Chapter 6

Fig. 12.Two types of bryozoans or “moss animals.” (a) Section of the lacy type bryozoan. (b) The spiral axis ofArchimedes(Mississippian).

Bryozoans arecolonialanimals 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-likeexoskeletoncomposed ofcalcareousorchitinousmaterial. These little chambers, known aszooecia(orautopores), 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 somefossilcolonies 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 thefossiliferousstrata of Texas. Undoubted bryozoan fossils have been recorded in rocks of LowerOrdovicianage, but questionableCambrianforms have also been reported. Bryozoans are abundant in the seas of today, but only a few forms inhabit fresh waters.

Plate 14

SPONGE SPICULES(GREATLY ENLARGED)

PALEOZOICSPONGES

Plate 15PENNSYLVANIANCORALS

Plate 16

CRETACEOUSCORALS

TERTIARYCORALS

In Texas one may expect to find bryozoan remains in thePennsylvanianrocks 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 someCretaceousandTertiarybeds, but their remains are small and fragmental and they are easily overlooked. Bryozoans have also been found matted on the shells offossilmollusks and other invertebrates.

The brachiopods are a large group of exclusively marine organisms with shells composed of two pieces calledvalves(fig. 13). These valves are usually composed ofcalcareousorphosphaticmaterial and enclose and protect the soft parts of thebrachiopodanimal. The soft parts are composed of muscles, themantle(which secretes the valves), digestive,respiration, reproductive, and excretory organs, and the tentacle-bearinglophophore.

In adult life thebrachiopodis attached to the sea bottom by means of a fleshy stalk called thepedicle(fig. 14), and this is usually extruded through a hole (thepedicleforamen) which is located in theventralorpediclevalve. The upturned area which is usually present on the pedicle valve is called thebeak. The other valve, known as thedorsalorbrachialvalve, 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,fossilbrachiopods are typically found with valves closed.

Brachiopods vary greatly in size and shape and exhibit a widevarietyof ornamentation, such as spines, ribs, nodes, and other structures. They are abundant fossils in many of thePaleozoicrocks of Texas but are relatively rare inMesozoicandCenozoicformations.

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

Fig. 13.Morphology and principal parts of articulate brachiopods.

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 ahinge-lineis lacking (fig. 14). Most inarticulate brachiopods are circular or tongue-like in shape and commonly composed ofchitinousandphosphaticmaterial. Inarticulate brachiopods range from LowerCambrianto 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 severalPaleozoicformations in Texas (Pl. 17,Lingula,Apsotreta,Angulotreta).

Plate 17

PENNSYLVANIANBRYOZOANS

CAMBRIANBRACHIOPODS

MISSISSIPPIANBRACHIOPODS

Fig. 14.Lingula, a typical Recent inarticulatebrachiopodshowing extended pedicle.

Articulate brachiopods have a well-definedhinge-line(fig. 13a). Onevalvehas well-developed teeth which articulate with sockets in the opposing valve, and there is a well-developed musclesystemwhich aids in opening and closing the shell. Articulate brachiopods are characterized bycalcareousshells which are typically of unequal size and a widevarietyof shapes (Pls.18,19). The class has been divided into several orders which have been established primarily on the nature of the pedicleforamenand the nature of shell growth.

Articulate brachiopods range from LowerCambrianto Recent in age and are particularly abundant in certainPennsylvanianformations of north-central and Trans-Pecos Texas. They are also present in certain otherfossiliferousstrata ofPaleozoicage but are less abundant and not as well preserved. The onlyCretaceousbrachiopodthat is found in large numbers isKingena wacoensis(Roemer) (fig. 15), which is particularly abundant in certain formations in the upper part of the Comancheseries.

Fig. 15.Kingena wacoensis, a commonCretaceousbrachiopod. (a)Dorsalview. (b)Lateralview. (c)Ventralview.

ThephylumMollusca 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 acalcareousshell that serves as anexoskeleton, 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 greatvariety, mollusks are particularly useful fossils. Moreover, the remains of certain mollusks, such as the oysters, are importantrockbuilders.

ThephylumMollusca has been divided into five classes:

1.Amphineura—the chitons or sea-mice; shell composed of eight valves or plates; not a commonfossil.Ordovicianto Recent.

2.Scaphopoda—the tusk-shells; shell composed of a single tusk-likevalve; generally not a commonfossilbut locally abundant in certainCenozoicformations.Devonianto Recent.

3.Gastropoda—the snails and slugs; slugs are without shells, snails have a single-valved shell which is typically coiled; common fossils inPaleozoic,Mesozoic, andCenozoicrocks.Cambrianto Recent.

4.Pelecypoda—clams, mussels, oysters, scallops; shells composed of two valves, usually, but not always, of equal size; common fossils, especially inMesozoicandCenozoicrocks.Cambrianto Recent.

5.Cephalopoda—squids, octopuses, the pearly nautilus, and the ammonoids (extinct); shell of onevalve, usually coiled and partitioned by septa; valuable fossils, especially inPaleozoicandMesozoicrocks. ?Cambrian,Ordovicianto Recent.

Plate 18PENNSYLVANIANBRACHIOPODS

Plate 19PENNSYLVANIANBRACHIOPODS

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

The typicalgastropodhas 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.

Gastropodshells, both Recent andfossil, exhibit a greatvarietyof 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 (thecolumella). The closed pointed end of the shell is called theapex, and each turn of the shell is called awhorl(fig. 16). The last-formed and largest whorl is called thebody whorl, and this whorl contains theaperture—the opening of the shell. The combined whorls exclusive of the body whorl are known as thespire. The inner and outer margins of the aperture are designated theinner lipand theouter lip, respectively. In some snails the aperture is closed by means of theoperculum—acalcareousor 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 theumbilicus(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 thecallus.

Many gastropods, particularly those of the TexasCretaceous, 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 withsediment. This filling later becomes solidified, and the outer shell may eventually be removed by weathering or solution. This type of internal mold is called asteinkernand normally does not reflect any external shell characteristics (Pl. 2). In some of thePennsylvanianandTertiaryformations, however, gastropods may be collected with the original shell in an excellent state of preservation.

Plates20-23 illustrate some typicalPaleozoic,Mesozoic, andCenozoicgastropods.

The pelecypods possess a shell composed of twocalcareousvalves (fig. 17) which enclose the soft parts of the animal. Members of this class live exclusively in an aquatichabitatand 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 orPecten, are swimmers. The Pelecypoda include such familiar saltwater forms as the clams and oysters, as well as the common fresh-water mussel. Pelecypods range fromCambrianto Recent in age but are more abundant inMesozoicandCenozoicrocks.

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

The typicalpelecypodvalves 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 thedorsal(top) side of the shell. In addition to the ligament,most forms haveteethandsocketswhich are located along thehinge-line. The teeth in onevalvearticulate with the sockets in the opposite valve, and this arrangement gives strength to the hinge.

Fig. 16.Morphology and principal parts ofgastropodshells. (a) Low-spired form withumbilicus. (b) Section of spirally coiled shell showingcolumella.

Most of thepelecypodshell is of calcium carbonate, but the outer layer, orperiostracum, of eachvalveis composed of horny material. The inner surface of the shell is lined with acalcareouslayer of pearly orporcelaneousmaterial.

Plate 20PENNSYLVANIANGASTROPODS

Plate 21

PENNSYLVANIANGASTROPODS

CRETACEOUSGASTROPODSINTERNAL MOLDS

Plate 22TERTIARYGASTROPODS

Plate 23TERTIARYGASTROPODS

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. Thebeakis commonly located on theanterior(front) end of the shell, and the end of the shell opposite this is designatedposterior(the rear). The hinge and ligament are located dorsally (along the top), and the lower margin of the shell where the valves open is calledventral(fig. 17a).

Fig. 17.Morphology and principal parts of a typicalpelecypodshell. (a) Exterior view. (b) Interior view.

The inner surface of the shell has certain markings which, along with the shell form anddentition(the nature and arrangement of the teeth and sockets), are important in classification. Muscle scars are present on the inside of most valves; theanteriormuscle scarsare located near the front of the shell, and theposteriormuscle scarsare 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 theventralmargin 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 themantlelineorpallial lineand marks the place of attachment of themantle—a soft membranous layer that enclosed the body of the animal. In some pelecypods thedorsalmargin of onevalvebears aseriesofhinge teethwhich articulate with a similar set of sockets on the other valve (fig. 17b). In addition to hinge teeth, certainspecieshavecardinal teethwhich are located below and in front of the hinge teeth.

The exterior of most shells is marked by aseriesofconcentricgrowth 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.

Fossilcollectors commonly find only onevalveof thepelecypodshell. 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 infossiliferousstrata of almost all ages. Some pelecypods ofPennsylvanian,Mesozoic, andCenozoicage 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, andTertiarysystems (Pls.24-31).

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 fromCambrianto 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 inPaleozoicandMesozoicrocks.

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.

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

In the livingNautilusthe shell is composed of calcium carbonate and is coiled in a flat spiral (fig. 18). The interior of the shell is divided into aseriesofchambersbycalcareouspartitions calledsepta. The point where eachseptumjoins the inner surface of the shell is known as thesuture. Thesesuture 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 manyfossilcephalopods 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).

Although the shell of the only type of living nautiloid is coiled, many of the early forms had straight cone-shaped shells (Orthoceras,Pl. 32), and these are common in some of thePennsylvanianformations of Texas.Fossilcoiled nautiloids may be collected in certain of theCretaceousandTertiarystrata of the State, but their remains are not common.Cymatoceras(Pl. 32) is a coiled fossil nautiloid from the Cretaceous of north Texas.

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