Fossil was 'first walker'

Pederpes finneyae: It was originally unearthed in 1971

The most primitive foot to walk on land has been described by scientists.

It belonged to an animal that lived about 345 million years ago - in what is now Scotland.

This fossil fills in a huge (20-million-year) gap in the fossil record

Dr Jenny Clack

The skeletal remains are the oldest in the fossil record to show bones that had the ability to move on land.

Dr Jenny Clack, who has studied the specimen, says it illustrates how life on Earth made the transition from a purely water-borne existence to one where creatures were able to forage on the shoreline.

"This is the first proper, walking foot," she told BBC News Online. "We have earlier feet, but they were for paddling - for swimming."

'Sluggish crawler'

The fossil was unearthed in 1971 from limestone deposits north of Dumbarton. Held at the Hunterian Museum in Glasgow, it was thought to be a fish.

The back foot has a structure that aids walking

Only recently was the surrounding rock cleared away sufficiently to reveal a creature with legs. One hind limb has a near-complete foot attached with five digits.

It has been classified as Pederpes finneyae. It was a short-limbed, large-skulled predator. It was about a metre in length and may have had the look of an ungainly crocodile.

"It was probably quite a sluggish crawler through the swamps where it lived," Dr Clack says.

The identification helps close a hole in the early fossil record of a group of creatures called tetrapods - backboned animals with four legs or limbs.

Bone twist

The oldest-known tetrapods are from the Devonian Period (more than 360 million years ago), but the fossils so far discovered are of animals that were clearly all swimmers. These creatures would have scuttled around just under the water.

And later tetrapods, from the Upper Carboniferous (about 340 million years ago), are modern-looking amphibian-like animals whose appendages were well-evolved to walk on land. They were true landlubbers.

Dr Clack: "A link, if you like, which is no longer missing"

The significance of Pederpes finneyae is that it straddles the two - both in terms of time and in its bone structure. It probably spent time in the water and on land.

"[P. finneyae] has a kind of twist on its bones - an asymmetry that allows it to bring its feet forward for walking," Dr Clack says. "Previously, tetrapod feet either pointed up to the sides or backwards as a paddle for swimming. The locomotion of [P. finneyae] is quite different to what went before."

Later tetrapods have a more developed form of this bone construction, she says.

"This fossil fills in a huge (20-million-year) gap in the fossil record. It is a link, if you like, which is no longer missing."

Scientists say tetrapods were the first animals known to walk the Earth and are the ancestors of today's mammals, reptiles, amphibians and birds.

P. finneyae is described in the journal Nature.