Why Did Snakes Forsake Their Legs?

Animals do the most amazing things. Read about them in this series by Janaki Lenin.

An American burrowing snake. Credit: Wikimedia Commons

An American burrowing snake. Credit: Wikimedia Commons

The origin of snakes was a mystery for a long time. Did they evolve from burrowing lizards or from marine reptiles? In recent years, evidence for a land origin has steadily mounted.

Researchers from University of Edinburgh, UK, and the American Museum of Natural History have added another dimension to the investigation of snake origins. They analysed the skull of the 90-million-year-old Dinilysia patagonica. Although scientists knew of the fossil since 1901, it’s only now they were able to use advanced techniques to examine it.

Herpetologists can tell a snake’s habits by looking at its physical characters. Tree snakes have long tails for their length, slender bodies, and large eyes. Burrowing snakes have relatively short tails, shovel-like or pointed snouts, and small eyes. Since the fleshy parts of an animal don’t fossilise, scientists couldn’t tell whether Dinilysia was a climber or a burrower.

Hongyu Yi, from the University of Edinburgh, and Mark Norell, from the museum, used computerised tomography to scan the skulls of the fossil, 33 other species of snakes, and 10 species of lizards. And then they built 3D virtual models of the labyrinthine inner ear of their samples. The convoluted canals and cavities of the inner ear help animals hear and maintain their balance. Snakes feel ground-borne vibrations through their inner ear. They cannot hear air-borne sounds no matter how well snake charmers play their beens (wind instruments made of gourds).

By comparing the models, the scientists were able to pick the differences. Burrowing animals have a unique large spherical structure within the inner ear, bound by three semicircular canals. The vestibules in the inner ears of burrowing snakes appear to be larger than burrowing lizards, like monitor lizards.

The position of the vestibule inside the Dinilysia patagonica. The white bar at the bottom represents 5mm. Credit: Hongyu Yi

The position of the vestibule inside the Dinilysia patagonica. The white bar at the bottom represents 5mm. Credit: Hongyu Yi

The structure contains a large otolith, also called an ‘ear stone,’ in the saccular region. This calcareous bone-like ‘stone’ not only amplifies ground-borne vibrations but is an accelerometer, converting head movements into neural impulses for the brain to process. The larger the otolith, the better the snake hears low frequency vibrations. But a big ‘ear stone’ slows the snake’s movements.

The vestibules of aquatic, general and burrowing snakes. Credit: Hongyu Yi

The vestibules of aquatic, general and burrowing snakes. Credit: Hongyu Yi

Aquatic and terrestrial snakes don’t have this special structure. Some generalists like the common king snake, found in the U.S., hunt above ground but also burrow. They don’t have a spherical vestibule in their inner ears; instead, theirs are flattened and oval, and not as large.

Since this enlarged structure is found only in burrowers, the scientists surmise it probably helps the animals hear underground prey better. And the structure within the inner ear of Dinilysia resembles that of modern-day burrowers.

At two metres long, the Dinilysia is the largest burrowing snake known. Most modern underground-dwelling snakes are less than a metre in length; the largest is the Mexican burrowing python that grows up to 1.6 metres long. Dinilysia may have hunted for buried eggs of other reptiles and small amphibians, reptiles, birds, and rodents above ground.

“We know that Dinilysia was limbless from its skeleton,” says Yi, the lead author of the study. “Other ancestral snakes, like Najash from Argentina, had a pair of hind legs. We can then reconstruct the evolutionary loss of limbs by linking the morphology of limbs and the environment inhabited by snakes.”

Although Najash had legs, it is thought to be a burrower like Dinilysia. All this evidence suggests that burrowing was the main lifestyle of ancient snakes, says Yi.

Yi goes on to say that snakes lost their legs when their ancestors became adept at burrowing. “The inner ears of fossils can reveal a remarkable amount of information, and are very useful when the exterior of fossils are too damaged or fragile to examine.”

The only burrowers that don’t have this specialised inner ear are blind snakes. Paradoxically, the earliest snakes were burrowing earthworm-like blind snakes (scolecophidians) according to another study published in March 2015.

If blind snakes were among the earliest, why don’t they have the same sort of structure as modern burrowing snakes?

“The unique shape of the vestibule in blind snakes is a specific characteristic of their lineage,” says Yi. “They not only have a small vestibule, but extremely ‘fat’ and short semicircular canals surrounding it. The combination of small vestibule and fat semicircular canals does not occur in any other lizard and snake we examined, so we suggest this represents special features of blind snakes, rather than the general trend of ear evolution in all snakes.”

Wanna burrow? Lose the legs.

Based on a reconstruction of the evolutionary tree of snakes, Yi and Norell theorise that the ancestral snake, closely related to Dinilysia, was a burrower too. This is one more in the increasing number of clues that confirm early snakes evolved on land from leggy lizards.

In February 2011, French and German scientists scanned a 95-million-year old fossil of an ancient snake from Lebanon. The 50-cm-long snake had 2-cm legs. The researchers concluded that the structure of its tiny legs resembled those of a modern land-living lizard. Each leg was bent at the knee and had no feet or toes. The scientists speculated that the species eventually lost its legs because they grew more slowly or stopped growing after a short period of time.

The oldest snake, Tetrapodophis amplectus, a 110-million-year old fossil discovered in Brazil, had two pairs of legs. They weren’t much use in getting around. Instead, they were probably used for grasping prey or mates, said scientists in a paper published in July.

Surely snakes need legs with long claws to support their underground lifestyle. Pushing earth with snouts seems like an ineffective way of burrowing.

Dinilysia probably burrowed into loose soils like modern sunbeam snakes,” says Yi. Snakes can’t burrow into hard ground. Many species of underground-living snakes still shove their way through sand, leaf litter, and rotting vegetation.

We have tantalising glimpses of how early snakes lived. In another recently published study, researchers say the snake ancestor, a denizen of Gondwana, was nocturnal and didn’t constrict its prey like pythons. We also know that they didn’t lay eggs but gave birth to live young.

Snakes lost their limbs to become expert burrowers. But they emerged to colonise trees, land, and the seas with that same unique tubular body shape.

The study was published in the journal Science Advances on November 27, 2015.

Janaki Lenin is the author of My Husband and Other Animals. She lives in a forest with snake-man Rom Whitaker and tweets at @janakilenin.

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