The scientists have come to this conclusion after studying the new dinosaur, named Sarahsaurus, discovered in Arizona where this animal lived about 190 million years ago.
It is believed that dinosaurs originated in South America and then rapidly spread out to every corner of the world, overwhelming all the animals in their path.
But the new discovery of Sarahsaurus challenges that view. "Until recently, we have viewed dinosaurs as very successful animals that outcompeted other species wherever they went,'' according to Prof Robert Reisz of the University of Toronto, who studied the species with Prof Tim Rowe of the University of Texas and Hans-Dieter Sues of the National Museum of Natural History in Washington.
"But this study puts dinosaurs in a very different light-that they were more opportunistic creatures that moved into North America only when a mass extinction event made eco-space available to them,'' the Canadian scientist is quoted as saying.
Though one of the five great mass extinction events in earth's history that happened at about 200 million years ago is said to have wiped out many of the potential competitors to dinosaurs, new evidence from Sarahsaurus suggests that they migrated into North America long after the extinction and that no such dinosaurs migrated there before the extinction.
Just 4.3 metres long and weighing only about 113 kilogrammes, Sarahsaurus is very small compared to earlier giant sauropods, the largest land animals in history.
The research appears in the online edition of the journal Proceedings of the Royal Society B.