During the Middle to Upper Miocene (roughly 12.4 to 5.3 million years ago), a warm and swamp-rich South America produced some of the largest reptiles the continent has ever seen. Giant caimans (Purussaurus) stretched longer than school buses, and freshwater turtles (Stupendemys) grew nearly as wide as dining tables. Many of those giants disappeared as climates cooled. But a new fossil analysis shows that one lineage never budged in size at all: the anacondas.

Published in the Journal of Vertebrate Paleontology, a University of Cambridge–led team reports that ancient anacondas already spanned about four to five meters some 12.4 million years ago — almost identical to the size of the anacondas found across the Amazon today.

The discovery comes from a huge set of fossilized anaconda vertebrae discovered in Venezuela’s Falcón State. Over several seasons of fieldwork, researchers from Cambridge, the University of Zurich, and the Museo Paleontológico de Urumaco collected 183 backbones representing at least 32 individual snakes.

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The team measured these fossils and combined the measurements with fossil data from other South American sites, allowing researchers to calculate the size of anacondas living during that period. They estimated that the ancient anacondas measured four to five meters, matching the size of anacondas living today. That means anacondas have been almost the same size for about 12.4 million years.

“Other species, such as 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,” lead author Andrés Alfonso-Rojas said in a statement.

To double-check their findings, researchers used a second method called ‘ancestral state reconstruction’, in which they used a family tree of snakes to estimate the size of giant anacondas and related living species such as tree boas and rainbow boas. The results matched their previous calculations, confirming that the average body length of the anaconda was four to five meters in the Miocene.

These findings contradict previous thoughts that ancient anacondas would have been even bigger due to warmer climates and the snake’s high sensitivity to temperature.

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

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Their size stability may come down to their ecological niche. Miocene northern South America resembled a vast Amazonian wetland — exactly the sort of place anacondas thrive today. The snakes occupied swamps, marshes, and big river systems rich in food like fish and capybaras. While many Miocene giants vanished as habitats shrank, the ecological conditions that support anacondas remained relatively intact.

This study is important because it provides evidence that was previously missing. Researchers lacked enough fossil material to determine when anacondas became giants, and these Venezuelan specimens finally anchor that timeline, showing that the snakes established — and held — their enormous dimensions soon after appearing in the Miocene.

It also suggests that today’s anacondas, often reaching four to five meters and occasionally stretching toward seven, are essentially living representatives of a Miocene body plan whose size has remained largely unchanged.

The study was published in the Journal of Vertebrate Paleontology.