The Age of Reptiles

The first period of the Mesozoic Era gave rise to the "Age of Reptiles." On land, the synapsids, which later developed into mammals, began to decline, and the archosaurs, or "dominant reptiles," lived in various habitats. The earliest crocodiles began to develop, along with turtles and frogs, among others. The pterosaurs ruled the air and the ichthyosaurs the water. The dinosaurs—another order of archosaurs—appeared in the Middle Triassic, approximately 250 million years ago. Toward the end of the Triassic Period, many other reptiles declined dramatically, and the dinosaurs began their reign.

HERRERASAURUS

Herrerasaurus ischigualastensis

DIFFERENT SIZES

Size

13 feet (4 m)

Diet

Carnivorous

Habitat

Conifer Forests

Epoch

Late Triassic

Range

South America

The First Dinosaurs

^^ The most primitive dinosaurs were very small in comparison to their relatives of later epochs. Most of them have been found in South America. They were carnivores. Some were scavengers, and others were highly agile hunters. They shared very primitive morphological structures with other reptiles of their group, the archosaurs. Mixed with these primitive structures, however, were advanced bone forms similar to those of the predators that would dominate the Cretaceous Period. These predators were known as the theropods. Throughout the Triassic Period, the early dinosaurs were an uncommon subgroup of reptiles. Toward the end of the Triassic Period, the first large herbivores appeared.

HOLLOW SHAPE

is what Coelophysis means.

Eoraptor

Eoraptor fossils were discovered in 1991 in northwestern Argentina. This small carnivore lived 228 million years ago and measured up to 40 inches (1 m) long. It had sharp teeth and agile hind legs for running and chasing its prey. It may also have eaten carrion.

Herrerasaurus is one of the most ancient dinosaurs. It is considered a key to understanding the path dinosaurs took to dominate the following 160 million years. The first fossils were found in the early 1960s in the Ischigualasto valley in

Mussaurus

Eoraptor

Coelophysis

Herrerasaurus

Mussaurus

Eoraptor

Coelophysis

Herrerasaurus

A TRUE CARNIVORE

HOLLOW SHAPE

is what Coelophysis means.

A TRUE CARNIVORE

Eoraptor Dinosaur

The First Giant Herbivore his primitive saurischian was among the first to usher in the age of the dinosaurs in the late Triassic Period, about 210 million years ago. Dinosaurs had already been in existence for some time, but they were smaller predators thus far. The saurischian was clearly one of the first that fed exclusively on plants and that reached the immense sizes typical of herbivores. Many fossils have been found in over 50 separate locations. The secret of this dinosaur's survival is believed to have been the lack of competition for food, since no other herbivore of the time grew as large. Its name, which means "lizard-hipped," was given to it in 1837 by the German naturalist Hermann von Meyer. •

Plateosaurus engelhardti

^^ These prosauropods, of the suborder

Sauropodomorpha, were among the primitive herbivorous dinosaurs that were forerunners of the giant sauropods of the Jurassic Period. However, they were not actually ancestors of these animals. It is known that they associated with others of their own species because, in many areas, several specimens have been found together. Because of the hot, dry conditions that prevailed where they lived, it seems that they migrated constantly in search of food, which consisted of conifers and palm trees.

also evidence that males and females had different shapes.

BIPEDS

Its powerful hips supported the entire weight of its body when it stood up on its hind legs to reach food in the high branches of trees.

Defensive Claw

This animal had few defensive resources. However, one of the toes of its front feet had a powerful claw that it used to cut branches and for self-defense. In reality, however, its best defense was to run.

Hind Foot

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