Part 21

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

CLASS AMPHIBIA.—

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

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

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

CLASS REPTILIA.—

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

Cotylosaurs.—

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

Turtles.—

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

Pelycosaurs.—

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

Therapsids.—

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

DINOSAUR SAURISCHIANS ORNITHISCHIANS STOCKS

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

GROUP AGE TEETH TAIL HEAD SIZE EXAMPLE

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

GROUP AGE DESCRIPTION SIZE HABITAT EXAMPLE

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

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

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

RHAMPHORHYNCHUS × ⅙ PTERANODON × ¹/₄₀

Ichthyosaurs.—

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

Mosasaurs.—

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