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.