Age of the Dinosaurs II: Dinosaurs Walk the Earth

Around 225 million years ago, during the Triassic Period, a new type of small carnivorous animals emerged in Argentina. (1) They evolved from archosaurs and were probably no larger than a large dog. (2) This were the first dinosaurs to walk the Earth. Like all reptiles, these dinosaurs laid eggs. (1)

The world they lived in consisted of one big continent, called Pangaea, and the Earth was warm and dry. There were vast deserts and no polar icecaps. In this world where very wet and very dry periods followed each other, dinosaurs as a group diversified and grew at a very fast rate. By the end of the Triassic Period there were herbivores alongside the carnivores and they populated the entire Earth. (1)


1. Pangaea

The Jurassic Period that followed the Triassic Period saw the world get wetter, although it remained warm. Plants started to colonize the deserts and immense forests of gigantic trees, ferns and other plants emerged. (1) Plants began to diversify and angiosperms evolved. Eventually, this group of plants would dominate the terrestrial vegetation. (3) The continents started to drift apart and between Northern America and Europe, as well as between Europe and Africa large oceans appeared. (1)

In this green world the number of dinosaurs increased rapidly and a lot of new species evolved. Some were big, heavily armored carnivores, while others were large herbivores like the familiar sauropods with their long necks. Carnivorous dinosaurs either hunted alone or in groups, while herbivores used a large variety of protections against them. Some had their size, armor, or horns to defend themselves, while others relied on speed, their senses, or living in a herd. (1)


2. Artist impression of Alamosaurus, a sauropod

At the beginning of the Cretaceous Period that came after the Jurassic Period temperatures on Earth reached a high point, only to decrease again over the next millions of years. (1) Dinosaurs split into two main groups: Saurischia (‘with lizard hips’) and Ornithischia (‘with bird hips). (2) It is in this period that dinosaurs like Tyrannosaurus rex walked the Earth. The T. rex is perhaps the most well-known dinosaur of our age. With its 12 meters long and 6 meters high it was a fearsome predator in Northern America, although some scientists believe the T. rex was a scavenger and not an active hunter. For a long time the T. rex was the largest dinosaur known to man, but today that distinction goes to Spinosaurus, with estimates between 12.6 and 18 meters in length. (4)


3. Reconstruction of Tyrannosaurus rex in walking position

At the end of the Cretaceous Period the continents reached their current positions, although India was an island. Australia, Antarctica and Southern America were still connected together by narrow land-bridges. (1) The primal forests consisting of mainly fern species made place for more modern forests consisting of oaks, beeches, and other broadleaved trees. In between there were groups of coniferous trees and in the underbrush there were flowering herbs. (5) The world was starting to look more and more like it does today.

References:

1. Amersham, B., National Geographic Dinosaurs, 1999, Firecrest Books (page 13-15, 36-46)
2. Ackroyd, Peter, The Beginning, 2003, Dorling Kinderley, London (page 60-82)
3. Palmer, D., Atlas of the Prehistoric World, 1999, Marshall, London (page 99)
4. Dixon, D., Carnivores, 2001, Ticktock Publishing (page 6-16, 28-31)
5. Dixon, D., Herbivores, 2001, Ticktock Publishing

Picture credits:


All pictures from Wikipedia Commons