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A 270-million-year-old fossil that sheds light on the murky origins of amphibians has been named after Kermit - the world's most famous frog.

Scientists found the fossilized skull in a collection at the Smithsonian National Museum of Natural History.

They say it's a new species of proto-amphibian and have named it Kermitops gratus because it bears a likeness to the iconic Muppet.

The much-loved, but famously shy frog would doubtless blush at the attention.

The researchers said the fossil was part of a group of relatives of amphibians that lived for more than 200 million years, but was totally unique.

There were a lot of features that were different from those in older tetrapods, amphibians' ancient ancestors.

For example, the region of the skull behind the animal's eyes was much shorter than its elongated, curved snout. Unique features like these led scientists to conclude that it belonged to a new species.

'What really jumped out to us was how [the fossil looked] bug-eyed, and due to slight crushing during the preservation it gave it kind of like a lopsided, crooked smile, and it really evoked Kermit's smile," said Calvin So, a doctoral student at George Washington University and lead author of the study.

The prehistoric fossil had spent decades in the Smithsonian's National Fossil Collection waiting for a scientist to take a closer look at it.

That would likely resonate with Kermit, whose hit song "It's Not Easy Being Green" reflected on how it felt to be constantly overlooked.

But Kermitops gratus is a fossil of enormous significance, the scientists say.

The early fossil record of amphibians and their ancestors is largely fragmentary which makes it hard to figure out how frogs, salamanders and their kin evolved.

Discovering new species like Kermitops is essential for fleshing out the early branches of the amphibian family tree, the researchers said.