Dinosaurs weren't that big or clever... just really, really lucky

Dinosaurs didn't rule the Earth because they were the fittest - they just got really, really lucky, according to scientists.

Tyrannosaurus Rex and the rest of his species were actually dominated by a bunch of giant crocodiles. Dinosaurs were were just fortunate to avoid two mass extinctions that wiped out their rivals.

Crurotarsal archosaurs weighed ten tonnes and grew to 12m (40ft) but were wiped out by a massive storm in the Triassic period, leaving dinosaurs with free rein.

Scientists believe dinosaurs fared better than the crurotarsans because many of them walked on two legs, not four, and they may have been warm-blooded.

'There was no sign that dinosaurs were eventually going to succeed,' said researcher Steve Brusatte of Columbia University.

'If 210 million years ago, we had to bet on which group would dominate, all reasonable gamblers would go with crurotarsans.

'It's not that the dinosaurs weren't doing well. The crurotarsans were doing more.'

Two massive storms, one possibly caused by an asteroid, led to the extinction of many species, including almost all crocodiles.

'The dinosaurs not only got lucky, but they got lucky twice,' Mr Brusatte said.

But well before the Jurassic period, when dinosaurs ruled, the crurotarsans - with 45cm (18in) teeth - vastly outnumbered them, the team from the University of Bristol wrote in the journal Science.

'The fundamental question is why were the dinosaurs able to become so dominant,' Mr Brusatte said.

'Evolution on a big scale often times is a matter of luck.'

Apart from the crurotarsans, there were phytosaurs that looked and lived a lot like today's crocodiles, staying submerged in rivers or lakes until attacking a victim.

The North American phytosaur Smilosuchus grew to 12m (39ft) long.

Others called rauisuchians were land predators with four powerful legs, massive skulls and flesh-tearing teeth.

Both were far more impressive than the typical dinosaur predator of the time like Coelophysis, a relatively lightly built, two-legged hunter about 3m (10ft) long.

The dinosaur plant eaters of the time like Plateosaurus were getting big, but were less varied than the plant-eating crurotarsans such as the heavily armoured aetosaurs.