Paleontologists in China looking for clues to how an ancient class of ancient sea-going reptiles known as nothosaurs moved through water say newly discovered fossil trackways in mud may provide some answers.

Nothosaurs, considered the earliest reptiles to live in a water habititat, thrived around 250 million years ago in the Mesozoic era. Fossil examples show they possessed paddle-shaped limbs on an elongated body.

How they moved through the water has long been a matter of debate, with arguments over whether they "rowed" along using their limbs like oars in a forward and back movement or "flew" through the water like modern-day penguins do, "flapping" their limbs in a figure-eight movement.

Tracks discovered in what was once an ancient seabed in Yunnan province in southwest China are the first direct evidence the reptiles moved over the seafloor by rowing their forelimbs in unison, the researchers said.

A number of tracks were found preserved in the mud, displaying slots arrayed in pairs followed by sweeping curves and straight lines, allowing the researchers to construct the movement of the nothosaurs' limbs as the marine predators moved through ancient seas in search of prey.

The measurements of the tracks suggested a group of nothosaurs, ranging in length from around 10 feet down to less than a yard long.

The tracks also yielded clues to how these creatures of the ancient coastlines scavenged for their food, the researchers said.

"We interpret the tracks as foraging trails. The nothosaur was a predator, and this was a smart way to feed," says research leader Qiyue Zhang from the Chengdu Center of China Geological Survey. "As its paddles scooped out the soft mud, they probably disturbed fishes and shrimps, which it snapped up with needle-sharp teeth."

Although primarily marine creatures, nothosaurs may have fed in water but occasionally came ashore on coastal beaches, experts say.

The Yunnan site has great renown for exceptional preservation of fossils and has yielded thousands of intact and detailed sea creature fossils.

The pristine condition of the mud tracks was still a surprise, the researchers said.

"When I first saw the site, I couldn't believe the amazing quality of the fossils," said study co-author Michael Benton of the University of Bristol in Britain. "It's quite unusual to find skeletons of marine reptiles such as the nothosaurs so close to evidence of their tracks."

The research team of scientists from China and Britain collaborated on the study, described in the journal Nature Communications.