Turtles the size of a car once roamed the Earth. Scientists just found their fossils

Palaeontologists work together during the excavation of the giant turtle Stupendemys geographicus. Photo / Reuters

In the swamps of northern South America some 10 million years ago, quotidian life-or-death battles unfolded at epic scale.

Giant caimans, in the same family as alligators, stalked the wetlands of modern-day Venezuela and Colombia, slinking along at 30-feet, snout to tail. Among their most formidable prey: the Stupendemys geographicus, a colossal turtle about which little was known - until now.

New research, published Wednesday in the journal Science Advances, reveals important findings about the Stupendemys, a now-extinct freshwater turtle, and details the discovery of one of its shells - the largest known turtle shell found to date, at nearly nine-and-a-half-feet long. The animal would have resembled, in length and weight, a midsized car.

Venezuelan Palaeontologist Rodolfo Sanchez and a male carapace of the giant turtle Stupendemys geographicus Photo / University of Zurich/Edwin Caden

The hulking reptile was about 100 times the size of its closest living relative, the Amazon river turtle, and twice the size of the largest living turtle, the marine leatherback, the researchers estimated.

The new findings provide the most thorough accounting yet of the Stupendemys, helping scientists answer crucial questions about what may have been the largest turtle to ever live.

"For almost four decades, we didn't have new and excellently preserved fossils of this turtle," Edwin Cadena, a palaeontologist at the Universidad del Rosario in Colombia and one of the study's lead researchers, told The Washington Post.

"Many questions - about its diet, if there were differences between males and females, and even if we were dealing with one or more giant turtle species - were completely unknown."

But thanks to the recently unearthed fossils, dug up in northern Venezuela and Colombia's Tatacoa Desert, Cadena and his co-authors have been able to fill in some of those blanks, which have lingered since the 1970s, when the animal was first described.

It now appears likely that Stupendemys geographicus was the lone species of giant turtle living in the region at that time, he said, and there were differences between the sexes and their diet was diverse and omnivorous.

Among their most surprising discoveries, Cadena said, was the presence of sturdy, front-facing horns on the shells of the males - "something completely new for such a giant turtle," he said.

The researchers hypothesise that the horns were used as "weapons in male-male combat behaviours." Deep scrapes in the horn of one fossil indicated they may have been used by turtles tangling over territory, they said.

Cadena and his team found more marks, too, some that told of their fearsome, frantic fights with the Purussaurus, the giant caimans that roamed the northern Neotropics during the Miocene epoch, the same time and place as the Stupendemys. The scars from their skirmishes are still visible today.

Some of the Stupendemys fossils had bite marks and punctured bones, and one shell had a tooth embedded in it.

Earth's landscape at the time of the Stupendemys bore little resemblance to today's topography. The turtle's habitat has turned to desert, but then, it was humid and swampy. The Andes weren't yet fully formed and the Orinoco and Amazon rivers cut different paths.

A sprawling wetland and lake system meant plenty of room for massive animals - especially the Stupendemys, which spent most of its days at the bottom of freshwater streams and small lakes, Cadena said. They likely lived across the whole northern part of South America.

But those ideal conditions were not to last. Over time, plate tectonics pushed the Andes higher, disrupting the water systems and drastically reducing the scope of their habitat, the researchers wrote.

At some point, that which had come to signify their success on earth, the conquering of their environment - their huge size - was no longer enough to keep them alive. In the early Pliocene, around 5 million years ago, they went extinct.