Chapter IV: Introduction (3)

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

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

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).

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 the Pennsylvanian formations of Texas. Fossil coiled nautiloids may be collected in certain of the Cretaceous and Tertiary strata of the State, but their remains are not common. _Cymatoceras_ (Pl. 32) is a coiled fossil nautiloid from the Cretaceous of north Texas.

SCHIZODUS × 1
MYALINA × ½
ASTARTELLA × 1
NUCULOPSIS × 1
ALLORISMA × 1
NUCULANA × 1
YOLDIA × 1
PINNA × ½

PROTOCARDIA × 1
ALECTRYONIA LUGUBRIS × 1
PLICATULA × 1
PECTEN × 1

GRYPHAEA WASHITAENSIS × 1
GRYPHAEA GRAYSONANA × 1
INOCERAMUS × 1
TRIGONIA × 1

EXOGYRA ARIETINA × 1
EXOGYRA LAEVISCULA × 1
NEITHEA × 1
EXOGYRA PONDEROSA × 1
EXOGYRA TEXANA × 1

PACHYMYA × ½
OSTREA CARINATA × 1
OSTREA QUADRIPLICATA × 1
PHOLADOMYA × 1

LIMA × 1
OSTREA LISBONENSIS × 1
PITAR × 1
VENERICARDIA BULLA × 1
PACHECOA × 2
PHOLADOMYA × 2
OSTREA SELLAEFORMIS × 1
CRASSATELLA × 1

ORTHOYOLDIA × 1
TELLINA × 1
VOKESULA × 2
NUCULA × 2
VENERICARDIA × ½

PLICATULA × 1
PECTEN × 1
ANOMIA × 1
GLYCYMERIS × 2
CARYOCORBULA × 2
BARBATIA × 1

b
Living chamber
Aperture
Septa
Siphuncle
Protoconch
Chamber

Subclass Ammonoidea.—

The ammonoids are a group of extinct cephalopods which are related to the nautiloids but are characterized by more complex suture patterns. Members of this subclass have an external partitioned shell which is straight, curved, or spirally coiled (Pl. 33). This group of cephalopods first appeared in Devonian time, became extremely abundant and varied during the Mesozoic, and was extinct by the end of the Cretaceous period.

Most Paleozoic ammonoids are characterized by a combination curved and angular suture pattern, and this type of suture pattern is referred to as _goniatitic_ (fig. 19b). Sutures that are curved and crenulated (marked in places by a series of tooth-like indentations) are known as _ceratitic_ (fig. 19c). Ceratitic sutures first appeared in the Mississippian, became increasingly abundant during the Triassic but were much less abundant during the Cretaceous. The _ammonitic_ suture has a very complexly subdivided pattern (fig. 19d). Cephalopods with ammonitic sutures range from Pennsylvanian to Cretaceous in age and were the most abundant cephalopods of the Mesozoic.

PHANEROCERAS × 1
ORTHOCERAS × 1

METOICOCERAS × ½
CYMATOCERAS × ½

TEXANITES × ½
ACANTHOCERAS × ½
TURRILITES × ½
DUFRENOYIA × ½
OXYTROPIDOCERAS × ½
BACULITES × ½
BELEMNITE × ½

Ammonoids are locally abundant in many of the fossiliferous rocks of Texas and are among the more useful Mesozoic guide fossils. Goniatites may be found in the Pennsylvanian of north-central and Trans-Pecos Texas, and ammonoids with the ceratitic suture pattern can be collected from the Lower Cretaceous of many parts of the State. Cephalopods exhibiting the typical ammonitic suture pattern are abundant in many of the Cretaceous rocks of Texas, and these fossils have contributed much toward an understanding of the Cretaceous strata of this State.

Subclass Coleoidea.—

These are squid-like cephalopods characterized by an internal shell or no shell at all. Included in this group are the squids, cuttlefish, octopuses, and the extinct belemnoids, but of these only the belemnoids are useful fossils. Members of this subclass range from Mississippian to Recent in age.

Order Belemnoidea.—

The belemnoids appear to be the oldest and most primitive of the coleoid cephalopods. Their earliest known occurrence is in rocks of Mississippian age, and they were particularly abundant during the Mesozoic. They became extinct at the end of Cretaceous time but have left considerable evidence of their existence in the Mesozoic strata of many parts of the world. Certain forms, because of their abundance and relatively short geologic range, are excellent guide fossils. Belemnoids have been found in the Upper Cretaceous of Texas (Pl. 33) but in general are rare or unknown in most Texas formations.

Phylum Annelida

Members of the phylum Annelida include the segmented worms such as the common earthworm. Annelids are marine, fresh water, or terrestrial and have apparently been common through much of geologic time. Because of their lack of hard parts, most of these worms have left little direct fossil evidence of their activities in the geologic past. Some annelids secrete straight or coiled calcareous tubes, and fossil worm tubes of this sort (fig. 20) are commonly found attached to brachiopods, mollusks, and other objects. Tubes of this nature have been reported from Paleozoic, Mesozoic, and Cenozoic rocks in Texas.

Some annelids have small chitinous jaws and teeth which also may be preserved as fossils. These dental structures are called scolecodonts and are microfossils.

Phylum Arthropoda

The arthropods are one of the more advanced groups of invertebrates, and they are known from the Cambrian to the Recent (Pl. 34). Modern representatives of this group include the crabs, shrimp, crayfish, insects, and spiders. Arthropods vary greatly in size and shape and are among the most abundant of all animals. They have become successfully adapted to a wide variety of environments and live on land, in water, and in the air. The typical arthropod has a segmented body which is usually covered by a chitinous exoskeleton which, in some forms, contains additions of calcium carbonate. They are highly specialized and well-developed animals in which locomotion is by means of paired jointed appendages.

Although the arthropods are of great importance in nature today, only a few groups are of importance to the paleontologist. Only two of these, the trilobites and the ostracodes, are discussed herein.

Subphylum Trilobitomorpha

The members of this subphylum are extinct arthropods which were most abundant during early Paleozoic time.

CLASS TRILOBITA.—

The trilobites are a group of exclusively marine arthropods which derive their name from the typical three-lobed appearance of their bodies (fig. 21a). The trilobite body is divided into a _central_ or _axial_ lobe and two _lateral_ lobes. The body of the animal was encased in a chitinous exoskeleton. The top part of this exterior covering, the _carapace_, is very thick, and it is this part of the trilobite that is usually preserved.

FOSSIL INSECT × 1
FOSSIL CRUSTACEANS
ENOPLOCLYTIA × 1⅓
ASTACODES × ¾
NOTOPOCORYSTES × 2
OSTRACODES × 40

Cephalon
Thorax
Pygidium
Axial lobe
Lateral lobes

The body is also divided into three parts from front to back. Beginning at the front of the animal these divisions are the _cephalon_ or head, the _thorax_ or abdomen, and the _pygidium_ or tail (fig. 21a). The body segments of the thorax were arranged in such a manner as to permit the animal to roll up into a ball, and many trilobites are found in this position (fig. 21b).

Trilobites first appeared in the Cambrian and were extinct by the end of the Permian. They occur sparingly in certain of the Paleozoic rocks of Texas and when found are likely to be fragmental and in a poor state of preservation.

Subphylum Crustacea

The crustaceans are the crabs, shrimp, crayfish, and ostracodes. Although not abundant, fossil crabs have been described from certain Cretaceous and Tertiary formations of the State (_Notopocorystes_, Pl. 34). However, the most useful and abundant crustacean fossils are the members of the class Ostracoda.

CLASS OSTRACODA.—

The ostracodes are small, bivalved, aquatic crustaceans which have the external appearance of small clams (Pl. 34). The remains of these tiny animals are so small that they are best studied under a low-power microscope, and because of their small size they are particularly useful to the micropaleontologist.

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.

CLASS CYSTOIDEA.—

These are primitive attached echinoderms which were relatively common in early Paleozoic time. The typical cystoid has a somewhat globular or sac-like _calyx_ (the main body skeleton) composed of numerous, irregularly arranged, calcareous plates (fig. 22b). The plates composing the calyx are usually perforated by pores or slits which were probably used in excretion or respiration. The calyx was attached to the sea bottom by a short stem.

Cystoids range from Cambrian to Devonian in age and were especially abundant during Ordovician and Silurian time. Their remains are rare or absent in the rocks of Texas.

CLASS BLASTOIDEA.—

The blastoids are extinct short-stemmed echinoderms with a small, symmetrical, bud-like calyx. The blastoid calyx is composed of 13 calcareous plates arranged in a typical five-sided pattern (fig. 22a). The _mouth_ is located in the center of the calyx and is surrounded by five openings called _spiracles_. Five distinct _ambulacral_ or _food grooves_ radiate outward from the mouth.

Blastoids range from Ordovician to Permian in age and were especially abundant during the Mississippian period. No blastoids have been reported from any of the rocks of Texas.

CLASS CRINOIDEA.—

The crinoids are commonly called _sea-lilies_ because of their flower-like appearance. The _calyx_ is composed of symmetrically arranged calcareous plates, and most crinoids have a long stem. Other crinoids are free-swimming in the adult stage and are attached only during the earlier phases of their development.

The crinoid calyx is typically cup-shaped (fig. 23) and five grooves radiate out from its center. These grooves continue outward along the complexly segmented arms and are used as channels to convey food to the mouth.

Calyx
Arm
Plate
Stem
Columnal
Cirri
Holdfast (root)

The crinoid stem is attached to the base of the calyx and serves for purposes of support and attachment. This stem consists of a relatively long flexible stalk composed of numerous calcareous disk-shaped segments called _columnals_ (fig. 23; Pl. 35), each of which contains a round or star-shaped opening in its center. Many crinoids have very long stalks (some are as much as 50 feet in length), and when the animal dies the columnals become separated and are scattered about on the ocean floor. Many Paleozoic limestones contain such great numbers of crinoid columnals that they are referred to as _crinoidal limestones_ (fig. 8). Crinoidal limestones occur in some of the Mississippian and Pennsylvanian formations of central Texas and in the Pennsylvanian of north-central and Trans-Pecos Texas.

The stalk is attached to the sea floor or some other object by means of a root system called the _holdfast_ (fig. 23). This structure commonly branches out into the surrounding sediments, and in this manner the crinoid animal is firmly anchored to the bottom of the sea.

Crinoids, like most echinoderms, are gregarious animals—that is, they commonly live together in large numbers, and for this reason great numbers of crinoid remains are commonly found concentrated in relatively small local areas. Most fossil crinoids are found as stem fragments because the more fragile calyx and root system are less likely to be preserved.

The earliest known crinoids have been found in rocks of Ordovician age, and their remains are particularly abundant in Paleozoic rocks. Crinoids are living today but most of them are stemless free-swimming forms called “feather stars,” much less abundant than their Paleozoic ancestors.

Subphylum Eleutherozoa

The eleutherozoans are free-swimming, bottom-dwelling, echinoderms which have been divided into two classes. The class Asterozoa (star-shaped echinoderms) contains the subclasses Asteroidea (the starfishes) and the Ophiuroidea (the brittle stars). Although they are known as fossils, neither of these groups is of paleontological importance. The class Echinozoa (echinoderms without laterally directed arm-like extensions) contains the subclasses Echinoidea (the sea urchins and sand dollars) and Holothuroidea (the sea cucumbers). Of these two subclasses, only the Echinoidea are useful fossils.

CLASS ASTEROZOA.—

These are typical star-shaped free-moving echinoderms in which the body is divided into a central disk and radiating arms.

Subclass Asteroidea.—

This class contains the starfishes which, although not common fossils, illustrate well the typical echinoderm characteristics (Pl. 35). Fossil starfishes have been found sparingly in certain formations in Texas, but well-preserved specimens are quite rare. However, excellently preserved starfishes have been found in slabs of Cretaceous limestones from central and north-central Texas.

Subclass Ophiuroidea.—

The ophiuroids are echinoderms with a well-defined central disk and five long, slender, whip-like arms. They have been called brittle stars because of their ability to shed their arms when they are disturbed. Their long, slender, snake-like arms have also resulted in their being called serpent stars. Ophiuroids range from Ordovician to Recent in age, but because of the delicate nature of their bodies they are seldom found as fossils. Ophiuroid remains have been found in certain Mesozoic and Cenozoic rocks of Texas, but they consist largely of small segments of the arms or body fragments.

CLASS ECHINOZOA.—

The echinozoans are a group of unattached echinoderms whose bodies consist of numerous calcareous plates and spines, but they do not possess the radiating arm-like extensions which characterize the asterozoans.

Subclass Echinoidea.—

Echinoids are free-moving echinoderms with disk-shaped, heart-shaped, biscuit-shaped, or globular exoskeletons (Pl. 36). Modern representatives of this group include the familiar sea urchins, heart urchins, and the sand dollars.

CRINOIDS
CRINOID CALYX × ½
CRINOID COLUMNALS × 1
HOLOTHURIAN SCLERITES (GREATLY ENLARGED)
CRETACEOUS FOSSIL STARFISHES
PENTAGONASTER × 1
PENTACEROS × 1

SALENIA × 1
ECHINOID SPINES × 2
ECHINOID PLATE × 2
HEMIASTER × 1
HOLASTER × 1
HOLECTYPUS × 1

The echinoid _test_ (exoskeleton) is composed of many intricately fitting calcareous plates (Pl. 36) which enclose the animal’s soft parts. The exterior of the test is typically covered with large numbers of movable spines (Pl. 36) which vary greatly in size. These spines are of some aid in locomotion, support the skeleton of the animal, and provide a measure of protection from enemies.

The oldest known echinoids have been recorded from rocks of Ordovician age, but it was not until the Mesozoic that the group began to flourish. They were especially abundant during the Cretaceous and have been abundant and varied from that time until the present.

Echinoids are particularly numerous in many of the Lower Cretaceous formations of Texas where they are commonly found in an excellent state of preservation. Heart urchins and biscuit urchins may be found in large numbers in many areas of the State, and especially in areas where there are good exposures of fossiliferous Lower Cretaceous rocks.

Subclass Holothuroidea.—

Members of this class, commonly called _sea cucumbers_, have a rather elongate, sac-like, cucumber-shaped body and bear little resemblance to other members of the phylum Echinodermata. The sea cucumbers do not have a well-defined skeleton; rather the body is supported by many small, disconnected, calcareous plates or rods called _ossicles_ or _sclerites_ (Pl. 35). These minute structures are embedded in the leathery skin which covers the body of the sea cucumber and may be preserved as fossils. Such remains are locally abundant in certain formations in Texas, but because of their small size, scattered occurrence, and problems in classification, this group is of little use to most paleontologists.

Holothuroid body impressions have been reported from the Middle Cambrian, and sclerites from rocks as old as Mississippian.

Phylum Chordata

The chordates are the most advanced of all animals and are characterized by the presence of a well-developed nervous system and a body supported by a bony or cartilaginous _notochord_ and/or _spinal column_. In the higher chordates (the vertebrates) the notochord is normally replaced by bone, but in the lower chordates (for example, the graptolites) it remains in a cartilaginous condition.

The phylum Chordata contains only two subphyla of paleontological significance. These are the subphylum Hemichordata, composed of primitive chordates (including the graptolites which are important fossils), and the Vertebrata, which includes all animals with backbones.

Subphylum Hemichordata

The hemichordates are characterized by a well-defined notochord which runs the length of the body, but they do not possess a true backbone. Only one class, the Graptolithina, is of paleontological importance.

CLASS GRAPTOLITHINA.—

The graptolites are a group of extinct colonial animals which were very abundant during early Paleozoic time. They are characterized by a chitinous exoskeleton consisting of rows of cups or tubes which housed the living animal. These cups are attached to single or branching stalks (fig. 24) which in some forms were attached to sea weeds, rocks, or other foreign objects where they led a fixed existence. The stalks of the unattached graptolites grew on floats (fig. 24a) and these floating forms attained wide geographic distribution. It is also possible that some of the attached forms were fixed to floating objects, such as sea weed, and thus were distributed in this manner.

Previous classifications have recognized the graptolites as members of the phylum Coelenterata. As coelenterates they were assigned, at various times, to the classes Hydrozoa, Scyphozoa, and Graptozoa. In addition, they were also classified as bryozoans by certain of the early paleontologists. This publication, in keeping with recent changes in taxonomy, considers graptolites to be an extinct group of hemichordates. This classification is based upon research in which uncompressed graptolites were etched out of chert and studied in great detail. Information derived from these relatively undistorted specimens indicates a much higher degree of body organization than was previously suspected, and as a result of these studies most paleontologists now consider graptolites to be some form of primitive chordate.

The chitinous graptolite exoskeleton is commonly preserved as a flattened carbon residue; their remains may be locally abundant along the bedding planes of certain black or dark gray shales.

Graptolites are known from rocks that range from Cambrian to Mississippian in age, and they are among the most important guide fossils for Ordovician and Silurian rocks.

Graptolites have been reported from Cambrian rocks in central Texas and from the Ordovician of west Texas (fig. 24). The most abundant of these occur in certain Ordovician rocks in the Trans-Pecos area where they are common fossils in certain formations.

Subphylum Vertebrata

The vertebrates are the most advanced of all chordates. They are characterized by a skull and a bony or cartilaginous _internal skeleton_, with a _vertebral column_ of bone or cartilage. This subphylum is commonly divided into two superclasses, the Pisces (the fishes and their relatives) and the Tetrapoda (the four-footed animals).

As mentioned earlier, most amateur collectors collect very few vertebrate remains, and for this reason this group is not discussed in detail. However, the more important vertebrate classes are briefly reviewed to enable the reader to have some understanding of this important group of animals. This part of the handbook will also serve as an introduction to some of the interesting and unusual, but now extinct, animals that have inhabited Texas in the geologic past. Among these animals are giant fishes, primitive amphibians, and many different types of dinosaurs. Included also are such unusual mammals as the giant ground sloths, saber-tooth cats, mammoths, and mastodons, all of which are now extinct. The remains of these, and many other interesting extinct vertebrates, may be seen in the geological collections of the Texas Memorial Museum at Austin. Many of these displays are accompanied by drawings which depict the scientific restoration of the animal’s soft parts and show how the animal may have appeared in life.

Superclass Pisces

The members of this superclass are commonly called fishes and are the simplest and most numerous of all vertebrates. They are aquatic, free-moving, cold-blooded (their blood maintains the temperature of the surrounding water), and breathe primarily by means of gills. However, some forms (the lungfishes) breathe by means of a lung developed from the air-bladder.

The most recent fish classification recognizes four major classes, the Agnatha (primitive jawless fishes), the Placodermi (armored fishes with primitive jaws), the Chondrichthyes (sharks and related forms with cartilaginous internal skeletons), and the Osteichthyes (true bony fishes).

CLASS AGNATHA.—

Fishes belonging to this class are primitive, jawless, and represented by the living lampreys and hagfishes. The first agnathans appeared in the Ordovician and were armored by a bony covering on the front part of their bodies. These primitive fishes, called _ostracoderms_, are the earliest recorded fishes and, in addition, appear to be the first known vertebrate animals. The ostracoderms first appeared in late Ordovician time, increased in numbers in the Silurian, and were extinct by the end of the Devonian.

CLASS PLACODERMI.—

These are primitive jaw-bearing fishes, the majority of which were heavily armored (Pl. 37). The _placoderms_ were shark-like in appearance, and some of them grew to be as much as 30 feet in length. Members of this class appeared first in the Devonian and lasted into the Permian, at which time they became extinct. Placoderms are rare in Texas, but the fragmentary remains of these primitive fishes have been found in Devonian rocks in central Texas.

CLASS CHONDRICHTHYES.—

This class includes such modern forms as the sharks, rays, and skates. They are characterized by skeletons which are composed of cartilage and are very abundant in the marine waters of today. The earliest known representatives of this class are reported from rocks of Devonian age, and they have been relatively common up to the present time.

Shark teeth (Pl. 37) can be found in Texas in Pennsylvanian, Permian, Cretaceous, Paleocene, Eocene, and Miocene rocks. These are probably the most common vertebrate fossils to be found in Texas and are usually found in thin-bedded marine limestones or clays.

CLASS OSTEICHTHYES.—

The Osteichthyes includes the true bony fishes, which are the most highly developed and abundant of all fishes. They possess an internal bony skeleton, well-developed jaws, an air-bladder, and, typically, an external covering of overlapping scales.

Included in this class are a primitive group of fishes called _crossopterygians_. These were very abundant in the Devonian and are believed to be the ancestors of the amphibians. The modern lungfishes also belong to the class Osteichthyes, and these primitive fishes, which are now found only in Australia, South America, and Africa, breathe by means of gills and lungs which have been developed from the air-bladder. Although not abundant as fossils, the remains of these specialized fishes have added much to present knowledge concerning the development of certain of the higher vertebrates.

The remains of bony fishes have been collected at many localities in Texas, and fossils of this type have been found primarily in rocks of Cretaceous age but have been reported from other rocks as well. Fish fossils are more commonly found in the form of teeth (Pl. 37), vertebrae, scales, and an occasional well-preserved skeleton.

_Conodonts_ (Pl. 37) are small, amber-colored, tooth-like fossils which are believed to represent the teeth of some type of extinct fish. Although geologists do not know a great deal about the origin of these strange fossils they are of value in micropaleontology. Conodonts have been reported from several Paleozoic formations in Texas and are useful guide fossils in some areas.

Superclass Tetrapoda

The tetrapods are the most advanced chordates and are typified by the presence of lungs, a three- or four-chambered heart, and paired appendages. Included here are the classes Amphibia (frogs, toads, and salamanders), Reptilia (lizards, snakes, turtles, and the extinct dinosaurs), Aves (birds), and Mammalia (including the mammals, such as men, dogs, whales, etc.).

CLASS AMPHIBIA.—

The amphibians were the earliest developed four-legged animals and are represented by such living forms as the toads, frogs, and salamanders. Amphibians are cold-blooded animals that primarily breathe by lungs and spend most of their life on land, but during their early stages of development they live in the water where they breathe by means of gills.

The amphibians apparently developed from the crossopterygian fishes during late Devonian time and were relatively abundant in the Pennsylvanian, Permian, and Triassic.

Amphibian remains in Texas are confined largely to lower Paleozoic and upper Mesozoic rocks. Numerous interesting and important discoveries of fossil amphibians have been made in north and west Texas where their remains (Pl. 40) have been collected in association with early types of reptiles. The areas where Permian red beds are exposed in Archer and Baylor counties and where Triassic red beds are exposed from Big Spring north along the edge of the High Plains have furnished most of these specimens.

CLASS REPTILIA.—

The reptiles have become adapted to permanent life on land and need not rely on an aquatic environment. They are cold-blooded and are normally characterized by a scaly skin. Reptiles have been much more abundant in the past than they are today, and they assumed many different shapes and sizes in the geologic past. Modern classifications recognize a large number of reptilian groups, but only the more important of these are briefly reviewed here.

Cotylosaurs.—

These were a group of primitive reptiles which, although retaining some amphibian characteristics, became adapted to an exclusively land-dwelling existence. The cotylosaurs lived during the Pennsylvanian and Permian and apparently became extinct sometime during the late Permian. Cotylosaurs (Pl. 40) are well known from the Permian of north Texas.

Turtles.—

These are reptiles in which the body is more or less completely enclosed by bony plates. This group is first known as fossils from late Triassic rocks of Europe, and modern representatives of the group include the turtles and tortoises. Fragmentary remains of turtle shells are among the most common vertebrate fossils found in the Tertiary. Some of the late Tertiary land tortoises were 3 to 4 feet long. The earliest known turtles in Texas have been found in Cretaceous rocks.

Pelycosaurs.—

The pelycosaurs were a group of late Paleozoic reptiles some of which were characterized by the presence of a tall fin on their back (Pl. 40). The fossils of these unusual creatures are well known from the Permian red beds of north-central Texas.

Therapsids.—

The therapsids were a mammal-like group of reptiles which were well developed for a terrestrial existence. Although the remains of these primitive reptiles are not particularly important fossils, study of the therapsids has provided much valuable information about the origin of the mammals. Members of this group appeared first in the middle Permian and persisted until the middle Jurassic, but therapsid remains have not been reported from Texas.

DINOSAUR SAURISCHIANS ORNITHISCHIANS
STOCKS

DINOSAUR THEROPOD SAUROPOD ORNITHOPOD STEGOSAUR CERATOPSIAN ANKYLOSAUR
TYPES
POSTURE BIPEDAL QUADRUPEDAL BIPEDAL AND QUADRUPEDAL QUADRUPEDAL QUADRUPEDAL
QUADRUPEDAL
ARMOR UNARMORED EXCEPT UNARMORED UNARMORED (SPEED BONY PLATES HORNS, BONY KNOBS AND SPIKES
FOR HUGE SHARP WAS CHIEF ALONG BACK, PLATE OVER NECK OVER DORSAL AREA,
TEETH DEFENSE) SPIKED-TAIL CLUB-LIKE TAIL
DIET CARNIVOROUS HERBIVOROUS HERBIVOROUS HERBIVOROUS HERBIVOROUS HERBIVOROUS
OTHER LARGE HEAD WITH HUGE BODY, LONG SLENDER-BUILD, SHORT NECK, SHORT NECK, “ARMADILLO-LIKE”
DESCRIPTIVE POWERFUL JAW, NECK AND TAIL, “DUCK-BILLED” LONG TAIL, STOCKY BUILD APPEARANCE
REMARKS GREATLY REDUCED SMALL HEAD SMALL HEAD
FORE-LIMBS
EXAMPLES CERATOSAURUS-J BRACHIOSAURUS-J PARASAUROLOPHUS-K _STEGOSAURUS-J_ STYRACOSAURUS-K PALEOSCINCUS-K
(AND AGE) ALLOSAURUS-J DIPLODOCUS-J CORYTHOSAURUS-K PROTOCERATOPS-K _ANKYLOSAURUS-K_
J-JURASSIC _TYRANNOSAURUS-K_ _BRONTOSAURUS-J_ _TRACHODON-K_ _TRICERATOPS-K_
K-CRETACEOUS

SCALE IN FEET 10 10 5 5 5 5

GROUP AGE TEETH TAIL HEAD SIZE EXAMPLE

RHAMPHORHYNCHOIDS JURASSIC WELL LONG TAIL WITHOUT SMALL
DEVELOPED WITH CREST MAXIMUM _RHAMPHORHYNCHUS_
FLATTENED WINGSPAN
RUDDER AT OF 2 FEET
END
PTERODACTYLOIDS JURASSIC JAW PARTLY SHORT OR WITH SMALL FROM SIZE
AND OR NO TAIL OR LARGE OF _PTERANODON-K_
CRETACEOUS COMPLETELY CREST SPARROW
TOOTHLESS, TO GIANTS
HORNEY BEAK WITH SPAN
OF 25′

GROUP AGE DESCRIPTION SIZE HABITAT EXAMPLE

ICHTHYOSAUR TRIASSIC FISH-LIKE BODY, AVERAGE MARINE
TO HEAD LONG AND LENGTH = _ICHTHYOSAURUS-J_
CRETACEOUS POINTED, NO 7′
DISTINCT NECK MAXIMUM
LENGTH =
43′
MOSASAUR CRETACEOUS LIZARD-LIKE AVERAGE MARINE
BODY, FLATTENED LENGTH = _PYLOSAURUS_
TAIL, 15′-20′
DOUBLE-JOINTED MAXIMUM
JAW, RECURVED LENGTH =
TEETH 50′
PLESIOSAUR TRIASSIC LONG NECK AND MAXIMUM MARINE
TO SMALL HEAD OR LENGTH = _ELASMOSAURUS-K_
CRETACEOUS LONG HEAD AND 50′ _TRINACROMERUM-K_
SHORT NECK,
POWERFUL FLIPPERS
CHELONIA TRIASSIC SHAPED LIKE MAXIMUM MARINE,
(TURTLES) TO PRESENT MODERN TURTLES, SIZE = STREAMS, _ARCHELON-K_
BODY COVERED 11′ × 12′ AND
WITH BONY PLATES TERRESTRIAL
PHYTOSAUR TRIASSIC CROCODILE-LIKE MAXIMUM STREAMS
BODY, NOSTRILS LENGTH = AND SWAMPS _RUTIODON_
ON A “HUMP” 25′
ALMOST BETWEEN
THE EYES

PELYCOSAUR × ¹/₁₂
DIMETRODON
PRIMITIVE AMPHIBIAN × ¹/₂₀
ERYOPS
COTYLOSAUR
SEYMOURIA × ⅕

ICHTHYOSAUR × ¹/₁₂₀
MOSASAUR × ¹/₆₀
PLESIOSAUR × ¹/₆₀

RHAMPHORHYNCHUS × ⅙
PTERANODON × ¹/₄₀

Ichthyosaurs.—

These were extinct, short-necked, marine reptiles that were fish-like in appearance. Ichthyosaurs resemble the modern dolphins, and some of them attained lengths of 25 to 40 feet (Pl. 41), though the average was much less. The group is known from rocks ranging from middle Triassic to late Cretaceous in age.

Mosasaurs.—

The mosasaurs are another group of extinct marine lizards which lived in Cretaceous seas. Some of these great reptiles grew to be as much as 50 feet long, and their great gaping jaws were filled with many sharp recurved teeth (Pl. 41). Mosasaurs were present in the great Cretaceous seas which covered many parts of Texas, and their remains have been reported from both north and central Texas. One such skeleton was found near Austin, and its skull is on display in the Texas Memorial Museum.

Plesiosaurs.—

The plesiosaurs were marine reptiles which were characterized by a broad turtle-like body, paddle-like flippers, and a long neck and tail (Pl. 41). These reptiles were not as streamlined or well equipped for swimming as the ichthyosaurs or mosasaurs, but the long serpent-like neck was probably very useful in helping the reptile catch fish and other small animals for food. Plesiosaur remains range from middle Triassic to late Cretaceous in age, and they have been found in Cretaceous rocks in Texas. A short-necked plesiosaur which was collected from Upper Cretaceous rocks near Waco is on display in the Strecker Museum at Baylor University in Waco.

Phytosaurs.—

The phytosaurs were a group of crocodile-like reptiles which ranged from 6 to 25 feet in length (Pl. 42). They resembled the crocodiles both in appearance and in their mode of life, but this similarity is only superficial, and the phytosaurs and crocodiles are two distinct groups of reptiles.

The phytosaurs are exclusively Triassic in age and their remains have been collected from Triassic rocks along the eastern margin of the High Plains of Texas.

Crocodiles and alligators.—

These reptiles adapted themselves to the same type habitat that was occupied by the phytosaurs, which preceded them. Crocodiles and alligators were much larger and more abundant during Cretaceous and Cenozoic time than they are today; the crocodiles first appeared in the Cretaceous and the alligators in the Tertiary. The remains of both crocodiles and alligators have been found in Texas, and one such crocodile (_Phobosuchus_) represents the remains of the largest crocodile yet discovered (Pl. 43). This specimen probably had an overall length of 40 to 50 feet, and its massive skull was 6 feet long and possessed exceptionally strong jaws. The remains of this great beast were collected from exposures of Upper Cretaceous rocks along the Rio Grande in Trans-Pecos Texas.

Pterosaurs.—

These were Mesozoic reptiles with bat-like wings supported by arms and long thin “fingers” (Pl. 42). The pterosaurs were well adapted to life in the air, and their light-weight bodies and wide skin-covered wings enabled them to soar or glide for great distances. The earliest known pterosaurs were found in lower Triassic rocks, and the group became extinct by the end of the Cretaceous. During this time certain of these creatures attained a wingspread of as much as 27 feet, but their bodies were small and light.

Dinosaurs.—

The collective term “dinosaurs” (meaning terrible lizards) has been given to that distinctive group of reptiles prominent in Mesozoic life for some 140 million years. In size, the dinosaurs ranged from as little as 1 foot to as much as 85 feet in length and from a few pounds to perhaps 45 tons in weight. Some were _carnivorous_ (meat-eaters) but the majority were _herbivorous_ (plant-eaters). Some were _bipedal_ (walked on their hind-legs) while others were _quadrupedal_ (walked on all fours), and although most of the dinosaurs were terrestrial in habitat, aquatic and semi-aquatic forms were also present.

According to the structure of their hip bones, the dinosaurs have been divided into two great orders. These are the Saurischia (forms with lizard-like pelvic girdle) and the Ornithischia (dinosaurs with a bird-like pelvic girdle).

Order Saurischia.—

Dinosaurs belonging to this order were particularly abundant during the Jurassic and are characterized by hip bones that are similar to those of modern lizards. These dinosaurs were first discovered in rocks of Triassic age and did not become extinct until the end of the Cretaceous. The lizard-hipped reptiles are divided into two rather specialized groups of dinosaurs: the _theropods_ (carnivorous bipedal dinosaurs that varied greatly in size) and the _sauropods_ (herbivorous, quadrupedal, semi-aquatic, usually gigantic dinosaurs).

SUBORDER THEROPODA.—

This type of saurischian dinosaur walked on bird-like hind limbs, and they were exclusively meat-eating forms, such as _Allosaurus_ (Pl. 44) of Jurassic age. Some theropods were exceptionally large and were undoubtedly vicious beasts of prey. This assumption is borne out by such anatomical features as the small front limbs with long sharp claws for holding and tearing flesh, and the large strong jaws which were armed with numerous, sharp, dagger-like teeth. The largest of all known theropods was _Tyrannosaurus rex_ which, when standing on his hind limbs, was almost 20 feet tall. Some individuals were as much as 50 feet long, and _Tyrannosaurus_ is believed to have been among the most vicious animals to ever inhabit our earth. A cast of the skull of one of these great beasts is on display in the Texas Memorial Museum at Austin, and a _Tyrannosaurus_ tooth has been found in the Big Bend National Park in Trans-Pecos Texas.

SUBORDER SAUROPODA.—

The sauropods were the largest of all dinosaurs, and some attained a length of 85 feet and probably weighed 40 to 50 tons (_Brontosaurus_, Pl. 44). They were primarily herbivorous dinosaurs which had become adapted to an aquatic or semi-aquatic type of existence and probably inhabited lakes, rivers, and swamps. The tracks of sauropod dinosaurs have been collected from Lower Cretaceous rocks in central Texas (Pl. 4) and Upper Cretaceous beds in Big Bend National Park in Trans-Pecos Texas.

Order Ornithischia.—

The ornithischian, or bird-hipped dinosaurs, were herbivorous reptiles which were quite varied in form and size and appear to have been more highly developed than the saurischians. This order includes the duck-billed dinosaurs (ornithopods), the plate-bearing dinosaurs (stegosaurs), the armored dinosaurs (ankylosaurs), and the horned dinosaurs (ceratopsians). Ornithischian tracks are known from Cretaceous rocks in central and Trans-Pecos Texas.

SUBORDER ORNITHOPODA.—

These unusual dinosaurs were predominantly bipedal, semi-aquatic, and some (like the duck-billed dinosaurs) were highly specialized (_Trachodon_, Pl. 45).

SUBORDER STEGOSAURIA.—

The stegosaurs were herbivorous, quadrupedal ornithischians with large projecting plates down the back and heavy spikes on their tails. The Jurassic dinosaur _Stegosaurus_ (Pl. 45) is most typical of the plate-bearing forms. This creature weighed about 10 tons, was some 30 feet long, and stood about 10 feet tall at the hips. _Stegosaurus_ is characterized by a double row of large, heavy, pointed plates which run along the animal’s back. These plates begin at the back of the skull and stop near the end of the tail. The tail was also equipped with four or more long curved spikes which were probably used as a means of defense. The animal had a very small skull which housed a brain that was about the size of a walnut, and it is assumed that these, and all other dinosaurs, were of very limited intelligence.

_Stegosaurus_ remains have not been discovered in Texas, but these, like certain other of the extinct vertebrates, are mentioned because of their interesting and unusual form.

SUBORDER ANKYLOSAURIA.—

The ankylosaurs were four-footed, herbivorous, Cretaceous dinosaurs which had relatively flat bodies. The skull and back of the animal were protected by bony armor, and the club-like tail was armed with spikes. _Paleoscincus_ (Pl. 45), a typical ankylosaur, had large spines projecting from along the sides of the body and tail. The armored spiked back and the heavy club-like tail probably provided _Paleoscincus_ with much-needed protection from the vicious meat-eating dinosaurs of Cretaceous time.

ALLOSAURUS × ¹/₁₈₀
BRONTOSAURUS × ¹/₂₅₀

STEGOSAURUS × ¹/₉₀
TRACHODON × ¹/₁₀₀
PALEOSCINCUS × ¹/₂₅
TRICERATOPS × ¹/₁₂₀

SUBORDER CERATOPSIA.—

The ceratopsians, or horned dinosaurs, are another group of dinosaurs that are known only from rocks of Cretaceous age. These plant-eating dinosaurs possessed beak-like jaws, a bony neck frill which extended back from the skull, and one or more horns. _Triceratops_ (Pl. 45) is the largest of the horned dinosaurs (some forms were as much as 30 feet long), and the skull measured 8 feet from the tip of the parrot-like beak to the back of the neck shield.

CLASS AVES.—

Because of the fragile nature of their bodies, birds are seldom found as fossils. In spite of this, however, some interesting and important fossil bird remains have been discovered.

The oldest known bird was found in Upper Jurassic rocks exposed in Germany. This primitive bird, named _Archaeopteryx_, is little more than a reptile with feathers. _Archaeopteryx_ was a pigeon-sized creature which had scales as well as feathers, a lizard-like tail, a toothed beak, and other definitely reptilian characteristics.

During late Cretaceous time the birds underwent changes that resulted in forms similar to those that are living today, and most of the present-day birds had developed by the end of the Tertiary.

Although not commonly found, fossil birds have been recorded from certain of the Cenozoic rocks of Texas.

CLASS MAMMALIA.—

The mammals are animals that are born alive and fed with milk from the mother’s breast. They are warm-blooded, air-breathing, have a protective covering of hair, and are the most advanced of all vertebrates. The foregoing features are the more typical mammalian characteristics, but exceptions to these are found in certain mammals.