By Stephen Beech
Anacondas have been giants of the animal world for over 12 million years, reveals new research.
Analysis of fossils from South America shows that the tropical snakes reached their maximum size 12.4 million years ago – and have remained massive ever since.
Anacondas are among the world’s largest living snakes.
They can grow up to 30 feet (nine meters) long and weigh up to 550 pounds (250kilos), although most adults are smaller at around 16 feet (five meters) in length.
The University of Cambridge-led research team explained that many animal species that lived 12.4 to 5.3 million years ago – the period known as the ‘Middle to Upper Miocene’ – were much bigger than their modern relatives due to warmer global temperatures, extensive wetlands and an abundance of food.
While other Miocene giants – such as the 12-meter caiman and the 3.2-meter giant freshwater turtle – have since gone extinct, anacondas bucked the trend by surviving as a giant species.
The researchers measured 183 fossilized anaconda backbones, representing at least 32 snakes, discovered in Venezuela.
Combining the measurements with fossil data from other sites in South America allowed them to calculate that ancient anacondas would have been four to five meters long, matching the size of anacondas that exist today.
Study lead author Andrés Alfonso-Rojas said: “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.
“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.”
He double-checked his calculations using a second method called ‘ancestral state reconstruction’ using a family tree of snakes as a way to reconstruct the body length of giant anacondas and related species of living snakes, including tree boas and rainbow boas.
The findings, published in the Journal of Vertebrate Paleontology, showed that the average body length of anacondas was four to five meters when they first appeared during the Miocene.
Anacondas now live in swamps, marshes, and large rivers such as the Amazon.
The researchers say that, in the Miocene, the whole of northern South America resembled today’s Amazonian region, and anacondas were much more widespread than they are now.
But there is still enough of the right habitat, with the right food – such as capybaras and fish – to allow modern anacondas to stay big.
It was previously thought that anacondas must have been even bigger in the past when it was warmer, because snakes are particularly sensitive to temperature.
Alfonso-Rojas, a doctoral student in Cambridge’s department of zoology, added: “This is a surprising result because we expected to find the ancient anacondas were seven or eight meters long.
“But we don’t have any evidence of a larger snake from the Miocene when global temperatures were warmer.”
The fossils used in the study were collected over several seasons of fieldwork by researchers from the University of Zurich in Switzerland, and the Museo Paleontológico de Urumaco in Venezuela.
