This 1.5-metre-wide dinosaur footprint was found in Plagne, France, in April. (Associated Press/Hubert Ragueet/CNRS) Fossil hunters in France have found some of the largest dinosaur footprints ever documented, measuring 1.5 metres in diameter.

The prints were made about 150 million years ago by large, long-necked, herbivorous dinosaurs called sauropods, which were more than 25 metres long and weighed up to 40 tonnes.

Paleontologist Jean-Michel Mazin of France's National Centre of Scientific Research (CNRS) said 20 prints have been found on a 10-hectare site near Plagne in eastern France.

The site in the Jura Mountains was a flat, muddy area near a shallow sea when the sauropods lived there, in the Jurassic period, which takes its name from the mountains.

Mazin said there could be hundreds or even thousands more footprints still hidden.

From the prints, "we can calculate their size and speed, find out about their behaviour and learn how they got around," Mazin told The Associated Press.

Amateur fossil hunters Marie-Helene Marcaud and Patrice Landry found the prints in April on a hiking path. Researchers at the CNRS only recently authenticated them.

"They were very hard to see because there were a lot of little stones [on the ground], there was grass growing there, and you really had to have a trained eye to notice something," said Mazin.

Mazin said it's believed the dinosaurs left their tracks in the mud, which then dried in the sun and set like plaster. The sea then washed sediment into the prints, sealing and protecting them, essentially creating fossilized footprints.

With files from The Associated Press