The biggest family tree of dinosaurs mapped out so far suggests that the origin of dinosaurs may have been 20 million years earlier than previously thought.

A study published in the journal Biology Letters says that the origin of dinosaurs was most likely about 250 million years ago.

The probabilistic method that the study used found a small chance that the origin could have been even earlier, at about 260 million years ago at the end of the Permian period. However, a date of 10 million years later than that came out as the most likely date for the emergence of Dinosauria.

If this earlier date of 260 million years ago is correct, then it means that the very earliest dinosaurs would have survived a massive extinction about 250 million years ago, study author Graeme Lloyd of the Natural History Museum in London told IBTimes UK.

The Permo-Triassic extinction was even more devastating than the mass extinction at the end of the dinosaurs in terms of the numbers of species that were wiped out. Yet there is a slight chance that dinosaurs were already present on Earth when this earlier extinction happened and they made it through, Lloyd says.

This estimate was based on Nyasasaurus being the earliest dinosaur. However, it is contentious whether Nysasaurus was in fact a dinosaur or not.

"Most dinosaurs are known from a few scraps rather than nice complete skeletons like the ones we have of Tyrannosaurus Rex. So we have less information to go on. The Nyasasaurus comes from the about 240 million years ago. That's older than all the other dinosaur fossils we have."

Lloyd's method used the largest family tree of dinosaurs composed to date. Using this he calculated the date of the origin of dinosaurs to be earlier than the oldest dinosaur fossil found – as might be expected due to the patchiness of the fossil record, he says.

Lloyd and his colleagues used two probabilistic methods to put a date on the origin of dinosaurs, which honed in on the same period. "Both of the methods seem to agree quite closely. We can never know for sure when these things happened but if two methods agree that's the most encouraging evidence we've got."