An artist's rendition shows the giant snake lounging in the tropical jungle 60 million years ago, near primitive crocodiles and giant turtles. (Jason Bourque/Nature)

Researchers have found the remains of what they are billing as the biggest snake the world has ever known — an animal estimated to be longer than a city bus and heftier than a car.

The boa constrictor-like reptile lived in South America about 60 million years ago and its size provides valuable clues about what the climate was like in the equatorial tropics at that time, said a study published in Thursday's issue of the journal Nature by an international team of authors that included University of Toronto paleontologist Jason Head.

"If it was around today and it swallowed you, you wouldn't even be a bulge in its belly," said Head.

He said he first saw one of the snake's giant vertebrae, the bones in its backbone, while chatting online with co-author Jonathan Bloch, from the University of Florida, who held them up in front of webcam.

A vertebra (one bone of the spine) of a 5-metre long anaconda, one of the largest living snakes, looks tiny beside a similar bone from the 13-metre long Titanoboa. (Ray Carson/University of Florida News Bureau)

"I jumped out of my seat and got very excited and he started laughing and I started laughing … because it's just such a mind-bogglingly big animal," Head recalled.

Based on the size of the snake's vertebrae — the largest ever for either a living or extinct snake — the researchers estimate that the ancient snake could have grown to be 13 metres long and weigh about 1,135 kilograms.

Head said the largest modern snakes are reticulated pythons, which reach a length of about nine metres, and green anacondas, which can grow to be 7.5 metres long.

Researchers found the vertebrae and ribs for about 28 individual snakes of this species, which was given the name Titanoboa cerrejonensis to indicate its great size and the fact that it was found in the Cerrejon region of northeastern Colombia. The type of pollen found with it suggested that it lived about 58 to 60 million years ago, roughly six to eight million years after the dinosaurs went extinct.

Head said cold-blooded animals such as snakes can't generate their own heat, so they need an external heat source to power their metabolisms. Because larger animals have slower metabolisms, larger cold-blooded animals need more heat.

Hotter climate lets cold-blooded animals grow

Scientists have already studied the size of snakes living at different temperatures, and found that their maximum size is proportional to the average temperature. Based on what they know about that relationship, as well as the size and environment of the living anaconda, which is similar, the study estimated that the largest specimens of Titanoboa would have needed an environment where the average temperature was at least 33 C — about six degrees warmer than equatorial South America is today — in order to survive.

The Titanoboa fossils were found in an open pit coal mine, alongside fossilized giant turtles, as well as fossils of primitive crocodiles that the snake likely ate. The snake likely spent most of its time in the water, as anacondas do, Head said.

Fossils from the tropics are difficult to find, Head said, because so much of the area is covered by jungle rather than bare rock or sand. That means there is little data about what the ancient climate was like there.

"Fortunately, the owners of the Cerrejon mine had the presence of mind to be interested in the fossils they were finding," he said.

The mine owners worked with Carlos Jaramillo of the Smithsonian Tropical Research Institute in Panama and Bloch, who eventually consulted Head because his research specialty is fossil snakes.