The 15cm specimen, named Casineria kiddi, dates back almost 340 million years to a dark age of animal evolution about which little is known.
All life on Earth can be traced back to the sea. Creatures with limbs and digits had evolved from fish by the end of the Devonian period 365 million years ago. But little is known about the time between this event and the appearance of fully terrestrial animals in the late Carboniferous period, about 335 million years ago.
But the fossil, found in a rock formation called the Cheese Bay Shrimp Bed, near Edinburgh, provides a clue to what was happening during those 30 million years.
The creature's remains consist of a number of fossilised skeletal fragments, but the skull is missing. Its most important feature is a five-digit limb - the earliest known in the fossil record.
This marks out C. kiddi as a land-dweller, placing it closer to amniotes (modern reptiles, mammals and birds) than to aquatic amphibians. The discovery suggests that amniotes date back a long way, to an era of rapid evolution early in the Carboniferous period.
A team of English and Scottish experts wrote in the journal Nature: "The degree of terrestriality exhibited by Casineria indicates that the transition to land-dwelling may have taken place within about 20 million years. The physical resemblance of Casineria to known true amniotes from the late Carboniferous period and its apparent phylogenetic relationship to these forms indicates that the split between amphibians and amniotes probably also occurred rapidly within this time-span."Reuse content