Prehistoric snake: 43 feet long, 2,500 pounds.

• Position: Reptiles

Scientists have recovered fossils from a 60,000,000-year-old South American snake whose length and weight might make today's anacondas and reticulated pythons seem a bit cuter and more cuddly. Named Titanoboa cerrejonensis by its discoverers, the size of the snake's vertebrae suggest it weighed 2,500 pounds and measured 43 feet nose to tall tip-and that is a conservative estimate. The scientists classify Titanoboa as a boine snake, a type of non-venomous constrictor that includes anacondas and boas.

"At its greatest width, the snake would have come up to about your hips," notes geologist David Polly of Indiana University, Bloomington, who identified the position of the fossil vertebrae, which made a size estimate possible. 'The size is pretty amazing, but our team went a step further and asked, how warm would the Earth have to be to support a body of this size?"

Paleontologists long have known of a rough correlation between a period or epoch's temperature and the size of its poikilotherms (cold-blooded creatures). As Earth's temperature increases, so does the upper size limit on poikilotherms. "There are many ways the anatomy of a species is correlated with its environment on broad scales," Polly explains. "If we understand these correlations better, we will know more about how climate and climate change affect species, as well as how we can infer things about past climates from the...