that is found in large numbers is Kingena wacoensis (Roemer) (fig. 15), which is particularly abundant in certain formations in the upper part of the Comanche series.

Phylum Mollusca

The phylum Mollusca encompasses a large group of aquatic (water-dwelling) and terrestrial (land-dwelling) invertebrates which includes such familiar forms as the snails, clams, oysters, squids, and octopuses. Most mollusks possess a calcareous shell that serves as an exoskeleton, and these hard parts are well adapted for preservation as fossils. However, some mollusks (the slugs) have no shells, and others (the squids) have an internal calcareous shell. Because of their relative abundance and great variety, mollusks are particularly useful fossils. Moreover, the remains of certain mollusks, such as the oysters, are important rock builders.

The phylum Mollusca has been divided into five classes:

1. Amphineura—the chitons or sea-mice; shell composed of eight valves or plates; not a common fossil. Ordovician to Recent.

2. Scaphopoda—the tusk-shells; shell composed of a single tusk-like valve; generally not a common fossil but locally abundant in certain Cenozoic formations. Devonian to Recent.

3. Gastropoda—the snails and slugs; slugs are without shells, snails have a single-valved shell which is typically coiled; common fossils in Paleozoic, Mesozoic, and Cenozoic rocks. Cambrian to Recent.

4. Pelecypoda—clams, mussels, oysters, scallops; shells composed of two valves, usually, but not always, of equal size; common fossils, especially in Mesozoic and Cenozoic rocks. Cambrian to Recent.

5. Cephalopoda—squids, octopuses, the pearly nautilus, and the ammonoids (extinct); shell of one valve, usually coiled and partitioned by septa; valuable fossils, especially in Paleozoic and Mesozoic rocks. ?Cambrian, Ordovician to Recent.

MARGINIFERA × 1 AMBOCOELIA × 1 SQUAMULARIA × 1 DERBYA × 1 MESOLOBUS × 1 CHONETES × 1 LINOPRODUCTUS × 1 PUNCTOSPIRIFER × 1 COMPOSITA SUBTILITA × 1 NEOSPIRIFER × 1

JURESANIA × 1 SPIRIFER ROCKYMONTANUS × 1 NEOSPIRIFER CAMERATUS × 1