Fossils: Armless remains shed light on how animals like snakes lost their limbs

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A small snake-like animal that lived around 308 million years ago evolved to lose its front limbs

The life


March 28, 2022


A reconstruction of Nagini mazonense, one of the first armless animals

Henry Sutherland Sharpe

Less than 100 million years after vertebrates developed their first legs, some of their descendants evolved to lose them again, fossils reveal. The discovery shows that terrestrial vertebrates began to evolve into a snake-like form at least 308 million years ago.

Arjan Mann at the Smithsonian National Museum of Natural History in Washington D.C. and colleagues have found two fossils of an ancient animal, both from rocks in Illinois well known to paleontologists for preserving in fine detail the remains of ancient land animals .

The animal was nicknamed Nagini mazonense, representing a new genus and a new species, and it belongs to a group called the Molgophids. It may have reached about 10 centimeters in length and had a snake-like body without forelimbs. It also lacked the bony structures that support the attachment of the forelimbs to the body, known as the pectoral girdle.

However, N. mazonense had a pair of small but fully formed hind legs, with four toes on each foot.

In addition to nearly complete skeletons, around 308 million years old, there were also traces of soft tissue, revealing that N. mazonense had a round snout and a long body with about 85 vertebrae and ribs. There was no sign of soft tissue in the area where you would expect forelimbs, Mann said.

“They rely on body-based locomotion like sidewinding and don’t really rely on limb-powered locomotion anymore,” Mann says.

It’s a fascinating discovery, says Rolf Zeller at the University of Basel in Switzerland. “Snake embryos, like pythons, still form hind limb buds that disappear during development,” he says. “The discovery of an ancient snake-like fossil that lacks forelimbs but retains hindlimbs is a fantastic discovery, as it reveals the existence of transitional forms before the complete loss of limbs during evolution.”

Modern snakes also lost their upper limbs and chest girdle first, about 170 million years ago, he adds. Corn N. mazonense and other molgophids are not direct ancestors of modern snakes, Mann says. “They are sort of an experimental first line of reptiles.”

The fact that molgophids evolved to lose at least a few limbs is an important finding, he says, because it shows that this ability is present in most legged vertebrates. Apart from snakes and some lizards, the only legless vertebrates are amphibians including salamanders and caecilians. But it appears other groups, such as mammals and birds, may have the ability to evolve into legless forms, he says.

A related molgophid was discovered in the same rocks in 2019, but in this case the animal – called Steenae innovator – had four legs.

“[The rock site in Illinois] is becoming something of a home to study early reptile evolution and all the different body plans that were experimented with early on,” Mann says.

Journal reference: Nature ecology and evolution, DOI: 10.1038/s41559-022-01698-y

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