Part 22

Plesiosaurs.—

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

Phytosaurs.—

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

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

Crocodiles and alligators.—

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

Pterosaurs.—

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

Dinosaurs.—

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

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

Order Saurischia.—

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

SUBORDER THEROPODA.—

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

SUBORDER SAUROPODA.—

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

Order Ornithischia.—

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

SUBORDER ORNITHOPODA.—

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

SUBORDER STEGOSAURIA.—

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

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

SUBORDER ANKYLOSAURIA.—

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

ALLOSAURUS × ¹/₁₈₀ BRONTOSAURUS × ¹/₂₅₀