Texas Fossils: An Amateur Collector's Handbook

by William Henry Matthews

ANIMAL FOSSILS

The fossilized remains of animals are very common in many of the sedimentary rocks of Texas. These remains are of many different kinds and represent the fossils of such diverse organisms as the shell of a tiny one-celled animal or the bones or tusk of a huge elephant. The fossils most commonly found, however, are the remains of invertebrate animals such as clams, snails, and corals, and it is this type of [fossil] that attracts the interest of most amateur collectors.

It is not always easy to tell whether certain organisms are plants or animals, and because of this some scientists have suggested that these “in-betweens” be placed in a separate kingdom—the [Protista]. The protistans are primarily [unicellular] organisms and are represented by such forms as bacteria, algae, diatoms, and the protozoans (see [below]). But in this publication, only the plant and animal kingdoms are recognized.

[Phylum] Protozoa

This [phylum] is composed of simple one-celled animals many of which have no shell or external body covering. Some, however, have external hard parts that can become fossilized, and these forms are quite useful microfossils.

CLASS SARCODINA.—

This class contains a group of one-celled animals which may secrete an [exoskeleton] (external protective covering) of [chitin], [silica], or calcium carbonate. Included in this class are foraminiferans (commonly called forams) and radiolarians.

Order Foraminifera.—

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. 9]a), 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.

Fig. 9. Typical Texas Foraminifera (all greatly enlarged). (a) Fusulina ([Pennsylvanian]). (b) Robulus. (c) Globigerina. (d) Frondicularia. (b-d, [Cretaceous]).

Fig. 10. Typical radiolarians (greatly enlarged). (a) Actinomma (Recent). (b) Porodiscus ([Eocene]).

[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. 11]a).

[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. 11]b), 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].

Fig. 11. Morphology and principal parts of corals. (a) [Solitary] or “horn” [coral]. (b) [Colonial] or compound coral.

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

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

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. 12]b), branching, or lace-like ([fig. 12]a), 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.

Plate 14

SPONGE SPICULES
(GREATLY ENLARGED)

[PALEOZOIC] SPONGES

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

Plate 15
[PENNSYLVANIAN] CORALS

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

Plate 16

[CRETACEOUS] CORALS

CLADOPHYLLIA × 1 PARASMILIA × 1 PLEUROCORA × 1

[TERTIARY] CORALS

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. 13]b). 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.

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

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).

Plate 17

[PENNSYLVANIAN] BRYOZOANS

FISTULIPORA × 6 POLYPORA × 5 RHOMBOPORA × 8

[CAMBRIAN] BRACHIOPODS

APSOTRETA × 10 LINGULA × 4 ANGULOTRETA × 10

[MISSISSIPPIAN] BRACHIOPODS

RHIPODOMELLA × 1 DICTYOCLOSTUS × 1 CAMAROTOECHIA × 1

Fig. 14. Lingula, a typical Recent inarticulate [brachiopod] showing extended pedicle.

Pedicle [Valve]

CLASS ARTICULATA.—

Articulate brachiopods have a well-defined [hinge-line] ([fig. 13]a). 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].

Fig. 15. Kingena wacoensis, a common [Cretaceous] [brachiopod]. (a) [Dorsal] view. (b) [Lateral] view. (c) [Ventral] view.

[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.

Plate 18
[PENNSYLVANIAN] BRACHIOPODS

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

Plate 19
[PENNSYLVANIAN] BRACHIOPODS

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. 16]a). 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.

Fig. 16. Morphology and principal parts of [gastropod] shells. (a) Low-spired form with [umbilicus]. (b) Section of spirally coiled shell showing [columella].

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.

Plate 20
[PENNSYLVANIAN] GASTROPODS

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

Plate 21

[PENNSYLVANIAN] GASTROPODS

EUPHEMITES × 1 STROBEUS × 1 PLATYCERAS × 1

[CRETACEOUS] GASTROPODS
INTERNAL MOLDS

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

Plate 22
[TERTIARY] GASTROPODS

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

Plate 23
[TERTIARY] GASTROPODS

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. 17]a).

Fig. 17. Morphology and principal parts of a typical [pelecypod] shell. (a) Exterior view. (b) Interior view.

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. 17]b). 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. 17]a) 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. 19]a) 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. 19]d).

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.

Plate 24
[PENNSYLVANIAN] PELECYPODS

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

Plate 25
[CRETACEOUS] PELECYPODS

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

Plate 26
[CRETACEOUS] PELECYPODS

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

Plate 27
[CRETACEOUS] PELECYPODS

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

Plate 28
[CRETACEOUS] PELECYPODS

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

Plate 29
[TERTIARY] PELECYPODS

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

Plate 30
[TERTIARY] PELECYPODS

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

Plate 31
[TERTIARY] PELECYPODS

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

Fig. 18. Morphology and principal parts of the pearly nautilus. (a) Exterior view of a Recent shell. (b) Sectioned view of the same shell to show internal structures.

b [Living chamber] [Aperture] Septa [Siphuncle] [Protoconch] Chamber

Fig. 19. Characteristic features of the various types of [cephalopod] sutures. (a) Nautiloid type. (b) [Goniatite] type. (c) [Ceratite] type. (d) [Ammonite] type.

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. 19]b). Sutures that are curved and crenulated (marked in places by a [series] of tooth-like indentations) are known as ceratitic ([fig. 19]c). 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. 19]d). Cephalopods with ammonitic sutures range from [Pennsylvanian] to Cretaceous in age and were the most abundant cephalopods of the [Mesozoic].

Plate 32

[PENNSYLVANIAN] CEPHALOPODS

PHANEROCERAS × 1 ORTHOCERAS × 1

[CRETACEOUS] CEPHALOPODS

METOICOCERAS × ½ CYMATOCERAS × ½

Plate 33
[CRETACEOUS] CEPHALOPODS

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.

Fig. 20. Types of typical annelid worms. (a) Serpula (×1) (b) Hamulus (×2). (c) Spirobis (×15).

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. 21]a). 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.

Plate 34
[FOSSIL] ARTHROPODS

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

Fig. 21. Morphology and principal parts of trilobites.

[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. 21]a). 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. 21]b).

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. 22]b). 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.

Fig. 22. Two extinct attached echinoderms. (a) Pentremites ([Mississippian]). (b) Caryocrinites ([Silurian]).

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. 22]a). 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.

Fig. 23. Typical modern crinoid, or “sea lily,” showing principal parts.

[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.

Plate 35

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

Plate 36
[CRETACEOUS] ECHINOIDS

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. 24]a) 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.

Fig. 24. Graptolites. (a) Diplograptus (×2). (b) Dendrograptus (×3). (c) Phyllograptus (×2).

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.

Plate 37

SHARK TEETH × 1

CONODONTS
(GREATLY ENLARGED)

PRIMITIVE ARMORED FISH
PLACODERM ([DEVONIAN])
× ½

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.

Plate 38
Comparison of the dinosaurs. Reproduced with permission of Dr. J. W. Dixon, Jr., and Geology Department, Baylor University, Waco, Texas.

DINOSAUR STOCKS	SAURISCHIANS		ORNITHISCHIANS
DINOSAUR TYPES	THEROPOD	SAUROPOD	ORNITHOPOD	STEGOSAUR	CERATOPSIAN	ANKYLOSAUR
POSTURE	BIPEDAL	QUADRUPEDAL	BIPEDAL AND QUADRUPEDAL	QUADRUPEDAL	QUADRUPEDAL	QUADRUPEDAL
ARMOR	UNARMORED EXCEPT FOR HUGE SHARP TEETH	UNARMORED	UNARMORED (SPEED WAS CHIEF DEFENSE)	BONY PLATES ALONG BACK, SPIKED-TAIL	HORNS, BONY PLATE OVER NECK	KNOBS AND SPIKES OVER [DORSAL] AREA, CLUB-LIKE TAIL
DIET	CARNIVOROUS	HERBIVOROUS	HERBIVOROUS	HERBIVOROUS	HERBIVOROUS	HERBIVOROUS
OTHER DESCRIPTIVE REMARKS	LARGE HEAD WITH POWERFUL JAW, GREATLY REDUCED FORE-LIMBS	HUGE BODY, LONG NECK AND TAIL, SMALL HEAD	SLENDER-BUILD, “DUCK-BILLED”	SHORT NECK, LONG TAIL, SMALL HEAD	SHORT NECK, STOCKY BUILD	“ARMADILLO-LIKE” APPEARANCE
EXAMPLES (AND AGE) J-JURASSIC K-CRETACEOUS	CERATOSAURUS-J ALLOSAURUS-J TYRANNOSAURUS-K	BRACHIOSAURUS-J DIPLODOCUS-J BRONTOSAURUS-J	PARASAUROLOPHUS-K CORYTHOSAURUS-K TRACHODON-K	STEGOSAURUS-J	STYRACOSAURUS-K PROTOCERATOPS-K TRICERATOPS-K	PALEOSCINCUS-K ANKYLOSAURUS-K
SCALE IN FEET	10	10	5	5	5	5

Plate 39
Comparison of [Mesozoic] flying and swimming reptiles. Reproduced with permission of Dr. J. W. Dixon, Jr., and Geology Department, Baylor University, Waco, Texas.

FLYING REPTILES—PTEROSAURS

GROUP	AGE	TEETH	TAIL	HEAD	SIZE	EXAMPLE
RHAMPHORHYNCHOIDS	[JURASSIC]	WELL DEVELOPED	LONG TAIL WITH FLATTENED RUDDER AT END	WITHOUT CREST	SMALL MAXIMUM WINGSPAN OF 2 FEET	RHAMPHORHYNCHUS
PTERODACTYLOIDS	JURASSIC AND [CRETACEOUS]	JAW PARTLY OR COMPLETELY TOOTHLESS, HORNEY BEAK	SHORT OR NO TAIL	WITH SMALL OR LARGE CREST	FROM SIZE OF SPARROW TO GIANTS WITH SPAN OF 25′	PTERANODON-K

[MESOZOIC] SWIMMING REPTILES

GROUP	AGE	DESCRIPTION	SIZE	[HABITAT]	EXAMPLE
ICHTHYOSAUR	[TRIASSIC] TO [CRETACEOUS]	FISH-LIKE BODY, HEAD LONG AND POINTED, NO DISTINCT NECK	AVERAGE LENGTH = 7′ MAXIMUM LENGTH = 43′	MARINE	ICHTHYOSAURUS-J
MOSASAUR	CRETACEOUS	LIZARD-LIKE BODY, FLATTENED TAIL, DOUBLE-JOINTED JAW, RECURVED TEETH	AVERAGE LENGTH = 15′-20′ MAXIMUM LENGTH = 50′	MARINE	PYLOSAURUS
PLESIOSAUR	TRIASSIC TO CRETACEOUS	LONG NECK AND SMALL HEAD OR LONG HEAD AND SHORT NECK, POWERFUL FLIPPERS	MAXIMUM LENGTH = 50′	MARINE	ELASMOSAURUS-K TRINACROMERUM-K
CHELONIA (TURTLES)	TRIASSIC TO PRESENT	SHAPED LIKE MODERN TURTLES, BODY COVERED WITH BONY PLATES	MAXIMUM SIZE = 11′ × 12′	MARINE, STREAMS, AND TERRESTRIAL	ARCHELON-K
PHYTOSAUR	TRIASSIC	CROCODILE-LIKE BODY, NOSTRILS ON A “HUMP” ALMOST BETWEEN THE EYES	MAXIMUM LENGTH = 25′	STREAMS AND SWAMPS	RUTIODON

Plate 40

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

Plate 41
SWIMMING REPTILES

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

Plate 42

CROCODILE-LIKE REPTILE
× ¹/₅₀
PHYTOSAUR

FLYING DINOSAURS

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).

Plate 43
Dr. Brown, R. T. Bird, and Dr. Schaikjer with the skull of Phobosuchus, an extinct crocodile from the [Cretaceous] of Trans-Pecos Texas.
Photograph courtesy of the American Museum of Natural History.

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.

Plate 44
SAURISCHIAN DINOSAURS

ALLOSAURUS × ¹/₁₈₀ BRONTOSAURUS × ¹/₂₅₀

Plate 45
ORNITHISCHIAN DINOSAURS

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.

Mammals first appeared in the [Jurassic] and were probably derived from some form of mammal-like reptile. Although rare during the [Mesozoic], mammals underwent rapid development and expansion during the [Cenozoic], and during this era certain types of mammals became extremely large and assumed many bizarre shapes. The majority of these unusual forms lived but a short time but are well known from their fossils, and the remains of some of these animals which inhabited Texas during the Cenozoic may be seen in the Texas Memorial Museum at Austin.

Recent mammalian classification recognizes several subclasses and numerous orders and suborders, but the treatment of the mammals in a publication of this nature must of necessity be somewhat brief and no attempt at detailed classification is made.

Subclass Allotheria.—

The allotherians first appeared during the [Jurassic] and underwent considerable development in the late [Cretaceous] and early [Tertiary]. Included in this subclass are the multituberculates which are a group of small rodent-like animals that were probably the earliest of the herbivorous mammals. These animals were probably never very numerous, and they became extinct during the early part of [Eocene] time.

Subclass Theria.—

Members of this subclass are first known from rocks of [Jurassic] age, and they constitute the largest group of mammals that are living today. Therians undergo considerable development before they are born and at birth typically resemble the fully developed animal. This subclass has been divided into several orders but only the more important ones are discussed here.

Order Edentata.—

The edentates are a rather primitive group of mammals which are represented by such living forms as the anteaters, tree sloths, and armadillos. Members of this group were common in the southern part of the United States in [Pleistocene] and [Pliocene] time, and [fossil] edentates have been reported from rocks of this age in Texas. One such form was Mylodon ([Pl. 46]), one of the extinct giant ground sloths. These huge sloths were quite heavy and some of them stood as much as 15 feet tall; these great creatures were the forerunners of the modern tree sloths of South America. The mounted skeleton of one of these giant ground sloths is displayed in the Texas Memorial Museum.

Plate 46
[CENOZOIC] MAMMALS

ENTELODONT × ¹/₃₅ GLYPTODON × ¹/₅₀ MYLODON × ¹/₉₀

Another interesting representative of this order was the glyptodont. These peculiar mammals, which were ancestral to the present-day armadillos, developed at about the same time as the ground sloths. Glyptodon ([Pl. 46]), a typical glyptodont that has been reported from the [Pleistocene] of Texas, is quite characteristic of this group. This armadillo-like beast had a solid turtle-like shell that in some forms was as much as 4 feet high. From the front of the bone capped head to the tip of its tail, a large individual might be as much as 15 feet long. The thick heavy tail was protected by a [series] of bony rings, and in some [species] the end of the tail was developed into a bony heavily spiked club. The [carapace] (hard outer shell) of a large glyptodont is mounted at the Texas Memorial Museum.

Order Carnivora.—

Animals belonging to this order are called carnivores and are characterized by clawed feet and by teeth which are adapted for tearing and cutting flesh. The carnivores, or meat-eaters, were first represented by an ancient group of animals called creodonts, and this short-lived group first appeared in the [Paleocene] and were extinct by the end of the [Eocene]. They ranged from the size of a weazel to that of a large bear, and their claws were sharp and well developed. Their teeth, however, were not as specialized as those of modern carnivores, and the creodont brain was relatively small. It is assumed that these animals had a very low order of intelligence when compared to the more advanced carnivores of today.

These early meat-eaters were followed by more specialized carnivores which developed throughout [Cenozoic] time. Some examples of these are the saber-tooth cat Dinobastis ([Pl. 47]) and the dire wolf Canis diris ([Pl. 47]), both of which have been reported from the Texas [Pleistocene]. Some remains of these unusual forms, representing the cat and dog families, are on display at the Texas Memorial Museum.

Order Pantodonta.—

Pantodonts, known also as amblypods, were primitive, hoofed, herbivorous animals. They were distinguished by a heavy skeleton, short stout limbs, and blunt spreading feet. The pantodonts appeared first during [Paleocene] time and had become extinct by the end of the [Oligocene].

Order Dinocerata.—

The members of this order are an extinct group of gigantic mammals commonly called uintatheres. Uintatherium ([Pl. 48]), which is typical of the group, had three pairs of blunt horns, and the males had dagger-like upper tusks. Some of the uintatheres were as large as a small elephant and stood as much as 7 feet tall at the shoulders. The size of the brain in relation to the size of the body suggests that these animals were not as intelligent as most mammals. Uintatheres are known from rocks ranging from [Paleocene] to [Eocene] in age. Uintathere remains have been reported from Big Bend National Park in Trans-Pecos Texas.

Order Proboscidea.—

The earliest proboscideans, the elephants and their relatives, first appeared in the late [Eocene] of Africa and were about the size of a small modern elephant but had larger heads and shorter trunks. Proboscidean development is marked by an increase in size, change in skull and tooth structure, and elongation of the trunk. Two well-known [fossil] proboscideans are the mammoth and the mastodon, both of which inhabited Texas during [Pleistocene] time. The mastodons resembled the elephants, but the structure of their teeth was quite different ([fig. 25]). Moreover, the mastodon skull was lower than that of the elephant and the tusks were exceptionally large—some reaching a length of 9 feet.

Plate 47
[CENOZOIC] MAMMALS

DINOBASTIS × ¹/₂₀ CANIS DIRUS × ¹/₁₅ HYRACOTHERIUM × ¹/₁₀ PLIOHIPPUS × ¹/₂₀

There were several types of mammoths, and the woolly mammoth is probably the best known. This animal lived until the end of the [Pleistocene] and, like the woolly rhinoceros discussed [below], is known from ancient cave paintings and frozen remains. Information gathered from these sources indicates that this great beast had a long coat of black hair with a woolly undercoat ([Pl. 49]).

Fig. 25. Sketches of [Pleistocene] (a) mastodon tooth (×⅙) and (b) mammoth tooth (×⅙).

During the [Pleistocene], mammoths were widespread over the United States, and their remains are abundant in many stream deposits of this age. Proboscidean bones have been reported from Pleistocene rocks in many parts of Texas, where they are commonly found in sand and gravel pits.

Order Perissodactyla.—

The perissodactyls, or odd-toed animals, are mammals in which the central toe on each limb is greatly enlarged. Modern representatives include the horses, rhinoceroses, and tapirs. Extinct members of the Perissodactyla include the titanotheres, chalicotheres, and baluchitheres, all of which grew to tremendous size and took on many unusual body forms.

HORSES.—

One of the first perissodactyls was Hyracotherium (also called Eohippus), which is the earliest known horse ([Pl. 47]). This small animal, whose remains have been found in Big Bend National Park, was about 1 foot high and his teeth indicate a diet of soft food. Following the first horse, there is a long [series] of [fossil] horses which provide much valuable information on the history of this important group of animals.

The record of the development of the horse is well represented in Texas, and the bones and teeth of [fossil] horses are common in certain parts of the State. Fossils of this type have been reported from the [Tertiary] of the Trans-Pecos, Gulf Coastal Plain, and High Plains regions of Texas, and the teeth of [Pleistocene] horses have been found in sand and gravel pits in many parts of the State. Horse teeth ([fig. 26]) are particularly useful fossils as they may be accurately identified and used to determine the age of the rocks in which they are found.

Fig. 26. Typical [Pliocene] horse tooth. Top view (a) and [lateral] view (b) of molar tooth (×½).

TITANOTHERES.—

This group of odd-toed mammals appeared first in the [Eocene], at which time they were about the size of a sheep. By Middle [Oligocene] time they had increased to gigantic proportions but still had a small and primitive brain. Brontotherium ([Pl. 48]) was slightly rhinoceros-like in appearance and is believed to be the largest land animal that ever inhabited the North American continent. This animal was about 8 feet tall at the shoulders; a large bony growth protruded from the skull and this was extended into a flattened horn, which was divided at the top.

Plate 48
[TERTIARY] MAMMALS

UINTATHERIUM × ¹/₄₅ BRONTOTHERIUM × ¹/₃₅

Although the titanotheres underwent rapid development during the early [Tertiary], these huge beasts became extinct during the middle of the [Oligocene] epoch. Titanothere remains have been reported from the Trans-Pecos region of Texas.

CHALICOTHERES.—

The chalicotheres were in some ways like the titanotheres, but they also exhibited many peculiarities of their own. The head and neck of Moropus, a typical chalicothere, were much like that of a horse, but the front legs were longer than the hind legs, and the feet resembled those of a rhinoceros except that they bore long claws instead of hoofs. The chalicotheres lived in North America from [Miocene] until [Pleistocene] time but were probably never very numerous, and their remains have not yet been discovered in Texas.

RHINOCEROSES.—

The rhinoceroses are also odd-toed animals, and there are many interesting and well-known fossils in this group. The woolly rhinoceros ([Pl. 49]) was a [Pleistocene] two-horned form that ranged from southern France to northeastern Siberia. The woolly rhinoceros is well known from complete carcasses recovered from the frozen tundra of Siberia and from remains that were found preserved in an oil seep in Poland. These unusual specimens plus cave paintings made by early man have given a complete and accurate record of this creature. Although the woolly rhinoceros has not been reported from Texas, other [fossil] rhinoceroses have been found in the High Plains and Gulf Coastal Plain of Texas. These fossils have been found in rocks ranging from Middle [Oligocene] to late [Pliocene] in age.

Baluchitherium, the largest land mammal known to science, was a hornless rhinoceros that lived in late [Oligocene] and early [Miocene] time. This immense creature measured approximately 25 feet from head to tail, stood almost 18 feet high at the shoulder, and must have weighed many tons. Remains of these creatures have not been discovered in North America, and they appear to have been restricted to Central Asia.

Order Artiodactyla.—

The artiodactyls are the even-toed hoofed mammals and include such familiar forms as pigs, camels, deer, goats, sheep, and hippopotamuses. This is a large and varied group of animals, but the basic anatomical structure of the limbs and teeth show well the relationship between the different forms. Artiodactyls are abundant fossils in rocks ranging from [Eocene] to [Pleistocene] in age and are common in rocks of this age in Texas.

ENTELODONTS.—

These giant pig-like artiodactyls lived during [Oligocene] and early [Miocene] time and were distinguished by a long heavy skull that held a relatively small brain. The face was marked by large knobs which were located beneath the eyes and on the underside of the lower jaw, and although these knob-like structures were blunt they had the appearance of short horns. Certain of these giant swine attained a height of 6 feet at the shoulders and had skulls that measured 3 feet in length ([Pl. 46]). Entelodont remains have been found in the Miocene of the Texas Coastal Plain.

CAMELS.—

The first known camels have been reported from rocks of upper [Eocene] age, and these small forms underwent considerable specialization of teeth and limbs as they developed in size. Many of the camels that lived during the middle [Cenozoic] had long legs which were well adapted to running and long necks which would have allowed the animals to browse on the leaves of tall trees.

The earliest known Texas camels were found in rocks of [Oligocene] age, and camels, like horses, must have been abundant in Texas during the [Pleistocene] for their fossilized remains are common in many parts of the State.

Plate 49
[CENOZOIC] MAMMALS

WOOLLY RHINOCEROS × ¹/₂₀ WOOLLY MAMMOTH × ¹/₄₀