Fossils of animals from 383 million years ago found in Arctic Canada hold important information on how fish turned into land creatures.
According to the theory of evolution, today's animals got here after millions of years of growing and changing.
But little is known about the stage where creatures developed from living underwater to walking on land.
The new fossils are described as crocodile-like animals with scales on their backs and fins instead of limbs.
The fossils are a really important find because they fill in a gap in information about evolution.
Scientists were already aware of a fish called Panderichthys that that lived 385 million years ago and the first four-limbed land-living animals called Acanthostega from about 365 million years ago - but they couldn't work out what had happened in between.
A cast of the fossil has gone on display at the Science Museum in London.