Armless fossil sheds light on how animals like snakes lost their limbs

A tiny snake-like animal that lived about 308 million years ago had evolved to lose its forelimbs

By Christa Lesté-Lasserre

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

Arjan Mann at the Smithsonian National Museum of Natural History in Washington DC and his colleagues found two fossils of an ancient animal, both of which came from rocks in Illinois that are well-known among palaeontologists for preserving the remains of ancient land animals in fine detail.

The animal has been dubbed Nagini mazonense, representing a new genus and species, and it belongs to a group called the molgophids. It may have grown to be about 10 centimetres long, and had a snake-like body with no forelimbs. It also lacked the bony structures that support the attachment of forelimbs to the body, known as the pectoral girdle.

However, N. mazonense did have a pair of small but fully formed back legs, with four toes on each foot.

Along with the nearly complete skeletons, which are around 308 million years old, there were also impressions of soft tissue, revealing that N. mazonense had a round snout and a long body with about 85 vertebrae and ribs. There were no signs of soft tissue in the area where forelimbs might be expected, says Mann.

“They’re relying on body-based locomotion like sidewinding and not really relying on limb-propelled locomotion anymore,” says Mann.

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

Modern snakes also lost their upper limbs and pectoral girdle first, about 170 million years ago, he adds. But N. mazonense and other molgophids aren’t direct ancestors of modern snakes, says Mann. “They’re sort of an early experimental lineage of reptiles.”

The fact that the molgophids evolved to lose at least some limbs is an important discovery, he says, because it shows that this ability is present in most vertebrates with legs. Apart from snakes and some lizards, the only vertebrates without legs are amphibians including salamanders and caecilians. But it seems that other groups, such as mammals and birds, may have the capacity to evolve into legless forms – unless they have lost the associated genes, he says.

A related molgophid was discovered in the same rocks in 2019, but in that case, the animal – called Infernovenator steenae – had four legs.

“[The rock site in Illinois] is becoming a sort of a hotbed for looking at early reptile evolution and all the different body plans that were experimented on early on,” says Mann.

Journal reference: Nature Ecology & Evolution, DOI: 10.1038/s41559-022-01698-y

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