Anacondas have been giants for 12 million years

Giant anacondas reached their maximum size 12.4 million years ago and have maintained those proportions ever since, defying the fate of other prehistoric giants that either shrank or went extinct, according to research published in the Journal of Vertebrate Paleontology.

Giants from the very beginning

A team led by the University of Cambridge analyzed 183 fossilized anaconda vertebrae representing at least 32 snakes discovered in Falcón State, Venezuela. The ancient snakes measured between four and five meters in length—identical to modern anacondas—challenging expectations that they would have been larger during the warmer Miocene period.

“This is a surprising result, because we expected to find ancient anacondas seven or eight meters long,” says Andrés Alfonso-Rojas, lead author and PhD student at the University of Cambridge. “But we have no evidence of a larger snake from the Miocene, when global temperatures were warmer.”

Survivors among extinct giants

The middle to late Miocene, spanning from 12.4 to 5.3 million years ago, was an era in which many animals were significantly larger than their modern descendants, thanks to a warmer climate, extensive wetlands, and abundant food. While other giants, such as the 12-meter caiman Purussaurus or the 3.2-meter freshwater turtle Stupendemys, have already gone extinct, anacondas have bucked this trend, persisting as giants right up to the present day.

“Other species, such as giant crocodiles and enormous turtles, have gone extinct since the Miocene, probably due to global cooling and shrinking habitats, but giant anacondas survived—they are incredibly resilient,” explains Alfonso-Rojas.

The researchers verified their findings using ancestral trait reconstruction, a method that uses snake family trees to calculate the body length of ancient anacondas and related species, including tree boas and rainbow boas. Both approaches confirmed that anacondas had already reached four to five meters in length when they first appeared in tropical South America around 12.4 million years ago.

The key to survival

The persistence of anacondas at gigantic sizes likely stems from their aquatic lifestyle and non-specialized diet, the researchers suggest. During the Miocene, northern South America resembled today’s Amazon region, and anacondas were much more widespread than they are now. Despite the reduction in available habitat, enough swamps, wetlands, and rivers remained—along with prey such as capybaras and fish—to sustain their considerable body length.

Modern anacondas typically reach four to five meters, although rare individuals can grow up to seven meters. The fossils were collected over multiple field seasons by collaborators from the University of Zurich and the Venezuelan Museo Paleontológico de Urumaco.