Researchers from the University of Cambridge have analyzed fossils showing that anacondas reached their large body size around 12.4 million years ago and have maintained it since. Unlike other giant reptiles that went extinct, anacondas persisted through changing climates. The study reveals their resilience in shrinking wetland habitats.
A team led by the University of Cambridge studied fossils of giant anacondas from South America, determining that these snakes achieved their full size approximately 12.4 million years ago during the Middle Miocene. Their body length has remained consistent at four to five meters, similar to modern specimens, which occasionally reach seven meters.
The research focused on 183 fossilized backbone segments from at least 32 individuals discovered in Falcón State, Venezuela. By combining these with data from other South American sites, the scientists estimated ancient anacondas matched today's dimensions. This stability contrasts with the Miocene era (12.4 to 5.3 million years ago), when warmer global temperatures, extensive wetlands, and plentiful food led many animals to grow larger than their descendants. For instance, the 12-meter caiman Purussaurus and the 3.2-meter turtle Stupendemys eventually vanished, likely due to cooling temperatures and habitat loss.
Anacondas, however, endured as a large-bodied group. "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 lead author Andrés Alfonso-Rojas, a PhD student in Cambridge's Department of Zoology. 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."
To verify measurements, the team used ancestral state reconstruction on a snake family tree, including relatives like tree boas and rainbow boas, confirming early anacondas averaged four to five meters. Contrary to expectations that warmer Miocene conditions would produce even larger snakes, no evidence supports sizes beyond modern ones.
Today, anacondas inhabit wetlands, marshes, and rivers like the Amazon, where remnant habitats sustain them alongside prey such as capybaras and fish. The Miocene's northern South America resembled the current Amazon Basin, enabling wider distribution then. Fossils were collected over field seasons by collaborators from the University of Zurich and Venezuela's Museo Paleontológico de Urumaco.
The findings appeared on December 1, 2025, in the Journal of Vertebrate Paleontology.