.Ancient Snakes Had Limbs for 70 Million Years and They Were 'Big Bodied and Big Mouthed'

A species of ancient snake had hind limbs for around 70 million years before losing them, scientists have discovered. This shows these creatures lived successfully with legs for a very long time before developing the characteristic body shape seen today—adding to the complicated story of snake evolution.

It is generally thought snakes evolved from lizards. Over time, their legs got smaller and eventually they were lost altogether. Some snake species, including pythons and boas, still retain the remnants of their legs with tiny digits they use to grasp with while mating. Exactly how and when the first limbless snakes appeared on Earth is not known, but fossils show their limbed ancestors still existed about 100 million years ago.

Understanding how snakes evolved is difficult because their skeletons are small and fragile, meaning they are rarely well preserved. However, between 2012 and 2017 researchers working in northern Patagonia, Argentina, found a trove of ancient snake fossils, including a dozen skulls and half a dozen skeletons belonging to the species Najash.

Led by Fernando Garberoglio, a paleontologist from Universidad Maimónides, Buenos Aires, the team uncovered what is believed to be the biggest and most well-preserved collection of fossil snakes in the world. These fossils dated to different periods, and, because of their high level of preservation, the team was able to build a 3D picture of the species' anatomy.

Findings, published in Science Advances, showed they maintained back legs for about 70 million years. They were also found to have cheekbones—something their descendants have also lost. "Our findings support the idea that the ancestors of modern snakes were big-bodied and big-mouthed—instead of small burrowing forms as previously thought," Garberoglio said in a statement.

Why these snakes went on to lose their limbs is still a mystery. Garberoglio told Newsweek that this species of ancient snake is just one of many that lost its limbs—the development, he said, is a case of convergent evolution, where multiple species occupying different niches all go on to evolve the same features independently.

Artist impression of the ancient snake. The animal had small hind legs. Raúl Orencio Gómez

"Limb loss did not just happen once in some ancient snake and all snakes have been limbless since. The logical outcome from this stream of information is that limb loss and limb reduction, from four legs to no legs, likely has happened to different groups of snakes for very different reasons. It is not exactly clear what those different evolutionary pressures might have been," Garberoglio said.

Why being limbless is advantageous to snakes is also unclear. "Modern snakes, and the same likely applies to ancient snakes as well, live in a wide variety of environments—from oceans, to deserts, to underground, in rainforest tree canopies, and on the ground in all manner of environments," Garberoglio told Newsweek. "They move differently, some even glide through the air, feed differently, and are so broadly adapted to the environments that it is clear that being limbless is not impediment to their evolution in the least. It is difficult therefore to identify the advantages of being limbless. What is clear is that it happened, likely many times, and has not been a selective disadvantage, but instead has opened up numerous new niches into which snakes have radiated and adapted for much longer than 100 million years."

fossil snake skull
An image of one of the fossilized snake skulls. Researchers say this is the most complete and well preserved collection of snake fossils in the world. Fernando Garberoglio

Garberoglio said their findings, together with recent advances in snake paleontology, show snake evolution was related to specializations in their heads, rather than the body and its elongation and limb loss. "This contrasts with previous hypotheses that considered limb reduction and body elongation as the main drivers of snake evolution," he said.

The next step in the research is to find more snake fossils so they can compare this species with others. Garberoglio also hopes to look at snakes living on the ancient continent of Gondwana to understand the role they played in snake evolution. "How they were distributed, whom they are related to, and how this impacts the modern distribution of snakes, and how closely or not they are related to some relatively obscure living forms?"