Miocene Giants: Ancient Anacondas Reached 4-5 Meters and Persisted to the Present

During the Middle to Upper Miocene (roughly 12.4 to 5.3 million years ago), warm climates and lush wetlands favored giant fauna. New research examining 12.4-million-year-old Venezuelan fossils reveals ancient anacondas (Eunectes) averaged 4-5 meters in length—roughly the same size as modern individuals. Unlike many other megafauna that peaked in the Miocene and then vanished, large Eunectes persisted in tropical South America to the present day. The team analyzed 183 fossil backbones from at least 32 snakes and used ancestral state reconstruction to estimate body length, identifying a Miocene maximum around 12.4 mya and showing no evidence for seven- to eight-meter snakes in that era. The study, led by Andrés Alfonso-Rojas of the University of Cambridge and published in the Journal of Vertebrate Paleontology, highlights the enduring gigantism of these tropical snakes in a once Amazon-like ecosystem.

Overview

The Miocene period, from 12.4 to 5.3 million years ago, featured warmer temperatures, expansive wetlands, and abundant food that allowed giant animals to thrive. In this context, researchers investigated the size of ancient anacondas to determine whether these snakes were significantly larger in the Miocene than they are today. The study focuses on 12.4-million-year-old fossils from Venezuela and aims to quantify the length of giant Eunectes and compare it with present-day specimens. The key finding is that these ancient snakes averaged about 4 to 5 meters in length, aligning with the typical size of modern anacondas. This challenges the assumption that Miocene reptiles were necessarily larger due to warmer climates.

"This is a surprising result because we expected ancient anacondas were seven or eight metres long." - Andrés Alfonso-Rojas, lead author, University of Cambridge

Fossil Evidence and Size

To estimate body length, the researchers measured 183 fossilized backbones from at least 32 individual anacondas. Although anacondas can have more than 300 vertebrae, a single well-preserved vertebra provides a reliable proxy for overall length. When combined with fossil data from throughout tropical South America, the team inferred that maximum anaconda size occurred around 12.4 million years ago and that the snakes remained large in the Miocene even as other megafauna declined. The findings indicate that giant Eunectes has persisted in tropical South America to the present, contrasting with other giants that went extinct or reduced in size as climates cooled and habitats shrank.

"Large Eunectes has persisted in tropical South America to the present." - Andrés Alfonso-Rojas, lead author

Methods and Validation

The team employed ancestral state reconstruction, a method that uses a phylogenetic tree of extant snakes alongside fossil measurements to estimate historical lengths. This approach confirmed the Miocene snakes averaged 13 to 16 feet (4 to 5 meters) long, rather than the seven to eight meters some researchers anticipated. The authors note that there is no evidence for Miocene snakes of seven to eight meters, suggesting that even in a warmer Miocene world, there was no larger Miocene anaconda on record.

"But we don’t have any evidence of a larger snake from the Miocene when global temperatures were warmer." - Andrés Alfonso-Rojas

Findings in Context

During the Miocene, much of northern South America resembled the Amazon today, offering abundant swampy, marshy habitats ideal for large aquatic snakes. While many other giant species (such as crocodiles and turtles) went extinct due to cooling temperatures and shrinking habitats, giant anacondas endured. The researchers highlight the resilience of Eunectes and suggest that tropical environments facilitated sustained gigantism, even as other megafauna declined. The authors also note a provocative contrast with the past: if other megafauna had remained at their Miocene sizes, modern ecosystems might look very different. The study’s reflection on ecological persistence underscores how certain lineages adapt to long-term environmental stability or change.

"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." - Andrés Alfonso-Rojas

Publication and Implications

The research was published in the Journal of Vertebrate Paleontology, with Andrés Alfonso-Rojas as the lead author and affiliated with the Department of Zoology at the University of Cambridge. The study’s implications extend beyond paleontology, offering insights into how tropical ecosystems support long-term gigantism and how climate and habitat stability interact with body size in snakes. The authors emphasize that ongoing habitat and food availability in tropical South America facilitated the continued prominence of the giant anacondas, a niche that remains remarkably similar to their Miocene ancestors. A lighthearted aside in the discussion notes that, unlike the megafauna of the past, today’s anacondas still dominate their wetlands, reminding readers of the enduring presence of these tropical giants.