Scientists have unearthed the skull, shoulders and part of the pelvis of the most primitive four-legged creature in the earth's history.

The 365 million-year-old fossil of the water-dwelling Ventastega curonica was found in Latvia, researchers report in a study published in Thursday's issue of the journal Nature.

The find will help scientists better understand the evolution of fish to advanced animals that walk on land, the researchers said.

Even though Ventastega is likely an evolutionary dead-end, the finding sheds new details on the transition from fish to tetrapods.

Stubby limbs, unknown number of digits

Tetrapods are animals with four limbs or those whose ancestors had four limbs, including amphibians, birds, reptiles and mammals. While an earlier discovery found a slightly older animal that was more fish than tetrapod, Ventastega is more tetrapod than fish.

The prehistoric creature, which existed more than 100 million years before the first dinosaurs roamed the earth, probably swam through shallow brackish waters, was about one metre long and ate other fish.

It likely had stubby limbs with an unknown number of digits, scientists said.

"If you saw it from a distance, it would look like a small alligator, but if you look closer, you would find a fin in the back," said lead author Per Ahlberg, a professor of evolutionary biology at Uppsala University in Sweden.

Scientists don't think four-legged creatures are directly evolved from Ventastega.

"It's more likely that in the family tree of tetrapods, Ventastega is an offshoot branch that eventually went extinct, not leading to the animals we now know," Ahlberg said. "At the time, there were a lot of creatures around of varying degrees of advancement."

They all seem to have similar characteristics, so Ventastega's find is helpful for evolutionary biologists.

Out of sequence

Ventastega is the most primitive of these transition animals, but there are older ones that are oddly more advanced, said Neil Shubin, professor of biology and anatomy at the University of Chicago. Shubin was not part of the discovery team but helped find Tiktaalik, the fish that was one step earlier in evolution.

"It's sort of out of sequence in timing," Shubin said of Ventastega.

Ahlberg didn't find the legs or toes of Ventastega but was able to deduce that it was four-limbed because key parts of its pelvis and its shoulders were found. From the shape of those structures, scientists were able to conclude that they were attached to limbs, not fins.

One question that eludes scientists is why fish started to develop what would later become legs. Edward Daeschler, associate curator of vertebrate zoology at the Academy of Natural Sciences in Philadelphia, theorizes that the water in which they lived was so shallow that critters like Ventastega had an evolutionary advantage by walking instead of swimming.