Continued: From fish to four legs

Scientists unearthed a skull of the most primitive four-legged creature in Earth's history, which should help them better understand the evolution of fish to advanced animals that walk on land. The 365 million-year-old fossil skull, shoulders and part of the pelvis of the water-dweller, Ventastega curonica, were found in Latvia, researchers report in a study published in today's issue of the journal Nature. Even though Ventastega probably was an evolutionary dead end, the finding sheds new light on the evolutionary transition from fish to tetrapods. Tetrapods are animals with four limbs and include amphibians, birds and mammals.

While an earlier discovery found a slightly older animal that was more fish than tetrapod, Ventastega is more tetrapod than fish. The fierce-looking creature probably swam through shallow brackish waters, measured about 3 or 4 feet long and ate other fish. It probably 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 looked closer, you would find a fin in the back," said lead author Per Ahlberg, a professor of evolutionary biology at Uppsala University in Sweden. "I imagine this is an animal that could haul itself over sand banks without any difficulty. Maybe it's poking around in semi-tidal creeks picking up fish that got stranded."

This all happened more than 100 million years before the first dinosaurs roamed Earth.

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 died off, 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," Ahlberg said. They all seem to have similar characteristics; so Ventastega's find is helpful for evolutionary biologists.

Ventastega is the most primitive of these animals, but there are older ones that are oddly more advanced, said Neil Shubin, professor of biology and anatomy at the University of Chicago, who 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," he said.

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