Fossil evidence from Venezuela suggests that anacondas have maintained their large size for over 12 million years. Researchers analyzed 183 fossilized vertebrae from at least 32 individual snakes discovered in Falcón State, Venezuela. By combining these measurements with additional fossil data from other South American sites, the team estimated that ancient anacondas measured between four and five meters long, which is similar to the length of modern anacondas.
To verify these findings, Alfonso-Rojas used a technique called ancestral state reconstruction. This method involves using a family tree of snakes to estimate the body length of both extinct and living species such as tree boas and rainbow boas. The results confirmed that anacondas had average lengths of four to five meters when they first appeared during the Miocene epoch.
Anacondas currently inhabit swamps, marshes, and large rivers like the Amazon. During the Miocene period, northern South America resembled today’s Amazonian environment, providing suitable habitat and food sources for these snakes. Although their range has since decreased, modern habitats still support their large size.
It was previously believed that warmer climates in the past would have resulted in even larger anacondas because reptiles are sensitive to temperature changes. However, Alfonso-Rojas stated: “This is a surprising result because we expected to find the ancient anacondas were seven or eight metres long. But we don’t have any evidence of a larger snake from the Miocene when global temperatures were warmer.”
The fossils used in this study were collected over several field seasons by teams from the University of Zurich and the Museo Paleontológico de Urumaco in Venezuela.