This section is from the book "An Introduction To Geology", by William B. Scott. Also available from Amazon: An Introduction to Geology.
The Gnathodontia, which are very near to the Permian Pro-ganosauria, are represented by Telerpeton and Hyperodapedon. Superficially like Crocodiles, but belonging to a different order, the Parasuchia, are the little Aetosaurus and the formidable Belodon (Fig. 287), the latter found also in this country. The first of the dolphin-like Ichthyosaurs, which became so important in the Jurassic, are sparingly found in the Trias. Another group of sea-dragons, the Plesiosaurs, which attained such great development in Jurassic times, is represented in the Trias by small ancestral forms, Nothosaurus, etc. These are of extraordinary interest, as showing the descent of the purely marine Plesiosaurs, with their swimming paddles, from terrestrial reptiles which had feet adapted for walking. Another order of Triassic reptiles, the Thalattosauria, were already well adapted to a marine, pre-daceous life; as yet they are known only in Nevada.
One of the most characteristic of the Mesozoic groups of reptiles is the super-order Dinosauria, of which the Trias has many representatives; but clearly there were very many more than have yet been found, for the Newark sandstones of the eastern United States have preserved a great variety of Dinosaurian footprints, but very few bones have been found in these rocks. The Dinosauria were much diversified, adapted for very different habits of life: some were herbivorous, others carnivorous; some walked on all fours; others were occasionally or habitually bipedal, and walked upright after the manner of birds, with which they have many structural features in common. The gigantic size attained by some of these creatures, even in the Trias, is shown by the footprints, some of which are 14 to 18 inches in length. Of the few American forms of which the bones have been found, the best known is Anchisaurus from the Connecticut valley, and of the European genera, the much larger Zandodon.
Fig. 287. - Skull of Belodon kapffi v. Meyer, about 1/10 natural size. (Zittel).
The earliest Turtles are found in the Triassic of Europe, and these first-known members of the order are as typically differentiated as any of the later members. No doubt the Turtles originated in the Permian, in some region as yet unknown, and migrated to Europe in the Trias. The Theriodontia, which we found beginning in the Permian, culminated in the Trias, especially in southern Africa. Of this group there are two Triassic orders, the Anomodontia and the Therocephalia. The former have cutting jaws, like Turtles, and may or may not possess a pair of great tusks in the upper jaw. Dicynodon, a genus of this suborder, has been found in South Africa, India, Russia, and Scotland. The Therocephalia present extraordinary approximations to the mammals, and have left a great wealth of remains - some of them very large - in the Karroo beds of South Africa, and less abundantly in India. The earliest known members of the flying reptiles, or Pterosauria, occur in the Rhastic of Europe.
The Trias has, as yet, yielded no Lizards or Snakes, which became very important at a later date. No birds are known from this period, nor any reptiles which can be regarded as the ancestors of birds.
Still another great advance in the progress of life is registered in the first appearance of the Mammals, which occurred in the Trias. Mammals are the most highly organized forms of animals; but these, their earliest known representatives, were very small and very primitive, giving little promise of being the future conquerors of the world, as they were tiny creatures which, in a measure, represent the transition from lower vertebrates upward. Two American genera, Dromatherium and Microconodon, and one European genus, Microlestes, have been recovered.