Different Species

During the middle of the Jurassic Period, the planet was lush and green. The gradual splitting of Pangea created new ecological environments, which were more humid and more diverse. The increased humidity enabled the growth of large trees and dense vegetation. This flourishing environment powered the continued diversification of different dinosaur species. In contrast, these conditions forced a decline in the majority of synapsids, and the archosaurs —the group that includes crocodiles— largely disappeared. Other species also found their ecological niches and multiplied. These species included sea creatures, such as sharks and rays, that resemble their modern relatives, as well as ray-finned fish with sharp teeth, such as the fierce predator Aspiclorhynchus. m

Dryosaurus

The fossils of this ornithopod, of the suborder Ornithischia, were found in Tanzania and the United States at the same time during the 19th century, in the middle of the so-called "Bone Wars." This lightweight herbivore could reach up to 14.8 feet (4.5 m) long and weigh nearly 200 pounds (90 kg).

CAMARASAURUS

means "chambered lizard."

MEGALOSAURUS

means "large lizard."

Camarasaurus

Megalosaurus

In 1676, the bones of one of the first

Giants of the Mesozoic

This large, herbivorous sauropod lived on the plains of North America 159 million years ago. Its fossils were first found in 1877. It grew up to 65.5 feet (20 m) long; even so, it was easy prey for large predators such as Allosaurus. It could weigh up to 22 tons (20 metric tons), and it walked on four feet, which prevented it from running quickly enough to easily escape.

dinosaurs were found in southern England, although they were not identified as such until 1819. This theropod predator was highly intelligent in comparison to its peers. It lived 181 million years ago, grew up to 29.5 feet (9 m) long, and weighed 1.1 tons (1 metric ton). It walked on its two hind legs and had two powerful front claws.

^^ Giant herbivores dominated the Earth. However, increasing diversity also brought increasing competition. The large sauropods, such as Diplodocus, and ornithischians, such as the stegosaurids, had to watch out for larger theropods, such as Megalosaurus, as well as for hordes of small, swift predators, such as Compsognathus. The first bird to descend from small dinosaurs appeared.

DRYOSAURUS

means "oak reptile."

COMPARATIVE SCALE

Diet Herbivorous

Habitat Tree-lined Savannas

Epoch Late Jurassic

Range North America

Brachiosaurus

Brachiosaurus

Dryosaurus Megalosaurus

Camarasaurus

Camarasaurus

42.6 feet

Height of Brachiosaurus, because of its long front legs and long neck

Eye Sockets

Its eyes sat in large sockets.

Nasal Cavities

They are believed to have been resonance chambers.

vertebrae in its back. Its short tail had about 50 bones, which helped it move easily.

VERTEBRAE

Brachiosaurus's extremely long neck had 13 vertebrae, with deep and complex cavities covered by membranes. It had 11 or 12

Ease of Movement-

was made possible by a ball-and-socket joint.

Jugal (Yoke) Bone was located behind the upper jaw and under the eye.

Teeth were used for tearing off tree branches when the head was pulled backward.

Hollow Bones made the neck lighter.

Vertebral Joint

The vertebrae were fused to strengthen the neck.

Support

This area anchored the supporting muscles.

COMPARATIVE SCALE

The length of its neck, in proportion to the rest of its body, sets it apart from other sauropods of its time. By means of its neck, which could be up to 42.5 feet (13 m) long, it could reach the tops of trees.

vertebrae in its back. Its short tail had about 50 bones, which helped it move easily.

LEGS Its front legs were longer than its hind legs.

HEAD

Its head was small and had a crest between the eyes. This crest had large nasal cavities at the top. Its teeth were like pegs and had spaces between them.

BRACHIOSAURUS

Brachiosaurus

Diet Herbivorous

Habitat Tree-lined Savannas

Epoch Late Jurassic

Range North America

42.6 feet

TAIL

Its tail was small in comparison to its body and was an extension of its spinal column.

VERTEBRAE

Brachiosaurus's extremely long neck had 13 vertebrae, with deep and complex cavities covered by membranes. It had 11 or 12

Ease of Movement-

was made possible by a ball-and-socket joint.

Brachiosaurus

Brachiosaurus

For a long time, this sauropod was the largest dinosaur for which we had a complete skeleton. It was a four-footed herbivore with a small brain and a long neck. Specimens have been found in the United States, on the Iberian Peninsula, and in northern and southern Africa. The African species were from the Cretaceous Period and had slight anatomical differences.

Jugal (Yoke) Bone was located behind the upper jaw and under the eye.

Teeth were used for tearing off tree branches when the head was pulled backward.

Hollow Bones made the neck lighter.

Dryosaurus Megalosaurus

Camarasaurus

Height of Brachiosaurus, because of its long front legs and long neck

Vertebral Joint

The vertebrae were fused to strengthen the neck.

Support

This area anchored the supporting muscles.

Eye Sockets

Its eyes sat in large sockets.

Nasal Cavities

They are believed to have been resonance chambers.

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.

STEGOSAURUS

Stegosaurus armatus

Size

29.5 feet (9 m

)

Diet Herbiv<

jrous

Habitat Subtroi

pical Forests

Epoch

Late Jurassic

WHERE IT LIVED

The first fossils were

found in Colorado

Other specimens have

since been found

in

India, western Europe, southern Africa, and China

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

Head

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

SEt-

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

PLATES

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

Ii"

Caudal Plate

Dorsal Plate

Caudal Plate

Cervical Plate

Cervical Plate y

The Cretaceous Period

Was an age of expansion. The dinosaurs continued to diversify, and the first snakes appeared. The Earth began to look like the planet we know today. The movement of tectonic plates created folds that came to form some of the mountain ranges of today, such as the Appalachians in North America and the Alps in Europe. At the end of this period, another mass extinction event occurred, probably caused by the impact of a meteorite.

n t- jmEPaS

CRETACEOUS

The name is based on the Latin word creta, which means stone. The name comes from the layer of limestone found in the rock formations that define this geological system.

An Evolving Planet

^^ During this period of 80 million years, the Earth's climate changed. Its temperate climate, accompanied by snow in the polar regions during the winter, was transformed into a warm, mild climate with noticeably different seasons. The ocean levels rose, currents increased ocean temperatures, and marine fauna multiplied. On land, the first flowering plants (gymnosperms) appeared, and forests of willow, maple, and oak harbored the last large dinosaurs.

140 to 65

MILLION YEARS AGO

The Earth began to adopt an appearance similar to that of today. Africa and South America separated from one another, as did North America and Europe. The North and South American plates drifted westward and collided with the Pacific plate, raising both the Rocky Mountains in North America and the Andes in South America.

Reptiles Scale
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