Dinosaurs living more than 200 million years ago had some sizable competition. (Representational)

Dinosaurs living more than 200 million years ago may have dominated the plains and grasslands of prehistoric Earth, but it turns out they still had some sizable competition.

Scientists on Thursday unveiled the discovery of a gigantic mammal-like reptile the size of an elephant that they believe rubbed shoulders with large Triassic-era dinosaurs, including the relatives of sauropods such as Diplodocus and Brachiosaurus.

Researchers from Polish and Swedish universities said the 10-tonne creatures -- a distant "cousin" to today's mammals -- challenge the idea that dinosaurs were the only large land animals around the time Earth had a single, enormous landmass known as Pangea.

They unearthed fossilised remains belonging to a previously unknown species of dicynodont, herbivores whose size ranged from small burrowers to large grazers, and who were mostly toothless.

All mammals, including humans, are descended from dicynodonts, despite their reptilian origins.

They managed to survive the mass extinction known as The Great Dying around 250 million years ago that killed up to 90 percent of species on Earth.

But dicynodonts were thought to have died out before the late Triassic period, by which time dinosaurs had become the dominant land creature.

Analysis of limb bones discovered in the Polish village of Lisowice shows the creature -- named Lisowicia bojani -- lived around 210-205 million years ago -- around 10 million years later than previous findings of dicynodonts.

"The discovery of Lisowicia changes our ideas about the latest history of dicynodonts, mammal Triassic relatives. It also raises far more questions about what really makes them and dinosaurs so large," said Tomasz Sulej, of the Polish Academy of Sciences, who worked on the study.

"Such an important new species is a once in a lifetime discovery."

The study, published in the journal Science, said the Lisowicia would have been 40 percent larger than any previously identified dicynodont.

"Dicynodonts were amazingly successful animals in the Middle and Late Triassic," said Grzegorz Niedzwiedzki from Sweden's Uppsala University.

"Lisowicia is hugely exciting because it blows holes in many of our classic ideas of Triassic mammal-like reptiles."