A pair of researchers, one with Instituto Argentino de Nivología, the other Universidad Peruana Cayetano Heredia, has found fossil evidence of an ancient giant ground sloth living in proto-Amazonian swamps. The fossil has shinbone bite marks from a Miocene caiman Purussaurus, a large crocodilian species from the period. In their paper published in the journal Biology Letters, François Pujos and Rodolfo Salas-Gismondi describe the fossil and what they learned about it.

The fossil was found in the rocks of a Pebas formation (near the Napo River) of the Peruvian Amazon back in 2004, but until now, it had not been thoroughly studied. The researchers found it to have belonged to a giant ground sloth from approximately 13 million years ago. Prior research has suggested that the area where the fossil was found was a wetland system at that time. Giant sloths roamed the edges of the water, eating the grasses that grew there. Prior research has also found that there were at least seven crocodilian species in the region, though some were quite small. The size and shape of the bite marks ruled out all but Purussaurus.

The researchers note that during the time period of the fossil, the area had not yet been overtaken by mammals, as virtually all of the land-dwelling carnivores were marsupials. Purussaurus, the largest of the crocodilian species, inhabited the water and is believed to have been the largest of the species to have ever lived, growing to lengths of 20 feet and having a bite believed to have been twice as powerful as T Rex.

The researchers suggest that the Purussaurus sprang up out of the water, just as crocs do today, and grabbed the sloth by the hind leg. Once it had a firm grip, it likely went into a "death roll" dragging the sloth into the water and holding it there until it drowned. The researchers found evidence of 46 tooth marks in the fossil, which showed that the sloth had been repeatedly bitten by the Purussaurus as it sought to get a firmer grip. The Purussaurus left behind shallow pits and scores, as well as holes that had fully penetrated the tibia. There was also no evidence of bone regeneration, strongly suggesting the sloth did not survive the attack.