Fossils reveal anacondas have been giants for over 12 million years

(Press-News.org) A University of Cambridge-led team has analysed giant anaconda fossils from South America to deduce that these tropical snakes reached their maximum size 12.4 million years ago and have remained giants ever since.

Many animal species that lived 12.4 to 5.3 million years ago, in 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 - like the 12-metre caiman (Purussaurus) and the 3.2-metre giant freshwater turtle (Stupendemys) - have since gone extinct, anacondas (Eunectes) bucked the trend by surviving as a giant species.

Anacondas are among the largest living snakes in the world. They are usually four to five metres long and in rare cases can reach seven metres.

The team measured 183 fossilised anaconda backbones, representing at least 32 snakes, discovered in Falcón State in Venezuela, South America. Combining these measurements with fossil data from other sites in South America allowed them to calculate that ancient anacondas would have been four to five metres long. This matches the size of anacondas that exist today.

The study is published today in the Journal of Vertebrate Paleontology.

“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,” said Andrés Alfonso-Rojas, a PhD student and Gates Cambridge Scholar in the Department of Zoology at the University of Cambridge, lead author of the research.

He added: “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,” said Alfonso-Rojas.

Alfonso-Rojas 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. This confirmed that the average body length of anacondas was four to five metres when they first appeared during the Miocene.

Anacondas live in swamps, marshes, and big rivers like the Amazon. In the Miocene the whole of northern South America resembled today’s Amazonian region, and anacondas were much more widespread than they are today. But there is still enough of the right habitat, with the right food like capybaras and fish around, to allow modern anacondas to keep being 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 said: “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.”

Before this study it wasn’t clear when anacondas evolved to be so big because of a lack of fossil evidence. These snakes can have more than 300 vertebrae in their backbones, and measurements of the size of individual fossilised vertebrae can provide a reliable indication of how long a snake was.

The anaconda fossils used in the study were collected over several seasons of fieldwork by collaborators at the University of Zurich and the Museo Paleontológico de Urumaco in Venezuela.

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