"They belong to a group that scientists had thought had gone extinct a long time earlier," said Dr Christian Sidor, a palaeontologist at the New York College of Osteopathic Medicine in the United States.

"Our study highlights the interaction between climate and evolution of life on land," he told Reuters.

About 250 million years ago, much of the Earth's land mass was fused into one huge continent called Pangaea. Scientists had thought that the same type of animals could have existed and gone anywhere on the massive continent, but the newly found fossils were completely different.

Sidor and his team, who reported their findings in the science journal Nature, suspect the two new species were isolated in Niger, at the centre of Pangaea, by its climate.

Between the early and later Permian period, the Earth went through a long warming spell. Huge glaciers existed early on but by the end of the period the centre of Pangaea had developed into a large desert-like area, which is where the fossils of the creatures were found.

"We think our animals were basically isolated from the rest of what was going on in Pangaea by this large desert climate. So climate is really the driving force between what we see in Niger and basically the rest of the world," Sidor said.

"We found what looks to be a little pocket, or endemic fauna, right in the centre of Pangaea."

Scientists had thought the group had become extinct 40 million years earlier, so finding two new specimens was a complete surprise.

The species, dubbed Nigerpeton and Saharastega, measured about 7-8 feet (2.1-2.4 metres) and 5-6 feet long respectively and were aquatic to some degree.

The animals are amphibians so Sidor and his team suspect there must have been some water in the area. They hope to go back to Niger to test the climate hypothesis.

During that time, Africa was not dissected by large oceans or mountain ranges so the researchers struggled to understand why the animals were found nowhere else.