Published online 4 February 2009 | Nature | doi:10.1038/news.2009.80

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'Titanic' boa fossils provide clues to past tropical climate.

An artist's reconstruction of Titanoboa cerrejonensis . Jason Bourque

Researchers have found fossils of the biggest known snake in the world, a discovery that could shed light on the climate of the tropics in the past.

The scientists estimate the snake lived 58 to 60 million years ago and was around 13 metres long. The giant, found in northeastern Colombia, dwarfs modern pythons and anacondas which usually don't exceed 6-6.5 metres and are thought to be the largest living snakes.

Since snakes are poikilotherms that, unlike humans, need heat from their environment to power their metabolism, the researchers suggest that at the time the region would have had to be 30 to 34 degrees Celsius for the snake to have survived. Most large snakes alive today live in the South American and southeast Asian tropics, where the high temperatures allow them to grow to impressive sizes.

"We've taken the snake and turned it into a giant thermometer," says lead author and vertebrate palaeontologist Jason Head of the University of Toronto in Canada, who says he "just about screamed" when he first saw the size of the fossils.

“We've taken the snake and turned it into a giant thermometer.” Jason Head

University of Toronto

Head's colleagues discovered fossilized vertebrae and ribs from 28 individual snakes in an open-pit coal mine at Cerrejón. The vertebrae's structure suggests the snake is closely related to the boa constrictor, leading the team to name the species Titanoboa cerrejonensis, or 'titanic boa from Cerrejon'. By comparing the shapes and sizes of the two best-preserved vertebrae to those of living snakes, the researchers calculated that the snake was 12.8 metres long and weighed 1,135 kilograms.

King of snakes

"It is hands-down the largest snake ever confirmed," says Harry Greene, an evolutionary biologist at Cornell University in Ithaca, New York, who was not involved in the work. "I think it's really spectacular."

A vertebra of a modern Anaconda (left) and (right) a vertebra from the Titanoboa . Ray Carson - UF Photography

Using models1 based on the largest modern-day snakes and their estimate of the Titanoboa 's size, the team calculated how hot the tropics must have been 58 to 60 million years ago, a period known as the Palaeocene. The mean annual temperature would need to be at least 30-34 degrees Celsius to support the snake's metabolism, the researchers report in Nature2. This range matches previous estimates from Palaeocene climate models that assume high atmospheric carbon dioxide concentrations3.

The results support the idea that the temperature difference between the Palaeocene tropics and higher-latitude regions was as large as it is today, even though the higher latitudes were much warmer during that time. This counters the so-called 'thermostat' hypothesis, which predicts that tropical temperatures would stay fairly stable even as other parts of the world heated up.

The study offers a "really big piece of evidence" to researchers trying to estimate Palaeocene climates, says Lisa Sloan, a climate scientist at the University of California, Santa Cruz. But Greene cautions that the team based their temperature calculations partially on the largest known size of an anaconda today, which the study pegs at 7 metres. This number is "very conservative" and could be as high as 11 metres, Greene says, which would lower the corresponding temperature estimate for the Palaeocene tropics.