Part 16

Of these five classes, only the Gastropoda, Pelecypoda, and Cephalopoda are discussed herein.

CLASS GASTROPODA.—

The typical gastropod has a spirally coiled, single-valved, unchambered shell. This shell encloses a soft body possessing a well-defined head with a pair of eyes and one or two pairs of tentacles. Most gastropods have gills and live in shallow marine waters, but some inhabit fresh water. Others are land-dwelling forms and breathe by means of lungs.

Gastropod shells, both Recent and fossil, exhibit a great variety of size, shape, and ornamentation. Such shells may be cone-shaped, spirally coiled, flat, turreted, or cylindrical. The shell is commonly wound in a spiral around a central axial pillar (the columella). The closed pointed end of the shell is called the apex, and each turn of the shell is called a whorl (fig. 16). The last-formed and largest whorl is called the body whorl, and this whorl contains the aperture—the opening of the shell. The combined whorls exclusive of the body whorl are known as the spire. The inner and outer margins of the aperture are designated the inner lip and the outer lip, respectively. In some snails the aperture is closed by means of the operculum—a calcareous or horny plate attached to the foot of the animal. This plate effectively seals the aperture when the animal is withdrawn into its shell. Some gastropods have shells that are loosely coiled, and in these forms the columella is absent. If the whorls of such shells are not in contact on the inner surface, this leaves an open space which is called the umbilicus (fig. 16a). The umbilicus is commonly seen as an opening in the base of the gastropod shell, but in some forms the umbilical opening may be partially or completely covered by a thick growth of shell called the callus.

Many gastropods, particularly those of the Texas Cretaceous, are commonly preserved as internal or external molds. This type of preservation occurs after the death of the animal, and the decomposition of the soft parts enables the shell to become filled with sediment. This filling later becomes solidified, and the outer shell may eventually be removed by weathering or solution. This type of internal mold is called a steinkern and normally does not reflect any external shell characteristics (Pl. 2). In some of the Pennsylvanian and Tertiary formations, however, gastropods may be collected with the original shell in an excellent state of preservation.

Plates 20-23 illustrate some typical Paleozoic, Mesozoic, and Cenozoic gastropods.

CLASS PELECYPODA.—

The pelecypods possess a shell composed of two calcareous valves (fig. 17) which enclose the soft parts of the animal. Members of this class live exclusively in an aquatic habitat and are most abundant in marine environments. Most pelecypods are slow-moving bottom-dwelling forms, but some, like the oysters, are attached. Still others, for example, the scallop or Pecten, are swimmers. The Pelecypoda include such familiar saltwater forms as the clams and oysters, as well as the common fresh-water mussel. Pelecypods range from Cambrian to Recent in age but are more abundant in Mesozoic and Cenozoic rocks.

The living animal is aquatic, with well-developed soft parts and a muscular, commonly hatchet-shaped foot. The soft mantle encloses the body and secretes the shell, and in some pelecypods part of the mantle is developed into the incurrent and excurrent siphons. The incurrent siphons bring fresh water and food into the mantle cavity, and waste products are passed out through the excurrent siphons. Respiration is by means of gills within the mantle cavity.

The typical pelecypod valves are of equal size and form, but some, such as the scallops and oysters, have two valves of unequal size and shape. The valves are hinged and held together by a tough elastic ligament which runs along the dorsal (top) side of the shell. In addition to the ligament, most forms have teeth and sockets which are located along the hinge-line. The teeth in one valve articulate with the sockets in the opposite valve, and this arrangement gives strength to the hinge.

a Suture Whorl Body whorl Aperture Umbilicus b Apex Spire Columella Body whorl Inner lip Outer lip

Most of the pelecypod shell is of calcium carbonate, but the outer layer, or periostracum, of each valve is composed of horny material. The inner surface of the shell is lined with a calcareous layer of pearly or porcelaneous material.

STRAPAROLUS × 1 AMPHISCAPHA × 1 WORTHENIA × 1 TREPOSPIRA × 1 BELLEROPHON × 1 EUOMPHALUS × 1

EUPHEMITES × 1 STROBEUS × 1 PLATYCERAS × 1

GYRODES × 1 LUNATIA × 1 TURRITELLA × 1 CERITHIUM × 1 TYLOSTOMA × 1 NERINEA × 1

DISTORSIO × 1 MESALIA × 1 FUSUS × 1 COCHLESPIROPSIS × 1 TURRITELLA × 1 LATIRUS × 1 CONUS × 1 VERTAGUS × 1 PSEUDOLIVA × 1

ANCILLA × 1 ARCHETECTONICA × 1 TUBA × 1 CALYPTRAPHORUS × 1 SYCOSTOMA × 1 SURCULA × 1 VOLUTOLITHES × 1 NEVERITA × 1 LEVIFUSUS × 1

The outline of the shell may vary greatly, but most pelecypods are typically clam-like. However, certain forms are round, others are long and narrow, and some have wing-like structures. Most pelecypods have a beak which represents the oldest part of the shell. The beak is commonly located on the anterior (front) end of the shell, and the end of the shell opposite this is designated posterior (the rear). The hinge and ligament are located dorsally (along the top), and the lower margin of the shell where the valves open is called ventral (fig. 17a).

a Dorsal Beak Anterior Posterior Concentric growth rings Ventral b Hinge teeth Cardinal teeth Anterior muscle scar Posterior muscle scar Mantle line

The inner surface of the shell has certain markings which, along with the shell form and dentition (the nature and arrangement of the teeth and sockets), are important in classification. Muscle scars are present on the inside of most valves; the anterior muscle scars are located near the front of the shell, and the posterior muscle scars are situated near the rear of the shell. These scars mark the place of attachment of muscles which were used to close the shell and aid in locomotion. Along the ventral margin of some shells there is a line or scar which extends from the anterior muscle scar to the posterior muscle scar. This is known as the mantle line or pallial line and marks the place of attachment of the mantle—a soft membranous layer that enclosed the body of the animal. In some pelecypods the dorsal margin of one valve bears a series of hinge teeth which articulate with a similar set of sockets on the other valve (fig. 17b). In addition to hinge teeth, certain species have cardinal teeth which are located below and in front of the hinge teeth.

The exterior of most shells is marked by a series of concentric growth lines (fig. 17a) which mark points of periodic addition of shell material. The external surface of many shells is also marked by various types of ornamentation, such as ribs, nodes, spines, and grooves.

Fossil collectors commonly find only one valve of the pelecypod shell. This is because the shell normally opens when the animal dies, and the valves may easily become separated. Fossil pelecypods are also commonly preserved as external and internal molds, and these are found in fossiliferous strata of almost all ages. Some pelecypods of Pennsylvanian, Mesozoic, and Cenozoic age are found with original shell material that appears to have undergone very little change. Fossil pelecypods are abundant and varied in Texas and are found in most of the fossiliferous formations of the Pennsylvanian, Cretaceous, and Tertiary systems (Pls. 24-31).

CLASS CEPHALOPODA.—

These are marine mollusks with or without chambered or solid shells which may be internal or external. The living animal possesses a well-developed head with eyes, horny jaws, and many tentacles fused with the foot. Cephalopods are the most advanced of all mollusks and include the squid, octopus, pearly nautilus, and the extinct ammonoids. Members of this class range from Cambrian to Recent in age but were much more abundant in ancient seas than they are today. Their remains constitute a very useful group of fossils, particularly in Paleozoic and Mesozoic rocks.

Most paleontologists have divided the Cephalopoda into three subclasses, the Nautiloidea, Ammonoidea, and the Coleoidea (known also as subclass Dibranchiata and subclass Decapoda); each of these is discussed below.

Subclass Nautiloidea.—

The nautiloids are cephalopods with external chambered shells in which the septa (dividing partitions) are simple and have smooth edges. This subclass is represented by a single living genus, Nautilus, and a large number of fossil forms.

In the living Nautilus the shell is composed of calcium carbonate and is coiled in a flat spiral (fig. 18). The interior of the shell is divided into a series of chambers by calcareous partitions called septa. The point where each septum joins the inner surface of the shell is known as the suture. These suture lines (fig. 19a) are not visible from the outside unless the outer shell has been removed, but they are visible on the internal molds of many fossil cephalopods and are of great importance in nautiloid and ammonoid classification. Nautiloids have very simple smoothly curved suture patterns, but ammonoids are characterized by more complex and wrinkled sutures (fig. 19d).