Anacondas are among the largest living snakes in the world. They are usually 4 to 5 m long and in rare cases can reach 7 m. In new research, paleontologists 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.
The green anaconda (Eunectes murinus). Image credit: MKAMPIS / CC BY-SA 4.0.
“Living snakes have a wide range of body lengths, with a modal size of 1 m and a range from 10 cm in the smallest leptotyphlopid thread snakes to approximately 7 m in the anaconda genus Eunectes and giant pythonids,” said University of Cambridge Ph.D. student Andrés Alfonso-Rojas and colleagues.
“Within Eunectes, the green anaconda (Eunectes murinus) is the largest species, with an average body size of 4-5 m in length and verifiable maxima between 6 and 7.2 m.”
“The evolutionary history of anacondas can be minimally constrained to the Middle-Late Miocene based on limited fossil samples from Brazil, Colombia, Peru, and Venezuela.”
“However, little is known about the body size of the earliest anacondas, which coexisted with gigantic crocodylomorphs, turtles, ungulates, and xenarthrans.”
In the study, the authors 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 4 to 5 m long — the size of anacondas that exist today.
“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,” Alfonso-Rojas said.
“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.”
The reserchers double-checked their 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 4 to 5 m when they first appeared during the Miocene.
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.
“This is a surprising result because we expected to find the ancient anacondas were 7 or 8 m long,” Alfonso-Rojas said.
“But we don’t have any evidence of a larger snake from the Miocene when global temperatures were warmer.”
The findings were published this week the Journal of Vertebrate Paleontology.
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Andrés F. Alfonso-Rojas et al. An early origin of gigantism in anacondas (Serpentes: Eunectes) revealed by the fossil record. Journal of Vertebrate Paleontology, published online December 1, 2025; doi: 10.1080/02724634.2025.2572967