Even in the savage world of dinosaurs, the meek shall inherit the earth, a new discovery suggests.
A dwarf dinosaur that lived 190 million years ago challenges the view that the "terrible lizards" conquered the world by overpowering their rivals.
Instead it is more likely they simply stepped into the space left by departing competitors, say researchers.
Newly-discovered dinosaur Sarahsaurus was around 14 feet long and weighed about 250lb.
It may seem big by modern standards, but the four-footed plant-eater was a small ancestor of giant sauropods such as diplodocus - the largest land animals that ever lived.
Evidence from Sarahsaurus and two other similar "sauropodomorphs" indicates that each migrated north from their birthplace in what is now South America long after a major extinction event.
No such dinosaurs moved to North America before the disaster at the end of the Triassic Period, one of five great mass extinctions in the Earth's history.
Study leader Professor Tim Rowe, from the University of Texas at Austin, US, said: "We used to think of dinosaurs as fierce creatures that outcompeted everyone else. Now we're starting to see that's not really the case.
"They were humbler, more opportunistic creatures. They didn't invade the neighbourhood. They waited for the residents to leave and when no one was watching, they moved in."
The research is reported in the journal Proceedings Of The Royal Society B.
Sarahsaurus had physical traits usually associated with gigantic animals, including long, straight, pillar-like thigh bones.
Prof Rowe said he was also intrigued by its hands, which were small but powerful, with large claws.
"It's doing something with its hands that involved great strength and power, but we don't know what," he said.
Sarahsaurus is named after Sarah Butler, a philanthropist who raised money for an interactive children's exhibit at the Austin Nature and Science Centre called "Dino Pit".