By analyzing ancient seabed imprints left by marine reptiles that inhabited the Earth during the Mesozoic, a team of researchers based in the UK and China may have finally discerned how these enigmatic predators propelled themselves through the ocean. The study has been published in Nature Communications.

Predatory marine reptiles dominated the oceans of the Mesozoic, or “middle life”, era (~251-66 million years ago) which spans the Triassic, Jurassic and Cretaceous periods. Around 245 million years ago, during the Triassic period, a group of semi-aquatic predators called nothosaurs stalked the oceans. These ancient reptiles had long, slender bodies and paddle-like limbs to efficiently propel them through the water.

Although scientists have some idea about the lifestyle of these top predators their mode of locomotion in the sea remained a mystery, in part because no tracks had ever been found. Did these reptiles swim like penguins, swirling their paddles in figure eight patterns? Or did they draw their limbs back and forth like a rowing boat?

The answer was found in the Luoping localities in Yunnan, southwest China; a site renowned for its remarkable fossil preservation. Thousands of ancient sea creature fossils have previously been unearthed here and now, conveniently, ancient tracks left by a marine predator have been discovered.

The tracks consisted of a long series of paired imprints in the mud that followed both straight lines and curves. Researchers based at the University of Bristol and the Chengdu Center of China Geological Survey analyzed the tracks and judging by their size and spacing, the team think that they were most likely left by nothosaurs. In particular, the scientists believe they belonged to two different species; Nothosaurus and Lariosaurus.

“We interpret the tracks as foraging trails,” explained lead researcher Professor Qiyue Zhang in a news-release. The prints also revealed that the nothosaurs used their forelimbs to propel themselves through the water, rowing with both forelimbs in unison. By digging their forelimbs into the seabed, they may have been able to flush out prey hiding in the mud.

“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,” added Zhang.

Fossils discovered in sites like Luoping are helping scientists understand how ecosystems recovered after the great Permo-Triassic extinction which is thought to have wiped out over 90% of all species on Earth.

“Here we use a detailed snapshot of how life was within 8 million years of the mass extinction,” said co-author Professor Shixue Hu. “It took all that time for the Earth to settle down from the cataclysm, and the arrival of these large, complex marine predators shows us the ecosystems had finally rebuilt themselves, and life could be said to have recovered from the crisis.”