Chapter 8

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

Thechitinousgraptoliteexoskeletonis 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 fromCambriantoMississippianin age, and they are among the most important guide fossils forOrdovicianandSilurianrocks.

Graptolites have been reported fromCambrianrocks in central Texas and from theOrdovicianof 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.

The vertebrates are the most advanced of all chordates. They are characterized by a skull and a bony or cartilaginousinternal skeleton, with avertebral columnof 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 fewvertebrateremains, 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.

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).

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

These are primitive jaw-bearing fishes, the majority of which were heavily armored (Pl. 37). Theplacodermswere 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 theDevonianand lasted into thePermian, 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.

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 ofDevonianage, and they have been relatively common up to the present time.

Shark teeth (Pl. 37) can be found in Texas inPennsylvanian,Permian,Cretaceous,Paleocene,Eocene, andMiocenerocks. These are probably the most commonvertebratefossils to be found in Texas and are usually found in thin-bedded marine limestones or clays.

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 calledcrossopterygians. These were very abundant in theDevonianand 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 ofCretaceousage 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 FISHPLACODERM (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 severalPaleozoicformations in Texas and are useful guide fossils in some areas.

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.).

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 lateDevoniantime and were relatively abundant in thePennsylvanian,Permian, andTriassic.

Amphibian remains in Texas are confined largely to lowerPaleozoicand upperMesozoicrocks. Numerous interesting and important discoveries offossilamphibians 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 wherePermianred beds are exposed in Archer and Baylor counties and whereTriassicred beds are exposed from Big Spring north along the edge of the High Plains have furnished most of these specimens.

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.

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 thePennsylvanianandPermianand apparently became extinct sometime during the late Permian. Cotylosaurs (Pl. 40) are well known from the Permian of north Texas.

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

The pelycosaurs were a group of latePaleozoicreptiles 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 thePermianred beds of north-central Texas.

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 middlePermianand persisted until the middleJurassic, but therapsid remains have not been reported from Texas.

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

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

FLYING REPTILES—PTEROSAURS

MESOZOICSWIMMING REPTILES

Plate 40

Plate 41SWIMMING REPTILES

Plate 42

CROCODILE-LIKE REPTILE× ¹/₅₀PHYTOSAUR

FLYING DINOSAURS

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 middleTriassicto lateCretaceousin age.

The mosasaurs are another group of extinct marine lizards which lived inCretaceousseas. 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.

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 middleTriassicto lateCretaceousin 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.

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 exclusivelyTriassicin age and their remains have been collected from Triassic rocks along the eastern margin of the High Plains of Texas.

These reptiles adapted themselves to the same typehabitatthat was occupied by the phytosaurs, which preceded them. Crocodiles and alligators were much larger and more abundant duringCretaceousandCenozoictime than they are today; the crocodiles first appeared in the Cretaceous and the alligators in theTertiary. 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.

These wereMesozoicreptiles 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 lowerTriassicrocks, and the group became extinct by the end of theCretaceous. During this time certain of these creatures attained a wingspread of as much as 27 feet, but their bodies were small and light.

The collective term “dinosaurs” (meaning terrible lizards) has been given to that distinctive group of reptiles prominent inMesozoiclife 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 werecarnivorous(meat-eaters) but the majority wereherbivorous(plant-eaters). Some werebipedal(walked on their hind-legs) while others werequadrupedal(walked on all fours), and although most of the dinosaurs were terrestrial inhabitat, 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 43Dr. Brown, R. T. Bird, and Dr. Schaikjer with the skull ofPhobosuchus, an extinct crocodile from theCretaceousof Trans-Pecos Texas.Photograph courtesy of the American Museum of Natural History.

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

This type of saurischian dinosaur walked on bird-like hind limbs, and they were exclusively meat-eating forms, such asAllosaurus(Pl. 44) ofJurassicage. 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 wasTyrannosaurus rexwhich, when standing on his hind limbs, was almost 20 feet tall. Some individuals were as much as 50 feet long, andTyrannosaurusis believed to have been among the most vicious animals to ever inhabit our earth. Acastof the skull of one of these great beasts is on display in the Texas Memorial Museum at Austin, and aTyrannosaurustooth has been found in the Big Bend National Park in Trans-Pecos Texas.

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 LowerCretaceousrocks in central Texas (Pl. 4) and Upper Cretaceous beds in Big Bend National Park in Trans-Pecos Texas.

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 fromCretaceousrocks in central and Trans-Pecos Texas.

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

The stegosaurs were herbivorous, quadrupedal ornithischians with large projecting plates down the back and heavy spikes on their tails. TheJurassicdinosaurStegosaurus(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.Stegosaurusis 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.

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

The ankylosaurs were four-footed, herbivorous,Cretaceousdinosaurs 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 providedPaleoscincuswith much-needed protection from the vicious meat-eating dinosaurs of Cretaceous time.

Plate 44SAURISCHIAN DINOSAURS

Plate 45ORNITHISCHIAN DINOSAURS

The ceratopsians, or horned dinosaurs, are another group of dinosaurs that are known only from rocks ofCretaceousage. 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.

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

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

During lateCretaceoustime 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 theTertiary.

Although not commonly found,fossilbirds have been recorded from certain of theCenozoicrocks of Texas.

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 theJurassicand were probably derived from some form of mammal-like reptile. Although rare during theMesozoic, mammals underwent rapid development and expansion during theCenozoic, 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.

The allotherians first appeared during theJurassicand underwent considerable development in the lateCretaceousand earlyTertiary. Included in this subclass are themultituberculateswhich 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 ofEocenetime.

Members of this subclass are first known from rocks ofJurassicage, 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.

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 inPleistoceneandPliocenetime, andfossiledentates have been reported from rocks of this age in Texas. One such form wasMylodon(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.

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