A 370-million-year-old skull and shoulder bone from a walking fish have shed light on when and how our distant ancestors slithered from the sea to begin a new life on dry land.

The fossilised remains belong to a beast which had the head of a tetrapod - among the first animals more adapted to land than water - and a body and fins resembling its fish-like predecessor, Panderichthys.

A study, published in the British journal Nature, says the creature, known as Ventastega curonica, had an ample jaw and razor-like teeth, suggesting a ferocious predator the size of an adult crocodile.

The creature - fragments of which were uncovered at a site in western Latvia - also had primitive flippers, allowing it to explore shallow marshes for prey.

Other fossil specimens of this strange species exist, but none is as complete or intact, according to the study.

'Missing link'

Per Ahlberg, of Uppsala University in Sweden, led the team of palaeontologists which made the Latvian discovery. The group say the area was once part of a swampy semi-tropical continent straddling the equator.

The fossil suggests early amphibious animals of the Late Devonian period did not evolve in a simple linear fashion, as once thought, but diversified along differing branches.

"It is tempting to interpret Ventastega as a straightforward evolutionary intermediate," the Swedish-led authors say. "However, this simple picture should be approached with a degree of caution."

The discovery comes two years after the unveiling in 2006 of a previously unknown species - dubbed Tiktaalik roseae - also described as a "missing link" between its ocean-dwelling precedessors and full-fledged tetrapods.

The scientists say their Latvia find falls into the morphological gap between Tiktaalik and the first of the tetrapods, Acanthostega.