Anacondas have survived the era of giant crocodiles and turtles - and are still "monsters" today

Anacondas were already giants when prehistoric crocodiles and giant tortoises still roamed northern South America - and they haven't changed much since then. This is the conclusion reached by an international team led by scientists from the University of Cambridge, having analysed fossils of large snakes from Venezuela.

The study found that anacondas reached their current size about 12.4 million years ago and have remained large ever since, despite climate change and the extinction of other giant animals. The work is published in the Journal of Vertebrate Paleontology.

How ancient anacondas measured up

The team studied 183 fossil anaconda vertebrae belonging to at least 32 individuals found in the state of Falcon in northern Venezuela. The data was compared to fossils from other sites in South America and to the size of modern anacondas.

Based on the size of the vertebrae scientists calculated that the ancient anacondas reached 4-5 metres in length - that is, were comparable to the current. Today's anacondas typically grow to 4-5 metres, and in rare cases to around 7 metres.

"We have shown that anacondas evolved very early into large-sized animals - almost immediately after their appearance in tropical South America around 12.4 million years ago - and have not changed in size since then," said the paper's lead author, Andres Alfonso-Rojas, a PhD student in the Department of Zoology at the University of Cambridge.

The age of giants and "unchanging" anacondas

During the middle to late Miocene (12.4-5.3 million years ago), many animals were much larger than their modern relatives. The warm climate, extensive wetlands and abundant food allowed giants to evolve.

These included, for example:

• the giant caiman Purussaurus, up to ~12 metres long;

• the freshwater turtle Stupendemys, up to 3.2 metres long.

These species have long been extinct, probably due to cold weather and a decline in suitable habitats. But anacondas in the genus Eunectes survived this "filter".

"Other giants of the Miocene - huge crocodiles and turtles - disappeared, but the anacondas survived. They proved to be super-resilient," Alfonso-Rojas notes.

Why anacondas may have survived

Anacondas live in swamps, swamps, and slow rivers - like the Amazon and its tributaries. In the Miocene, the entire northern part of South America was similar to the modern Amazon region: lots of water, forests, and a rich food base. Anacondas were much more widespread then than they are now.

Although the climate has cooled since then, there is still enough suitable habitat - with warm water and plenty of prey (fish, capybaras and other mammals) - to keep these snakes large.

Previously, some scientists have suggested that in the warmer Miocene anacondas could have been even larger - 7-8 metres or more, because snakes are sensitive to the temperature of the environment. But the new data doesn't support that.

"We expected ancient anacondas to be noticeably longer than modern ones," Alfonso-Rojas admits. - But there is no evidence of larger snakes of this genus from the Miocene."

How to tell the length of a snake from its vertebrae

Large snakes can have more than 300 vertebrae, and the size of an individual vertebra correlates well with overall body length. Measuring fossilised vertebrae is therefore one of the most reliable ways to estimate the size of long-extinct snakes.

To verify the findings, the team also used a second approach - the ancestral reconstruction method. The scientists built a "family tree" of different snakes and used it to reconstruct the approximate body length of ancient anacondas and their closest relatives - tree and rainbow boa constrictors.

Both methods showed the same thing: the average body length of anacondas at their appearance in the Miocene was already 4-5 metres and has remained approximately the same ever since.

The fossils used in the paper were collected during several field seasons by scientists from the University of Zurich and the Museum of Palaeontology of Urumaco (Venezuela).