After studying fossils from an ancient ground sloth found in 2004, a study published in Biology Letters has concluded the tooth marks covering the bones were from the animal being preyed on by a 4-meter (13-foot) juvenile ancient crocodile species.

After an expedition to the Napo River in Peru revealed beds containing 13-million-year-old fossils from the Miocene era (~16-11.6 million years ago), researchers recovered damaged tibia bones belonging to a large mammal. A left tibia was identified as belonging to a mid-sized ground sloth (Pseudoprepotherium) that grew to around the size of a capybara, smaller than some of its giant relatives. The bones were littered with tooth marks akin to the shape of those left after a crocodile attack.

"The tibia discovered in the Peruvian Amazonia is the first one of a mammal bearing crocodylian tooth marks and therefore crucial for understanding the dynamic of ancient ecosystems," co-author Dr Rodolfo Salas-Gismondi told CNN.

The massive crocodylian, believed to be a giant caiman Purussaurus, managed to puncture deep into the bone and peppered the fossil with 46 puncture marks. The Purussaurus was one of the most fearsome predators of its time, growing 10.3 meters (34 feet) in length and carrying a bite that has no equal. According to the study, adult Purussaurus could crush anything in its jaws with a massive bite force of 69,000 Newtons, well over four times the most powerful bite ever recorded, belonging to the Nile crocodile. Impressively, the authors believe the creature that preyed on the sloth was just a juvenile, so whilst it was easily the length of an average modern-day American alligator, it was barely half the size of a fully-grown adult.

Despite its juvenile size, the giant caiman clasped around the leg and attempted to rip the limb from the struggling sloth, according to the authors. Unfortunately for the sloth, it did not survive the attack.

To decipher what predator was responsible for the attack, Dr Salas-Gismondi and co-author Dr François Pujos had to eliminate all other possibilities. Analyzing the tooth marks’ shape and depth eliminated various other genera of caiman, with many simply being too small to be able to inflict the damage seen on the bones. After careful inspection, the puncture marks appeared to match the "conical and blunt teeth" of the Purussaurus, which has both the size and tooth shape to be the likely perpetrator of the hunt.

Evidence for the interactions between reptiles and mammals in Miocene swamps is lacking, so this finding provides a fascinating insight into the hunting habits of large predators like Purussaurus. Present-day species of crocodile will latch onto the hind limbs of large prey before dragging them into water to drown, and the findings presented here suggest a similar behavior from prehistoric species.

Before this study, reptilian predators residing in swamps at this time were thought to mainly prey on large turtles. After the finding of the sloth remains, the researchers believe this represents an alternate dynamic in which the massive caimans would also ambush land animals, crushing them with their immense bite before dragging them to their demise.