Bristol University researchers have found out how ancient marine reptiles navigated through water.

During the Mesozoic era (252-66 million years ago), the seas were dominated by a variety of marine reptiles. Nothosaurs - which lived during the age of the dinosaurs - were one of the earliest groups. The group comprised of voracious semi-aquatic hunters with elongated bodies and paddle-like limbs. The ancient monsters were top predators of the Triassic coasts, some 245 million years ago.

Researchers long debated about their swimming style - whether they used their limbs to row back and forth or did they 'fly' underwater by sweeping their forepaddles.

For the study, researchers examined trackways formed on an ancient seabed in Yunnan, southwest China. The tracks featured slots in the mud, arranged in pairs and in long series of 10 to 50 that follow straight lines and sweeping curves.

The researchers said, looking at the size and space of the paired markings, that they were created by nothosaurs - ranging from more than 3 metres to less than a metre in length. The markings showed that these reptiles propelled themselves through water by rowing their forelimbs in harmony. 

The large Nothosaurus and the tiny Lariosaurus are likely behind the trackmakers.

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

The tracks were found in localities around Luoping in Yunnan that has produced thousands of exquisite fossils of sea creatures.

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

These findings could help researchers gain insight into the Permo-Triassic mass extinction that destroyed about 90 percent of all species on Earth.

The finding is published in the journal Nature Communications.