Dinosaurs - Chronology - 220 million years ago

When dinosaurs first appeared about 230 million years ago the world was very different to as we know it. There were very few representatives of any animal groups alive today - no mammals, birds or lizards (although there were some lizard-like reptiles).
What? No grass?
The difference was also apparent in the plant kingdom. There were no flowering plants, which includes most of the common trees and shrubs today.

The trees would not look very familiar to us today, although some were relatives of modern day ferns and podocarps. Plant life would have seemed very drab, just green and brown in colour. There was no grass, instead, low ground cover would have been ferns and mosses.

The Triassic world was unusual for another reason. About 20 million years before the appearance of the first dinosaurs, the biggest extinction the world had ever known had occurred. Over 90% of all plant and animal species then alive on land and in the sea had died out at this time. Even in the Late Triassic the world was still recovering and there was not the usual variety of life normally found on earth.

A giant desert
The continents were configured differently to today. All the land masses of the earth were joined together into one huge continent called Pangaea. This stretched from pole to pole and its central region was a vast inhospitable desert.

We can tell this, as the type of rocks that were deposited at this time have sedimentary features characteristic of a dry harsh climate. As all the continents were connected, the animals and plants found in the fossil record from that time are very similar all over the world.

It took more than 10 million years before ecosystems recovered and complex systems and larger animals took even longer. Most of the dominant land animals that were around when dinosaurs evolved were products of long and established lines of descent.

New life
The Late Triassic was an innovative time in the animal kingdom. By the end of the period not only the dinosaurs had appeared but also pterosaurs (flying reptiles), various kinds of marine reptiles, the first crocodiles, turtles and the earliest true mammals.

Towards the end of the Triassic, 220 million years ago, there was another extinction, which wiped out many of the non-dinosaurs including the dicynodonts such as Placerias and primitive archosaurs such as Postosuchus. It was after this that dinosaurs really started to radiate and diversify.

Dinosaurs gain the edge
It is likely that dinosaurs did not out-compete other animals as has often been assumed, due to their superior speed and agility, but that they were fortunate in that they were not hit as hard by the extinctions. Another extinction at the very end of the Triassic, wiped out the remaining primitive archosaurs and the dinosaurs were the only large land animals left.