The team measured 183 fossilized anaconda backbones, representing at least 32 snakes, discovered in Falcón State in Venezuela, South America.

A University of Cambridge-led team has analyzed giant anaconda fossils from South America to deduce that these tropical snakes reached their maximum size 12.4 million years ago and have remained giants ever since.

Many animal species that lived 12.4 to 5.3 million years ago, in the period known as the "Middle to Upper Miocene," were much bigger than their modern relatives due to warmer global temperatures, extensive wetlands and an abundance of food.

While other Miocene giants—like the 12-meter caiman (Purussaurus) and the 3.2-meter giant freshwater turtle (Stupendemys)—have since gone extinct, anacondas (Eunectes) bucked the trend by surviving as a giant species.

Anacondas are among the largest living snakes in the world. They are usually four to five meters long and in rare cases can reach seven meters.

Fossil evidence and modern comparisons

The team measured 183 fossilized anaconda backbones, representing at least 32 snakes, discovered in Falcón State in Venezuela, South America. Combining these measurements with fossil data from other sites in South America allowed them to calculate that ancient anacondas would have been four to five meters long. This matches the size of anacondas that exist today.

The study is published in the Journal of Vertebrate Paleontology.

The team measured 183 fossilized anaconda backbones, representing at least 32 snakes, discovered in Falcón State in Venezuela, South America.

"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 Andrés Alfonso-Rojas, a Ph.D. student and Gates Cambridge Scholar in the Department of Zoology at the University of Cambridge, lead author of the research.

He added, "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 Alfonso-Rojas.

Alfonso-Rojas double-checked his calculations using a second method called "ancestral state reconstruction," using a family tree of snakes as a way to reconstruct the body length of giant anacondas and related species of living snakes including tree boas and rainbow boas. This confirmed that the average body length of anacondas was four to five meters when they first appeared during the Miocene.

Anacondas' habitat and evolutionary history

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.

The study found that anacondas reached their maximum size 12.4 million years ago and have remained giants ever since. Anacondas are usually four to five meters long and in rare cases can reach seven meters.

Anacondas can have more than 300 vertebrae in their backbones, and measurements of the size of individual fossilized vertebrae can provide a reliable indication of how long a snake was.

Lead researcher Andrés Alfonso-Rojas loves working with living snakes as well as fossils.

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.

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 meters 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 fossilized 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.

More information: An early origin of gigantism in anacondas (Serpentes: Eunectes) revealed by the fossil record, Journal of Vertebrate Paleontology (2025). DOI: 10.1080/02724634.2025.2572967

Journal information: Journal of Vertebrate Paleontology

Provided by University of Cambridge