Scientists have unearthed new fossils of one of the largest turtles that ever lived: a car-sized reptile which prowled the lakes and rivers of what is now northern South America from about 13m years ago to 7m years ago.

The fossils of the turtle – Stupendemys geographicus – were found in Colombia’s Tatacoa Desert and Venezuela’s Urumaco region, and for the first time provide a comprehensive understanding of the creature which grew up to 13ft (4 meters) long and 1.25 tons in weight.

Stupendemys males boasted sturdy front-facing horns on both sides of its shell very close to the neck. Deep scars detected in the fossils indicated that these horns may have been used like a lance for fighting with other Stupendemys males over mates or territory. Females did not have the horns.

Fighting occurs among certain turtles alive today, particularly between male tortoises, according to palaeontologist Edwin Cadena of the Universidad del Rosario in Bogotá, who led the research published in the journal Science Advances.

Stupendemys is the second-largest known turtle, behind seagoing Archelon, which lived roughly 70m years ago at the end of the age of dinosaurs and reached about 15ft (4.6 meters) in length.

The first Stupendemys fossils were found in the 1970s but many mysteries remained about the animal. The new fossils included the largest-known turtle shell – 9.4ft (2.86 meters) long, even larger than Archelon’s shell – and the first lower jaw remains, which gave clues about its diet.

“Stupendemys geographicus was huge and heavy. The largest individuals of this species were about the size and length of a sedan automobile if we take into account the head, neck, shell and limbs,” Cadena said.

“Its diet was diverse, including small animals – fishes, caimans, snakes – as well as molluscs and vegetation, particularly fruits and seeds. Putting together all the anatomical features of this species indicates that its lifestyle was mostly in the bottom of large freshwater bodies including lakes and large rivers,” Cadena added.

Stupendemys – meaning “stupendous turtle” – inhabited a colossal wetlands system spanning what is now Colombia, Venezuela, Brazil and Peru before the Amazon and Orinoco rivers were formed.

Its large size may have been crucial in defending against formidable predators. It shared the environment with giant crocodilians including the 36ft-long (11-meter-long) caiman Purussaurus and the 33ft-long (10-meter-long) gavial relative Gryposuchus. One of the Stupendemys fossils was found with a two-inch-long (5cm) crocodile tooth embedded in it.