The Dinosaurs

For 165 million years, life on earth was dominated by the most spectacular animals that have ever existed – the dinosaurs.

The Mesozoic Era was a high point in the history of life, because it was the age of dinosaurs. This fantastic creatures included the biggest, heaviest, and the most terrifying of all land animals. They evolved into an amazing variety of forms ranging from huge, armoured leaf-eaters to nimble, feathered hunters,and gave rise to the birds that still flourish today

Family tree

All the dinosaurs of the Mesozoic Era except the very earliest species belonged to two main group known as the ornithischians and saurischians. The ornithischians evolved into three main types that were nearly all plant – eaters. The saurischians divided into the mainly meat – eating theropods and the big, plant – eating sauropodomorphs.


The word ornithischian means “bird-hipped”. It refers to the way that the pelvic bones of these dinosaurs resemble those of birds. The ornithischians also had beaks supported by special jawbones. But confusingly the birds themselves are small theropod dinosaurs, part of the saurischian group.

  • Marginocephalians

These consisted of ceratopsians, like this Pentaceratops with its huge horned neck – frill, and pachycephalosaurs with very strong, armoured skulls.

(a) Ornithopods

One of the most successful groups of dinosaurs, the ornithopods ranged from lightweight animals that ran on two legs to heavyweights like this Muttaburrasaurus. They were all plant – eaters.

(b) Thyreophorans

The thyreophorans included the heavily armoured ankylosaurs and stegosaurs such as this spiny Kentrosaurus.


The saurischian dinosaurs of the early Mesozoic Era had pelvic bones that resembled those of lizards, so the word saurischian means “lizard – hipped”. But many of the later forms evolved bird – like pelvic bones, which the birds were to inherit. Saurischians also had longer, more flexible necks than the ornithischians.

(a) Theropods

Spinosuarus was one of the biggest theropods – the group that included all the powerful hunters. They all walked on their hind legs, and many had feathers.

(b) Sauropodomorphs

The sauropodomorphs included the biggest dinosaurs – colossal animals such as Brachiosaurus. They were all plant – eaters that supported their immense weight on four pillar – like legs resembling those of elephants. They had very long necks, which were often balanced by equally long tails.

Strong legs

Unlike modern reptiles such as crocodiles and lizards, the dinosaurs stood with their legs beneath in much the same way as mammals. We also know that many of them were warm – blooded, which allowed them to be more active than modern cold – blooded reptiles.

Dinosaur stance

All dinosaurs stood tall on straight legs, so their weight – which could be immense – was fully supported.

Crocodile stance

Modern crocodiles are close relatives of dinosaurs,but their legs do not support their body quite as well.

Lizard stance

Lizards usually sprawl with their legs outspread and their bellies close to the ground, which slows them down.

Reptile neighbours

The dinosaurs were not only giant reptiles living during the Mesozoic Era. They shared their world with marine reptiles such as Mosasaurus, and pterosaurs like Pterodactylus. These animals were not dinosaurs, although the pterosaurs were close relatives. Most were hunters, and some marine reptiles had massively strong jaws. The pterosaurs were much more lightly built, with small bodies and long, efficient wings of structured skin.


At up to 15 m long, Mosasaurus was a powerful predator that lived at the very end of the age of dinosaurs.


Like all pterosaurs, Pterodactylus was a warm – blooded, furry – bodied animal that probably flew as well as a bird.

The Mesozoic Era

The age of dinosaurs begin 230 million years ago, during the Triassic period. This was the first of three periods that made up the Mesozoic Era. The earliest dinosaurs had to compete with other types of reptile, but many of these became extinct at the end of the Triassic,allowing dinosaurs to flourish during the Jurassic Period. The last period, the Cretaceous, saw the evolution of an amazing variety of dinosaurs, including the fearsome tyrannosaurs.

Triassic Earth

From 252 to 201 million years ago, all the continents were joined together in a huge supercontinent with a vast desert at its heart. Most plants and animals lived near the fringes.

Jurassic Earth

The supercontinent with in two near the beginning of the Jurassic Period. The deserts shrank, and the growth of lush forests provided food for huge plant – eating dinosaurs.

Cretaceous Earth

The Jurassic Period gave away to the Cretaceous 145 million years ago. The continents became more fragmented,allowing many different types of dinosaur to evolve in various lands.

References :

Knowledge Encyclopedia, DK Publications.