Nine out of 10 species that existed on the Earth were killed off by the Permian Mass Extinction.
After those catastrophic years, the most successful species were nature's loyal brigade of cleaners and recyclers – the fungi. When times get really tough, history shows it's best to be small – like insects, bacteria and fungi. For a while, the Earth belonged to them.
Only one species of mammal-like reptile survived at all: the Lystrosaurus. If this creature had perished, evolution would almost certainly have shunted yet-to-come mammals and humans into life's might-have-beens, consigning our ancestors to the waste bucket of life, an experimental cul-de-sac. No matter, then, that our reptilian saviour wasn't exactly blessed with good looks, being something like a cross between a hippopotamus and a pig.
No one knows quite why or how the Lystrosaurus survived the gigantic trauma of the Permian Mass Extinction, but for millions of years it thrived across all Pangaea. There is almost no record of any other vertebrate alive during this time, 230 million years ago, which is called the early Triassic Period.
Then, out of the barren sands of extinction, came a completely new generation of land-living reptiles – one that grew into the most fearsome and dominant prehistoric force that had ever trodden the Earth. Dinosaurs were reptiles that lived on land. They had rearward-pointing elbows and forward-pointing knees, with hips that allowed many of them to walk on two legs. Most were large – their average weight is estimated at 850kg, whereas the average weight of a modern mammal today is only 863g.
Like the Dimetrodon that thrived before the Permian Mass Extinction, dinosaurs benefited greatly from living in an era when it was possible to walk from pole to pole. Thanks to the arrangements of the Earth's continents into one giant landmass, the strongest land-based creatures of this era had a unique opportunity to evolve into a position of unassailable dominance. Natural barriers, such as oceans, never got in their way. It was almost inevitable that their success would lead to a reduction in the variety of other land-living things.
The dinosaurs were the first creatures ever to have their legs directly under their bodies at all times, whether walking, running, galloping or hopping. This is called a "fully improved stance", and meant that many of them could walk upright. This may have been the biggest single biological factor in the dinosaurs' success. Animals with a fully improved stance could grow bigger, move further and walk faster than other living creatures, allowing many body types and lifestyles to evolve.
Some of the biggest dinosaurs were the sauropods, such as the Diplodocus. These beasts, which walked on all four legs, could grow up to 27.5m long and weighed up to 11 tons. Their survival strategy was simple: they grew so large that few other creatures were big enough or strong enough to kill them. So they did very well. In 1994, a 147m-long fossilised sauropod trackway was found along an ancient muddy estuary in Portugal. The huge footprints showed scientists how these creatures walked, confirming that they held their bodies above their legs and lifted their long tails to stop them dragging along the ground, allowing them to walk more easily.
Others were fast. Hypsilophodon grew only to about the height of an adult human's waist, but could run as fast as a modern deer. More than 20 of their skeletons have been found fossilised on the Isle of Wight, where they unwittingly wandered into a pool of lethal quicksand. With long, thin feet and short thighbones for rapid forward and backward movement, these two-legged creatures had self-sharpening teeth and lived off plants. They survived purely thanks to being able to run away from danger so fast.
Other dinosaurs could walk on either two or four legs, such as the Iguanodon discovered by Gideon Mantell. This beast couldn't run out of harm's way. Instead, its thumb was cleverly crafted into a terrifyingly sharp dagger which it used to defend itself, usually standing upright on its hind legs to fend off attackers.
But one of the strongest beasts ever to stalk the Earth was Tyrannosaurus rex – T Rex for short. It belonged to the therapod family, which originated in what is now western North America.
T Rex walked on two legs, and had a massive skull, balanced by a long, heavy tail. Its hands had just two fingers, and its forearms were quite short compared with its massive legs and tail. At 12m long and weighing the same as a modern elephant, this was one big dinosaur. It dined off either dead carcasses or live prey – possibly both.
Some experts think T Rex could run fast despite its huge size – perhaps up to 30mph. Others think it lumbered around at a more modest 10mph. No one knows for sure how powerful its muscles actually were, because all that remains are skeletons reconstructed from the more than 30 specimens found in rocks around the world. But with jaws more than eight times as powerful as a lion's, the T Rex could literally pulverise bones to extract the nutritious marrow inside. Its teeth were like a shark's, continually being replaced throughout its life.
The most complete T Rex skeleton to date was discovered on 12 August 1990 in a place called the Hell Creek Formation in South Dakota by Sue Hendrickson, an amateur fossil-hunter. This almost complete skeleton, called Sue in honour of its finder, measures some 4m high and 13m long.
Finding fossils like this is serious business. After a protracted legal fight over its ownership, Sue was ultimately declared the property of landowner Maurice Williams. He eventually sold her at auction for $7.6m.