Part 23

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.

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.—