Anacondas live in swamps, marshes, and big rivers like the Amazon. In the Miocene the whole of northern South America resembled today’s Amazonian region, and anacondas were much more widespread than they are today. But there is still enough of the right habitat, with the right food like capybaras and fish around, to allow modern anacondas to keep being big.

It was previously thought that anacondas must have been even bigger in the past when it was warmer, because snakes are particularly sensitive to temperature.

Alfonso-Rojas said: “This is a surprising result because we expected to find the ancient anacondas were seven or eight metres long. But we don’t have any evidence of a larger snake from the Miocene when global temperatures were warmer.”

Before this study it wasn’t clear when anacondas evolved to be so big because of a lack of fossil evidence. These snakes can have more than 300 vertebrae in their backbones, and measurements of the size of individual fossilised vertebrae can provide a reliable indication of how long a snake was.

The anaconda fossils used in the study were collected over several seasons of fieldwork by collaborators at the University of Zurich and the Museo Paleontológico de Urumaco in Venezuela.

This research* was funded by The Gates Cambridge Trust and the Natural Environment Research Council.*