Amazon’s giant anacondas reached their peak size 12 million years ago, study shows

A study of anaconda fossils shows that these snakes reached their maximum size 12.4 million years ago and then stopped growing.

As some of the planet’s largest snakes, anacondas commonly measure 4 to 5 metres, but some can reach 7 metres.

In this new work, researchers led by the University of Cambridge examined 183 fossilized anaconda backbones collected in Falcón State, Venezuela. All these fossils belong to at least 32 distinct snakes.

The team calculated that the ancient snakes were four to five metres long. This is the same impressive, powerful size as the anacondas that coil through the Amazon today.

“By measuring the fossils, we found that anacondas evolved a large body size shortly after they appeared in tropical South America around 12.4 million years ago, and their size hasn’t changed since,” said Andrés Alfonso-Rojas, lead author.

Survived climatic changes

It shows that the anaconda survived the climatic changes without shrinking while other large animals disappeared.

During the Middle to Upper Miocene period, spanning 12.4 to 5.3 million years ago, various animal species were much larger than their present-day descendants. Warmer global temperatures, expansive wetlands, and abundant food sources fueled this gigantism.

Giants like the 12-metre caiman (Purussaurus) and the 3.2-metre freshwater turtle (Stupendemys) roamed the extensive wetlands of northern South America, yet these titans are now extinct.

But the anaconda (Eunectes) defied this pattern, uniquely surviving into the modern era while retaining its colossal size.

“Other species like giant crocodiles and giant turtles have gone extinct since the Miocene, probably due to cooling global temperatures and shrinking habitats, but the giant anacondas have survived – they are super-resilient,” said Alfonso-Rojas, who is a PhD student and Gates Cambridge Scholar in the Department of Zoology at the university.

The fossil calculations were validated using a second method called “ancestral state reconstruction.”

This method used a snake family tree to reconstruct the body lengths of ancient anacondas and related living species, such as tree boas and rainbow boas.

Notably, the reconstruction validated the initial discovery that anacondas averaged four to five metres in body length upon their first appearance in the Miocene era.

Aquatic lifestyle proved beneficial

This finding challenges the conventional wisdom that snakes, being cold-blooded and highly sensitive to temperature, must have reached their absolute largest size during the hottest periods of the past.

“This is a surprising result because we expected to find that 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,” noted Alfonso-Rojas.

He confirmed that no evidence of a larger specimen from this warmer period was found.

The real puzzle, however, is not the size ceiling but the species’s incredible resilience to subsequent global cooling.

According to the team, the key to their survival lies in their aquatic lifestyle and generalist diet.

The Miocene wetlands of northern South America were similar to the Amazonian region of today, providing a perfect, widespread habitat.

Even as global conditions changed, enough of the right swampy environment — complete with the right food, such as capybaras and fish — persisted to allow modern anacondas to maintain their impressive body length.

The findings were published in the Journal of Vertebrate Paleontology on December 1.