Page 23 - Reptiles & Dinosaurs (Britannica Illustrated Science Library)

18 DINOSAURS REPTILES AND DINOSAURS 19

Different Species COMPARATIVE SCALE

uring 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
D 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— HEAD
largely disappeared. Other species also found their ecological niches and multiplied. These species Its head was small and
included sea creatures, such as sharks and rays, that resemble their modern relatives, as well as ray- had a crest between the
eyes. This crest had
finned fish with sharp teeth, such as the fierce predator Aspidorhynchus. large nasal cavities at
Dryosaurus Megalosaurus Camarasaurus Brachiosaurus the top. Its teeth were
like pegs and had spaces
between them.
Giants of the Mesozoic

Giant herbivores dominated the Earth. However, BRACHIOSAURUS NECK
increasing diversity also brought increasing competition. Brachiosaurus The length of its neck, in feet
The large sauropods, such as Diplodocus, and ornithischians, proportion to the rest of its 42.6 (13 m)
such as the stegosaurids, had to watch out for larger Size 82 feet (25 m) body, sets it apart from other Height of Brachiosaurus,
theropods, such as Megalosaurus, as well as for hordes of Diet Herbivorous sauropods of its time. By because of its long front
small, swift predators, such as Compsognathus. The first bird means of its neck, which legs and long neck
to descend from small dinosaurs appeared. Habitat Tree-lined Savannas could be up to 42.5 feet
Epoch Late Jurassic (13 m) long, it could
Range North America reach the tops of Eye Sockets
MEGALOSAURUS trees. Its eyes sat in large
means “large lizard.” sockets.
DRYOSAURUS
means “oak reptile.” Nasal Cavities
They are
Brachiosaurus believed to have
been resonance
For a long time, this sauropod was the largest chambers.
dinosaur for which we had a complete skeleton.
It was a four-footed herbivore with a small brain
Megalosaurus and a long neck. Specimens have been found in
In 1676, the bones of one of the first the United States, on the Iberian Peninsula, and
in northern and southern Africa. The African
dinosaurs were found in southern England, species were from the Cretaceous Period
although they were not identified as such and had slight anatomical differences.
until 1819. This theropod predator was
highly intelligent in comparison to its peers. Jugal (Yoke) Bone Teeth
It lived 181 million years ago, grew up to was located behind were used for tearing
29.5 feet (9 m) long, and weighed 1.1 tons the upper jaw and off tree branches when
(1 metric ton). It walked on its two hind under the eye. the head was pulled
backward.
legs and had two powerful front claws.
TAIL VERTEBRAE
Its tail was small Brachiosaurus's extremely long vertebrae in its back. Its short
in comparison to
Dryosaurus its body and was neck had 13 vertebrae, with deep tail had about 50 bones, which
helped it move easily.
and complex cavities covered by
The fossils of this ornithopod, of the suborder an extension of its membranes. It had 11 or 12
Ornithischia, were found in Tanzania and the spinal column.
United States at the same time during the
19th century, in the middle of the so- CAMARASAURUS
called “Bone Wars.” This means “chambered lizard.” Support
lightweight herbivore could This area anchored the
reach up to 14.8 feet (4.5 m) Vertebral Joint supporting muscles.
long and weigh nearly The vertebrae were
fused to strengthen
200 pounds (90 kg). the neck.
Camarasaurus Hollow Bones
made the neck lighter.
This large, herbivorous sauropod lived on the plains of LEGS
North America 159 million years ago. Its fossils were Its front
first found in 1877. It grew up to 65.5 feet (20 m) long; legs were Ease of Movement
even so, it was easy prey for large predators such as longer than was made possible by a
Allosaurus. It could weigh up to 22 tons (20 metric its hind ball-and-socket joint.
tons), and it walked on four feet, which prevented it legs.
from running quickly enough to easily escape. 34.8 inches (88.5 cm)