A Docile Vegetarian

This striking dinosaur is one of the most widely studied in the history of paleontology. The first fossils were discovered by Othniel C. Marsh in 1877 in the American West, during the “Bone Wars.” This quadruped herbivore could measure up to 29.5 feet (9 m) long and weigh up to 2.2 tons (2 metric tons). Because of its small head, it has been used since the 19th century as a symbol of stupidity. It was later shown that most dinosaurs had small brains and that Stegosaurus's brain was larger than average.

Stegosaurus
was an ornithischian dinosaur that belonged to the family Stegosauridae. Its distinctive features included wide plates on its back and four spines, up to 24 inches (60 cm) long, on its tail. The function of these features is still under debate, but it is believed that they served mainly for self-defense. Stegosaurus was an easy victim for the great predators of its time, such as Allosaurus, but it is also believed that it may have been hunted by packs of small predators, such as Ornitholestes. It is doubtful that Stegosaurus could raise itself up on its hind legs, so it probably fed mostly on low bushes.

Head
Its lightweight head had
small teeth that were of
little use for chewing, so it
swallowed plants whole.

Legs
Its front legs were
half as long as its
hind legs. Each
foot had five wide,
short toes.

Tail
Stegosaurus's only
real defense was likely
the four spines on its
tail, which it swung
back and forth.

PLATES
These triangular bony structures were not very solid, but they had a complex network of veins.The plates were likely use to regulate the animal's body temperature or even for courtship.

STEGOSAURUS
Stegosaurus armatus
Size 29.5 feet (9 m)
Diet Herbivorous
Habitat Subtropical Forests
Epoch Late Jurassic
Range North America

WHERE IT LIVED
The first fossils were found in Colorado. Other specimens have since been found in India, western Europe, southern Africa, and China.

The “Age of Reptiles”

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.

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.

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.

EORAPTOR
Tiny predator whose name
means “thief of dawn”

HOLLOW SHAPE
is what Coelophysis means.

Mussaurus
The only known fossils of this species were
found alongside eggshells. They were no more
than 8 inches (20 cm) long. Their adult size is
unknown, but it is estimated to have been as
much as 6.5 to 8 feet (2-2.5 m). They are
known to have been herbivores.

MUSSAURUS
means “rat reptile.”

Coelophysis
This skilled carnivore could grow up to 9.2
feet (2.8 m) long. Two types of fossils have
been found. They are believed to be males
and females, respectively. Fossils of
this biped hunter have been found
in the United States in several
southwestern states.



HERRERASAURUS
Herrerasaurus ischigualastensis
Size 13 feet (4 m)
Diet Carnivorous
Habitat Conifer Forests
Epoch Late Triassic
Range South America

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 northern Argentina by an official paleontological commission headed by Osvaldo Reig. He named
the dinosaur in honor of the local guide who found it. Since then several complete skeletons have been found.

A TRUE CARNIVORE
One of the traits that defined this animal as a
dinosaur was its typical theropodian head. Its
narrow skull had nasal cavities in the front
part of its snout, and its eye sockets were part
of a hollow bony structure, making its skull
both lightweight and strong.

MOUTH
Its tubelike teeth were more
curved than those of other
carnivores that followed, but they
were sharp and serrated like those
of its theropod relatives.

LIMBS FOR HUNTING
Like its head, this dinosaur's limbs had
the same proportions as the later giant
predators of the Cretaceous Period. Its
small front limbs were designed for
capturing prey.

FRONT LIMBS
The relative size of its front limbs
suggests that this animal mainly
walked on two feet. Each hand had
three long claws and two short ones.
Its hands were able to grasp with the
help of “thumbs” slightly opposed to
the other claws. Its hand was a
formidable weapon for attacking and
holding prey, an earlier form of the
hands of other theropods.

HIND LEGS
On its lower limbs, this
dinosaur's toe bones show a
high degree of superposition.
The toe bones of the first
digit, although well
developed, are very short and
lightweight compared to the
second, third, and fourth
toes. These long, strong feet
enabled Herrerasaurus to
run while bearing the great
weight of its body.

PELVIS
Herrerasaurus was a very
early saurischian with a
primitive sacrum, ilium,
and hind legs but a highly
developed pubic bone and
vertebrae. These traits
show that this dinosaur
had unique
characteristics in its
spinal column.

SPINAL COLUMN
The central vertebrae are high and
short, and the neural arches have
square-shaped projections that are
thicker toward the back. There are
only two sacral vertebrae.

TAIL
Most predators used their
tails to keep their balance
while chasing their prey.