Scientists scanned the only snake with a fake spider on its tail to see exactly how it works

The spider-tailed horned viper lures birds with a spider-shaped tail tip, and CT scans show the lure is skin alone over ordinary bones.

One snake in the world has a tail that ends in a fake spider, with a plump body and long dangling legs. The snake twitches it across the rocks until a bird swoops in to eat the spider. This is when the bird becomes the snake's meal.

Scientists have now scanned the skeleton of the first specimen collected. They found the bones inside that tail turned out to be ordinary viper bones.

The snake is the spider-tailed horned viper, Pseudocerastes urarachnoides, which lives in Iran. Georgios Georgalis of the Polish Academy of Sciences led the study with Sara Ruane of the Field Museum in Chicago.

The result matters most to people who work on fossils. Once an animal is down to only its bones, a feature this dramatic leaves no trace.

Mistaken for a tumor

A Field Museum expedition to Iran collected the snake in 1968. It was filed as an odd Persian horned viper, a related species. "People thought that it had some sort of deformity at the tip of its tail, like a tumor," Ruane said.

The specimen stayed in the collection for decades. In 2003, another snake with the same kind of tail turned up, Ruane said. In 2006, scientists described the spider-tailed horned viper as a new species.

The 1968 snake became its holotype, the single reference specimen that defines what the species looks like.

Videos filmed in the wild soon showed what the tail was for. The snake holds still and moves nothing but its tail tip, so the fake spider appears to crawl across the rocks. A bird comes down to catch it, and the snake strikes.

The tail bones are ordinary

The Field Museum opened a new X-ray scanning lab in late 2024, and Ruane made sure the viper was among the first specimens through it.

The scanner takes thousands of X-ray images and stacks them into a 3D model of everything inside, so nobody has to cut a one-of-a-kind specimen open. This scan ran 44 minutes and collected 3,848 images.

Georgalis, Ruane and Kacper Węgrzyn of the Chinese University of Hong Kong then worked through the whole skeleton. It has 183 vertebrae plus a fused tip: two in the neck, 145 along the body, four at the vent, and 32 in the tail.

That tail could easily have held strange bones. "In some other snakes, like sand boas and rubber boas, the vertebrae at the tip of the tail have extreme shapes that look really alien," Georgalis said.

Instead, the 32 tail bones are typical viper vertebrae that get smaller toward the tip. No bone reaches into the spider's legs or its body.

In an interview with Earth.com, Ruane said the team had gone in expecting the bones to match the outside. "There was nothing inside the skeleton that would give you any clue that this snake had this extraordinary spider-like structure on the outside," she said.

The fake spider is made of scales

The whole lure is skin. On the body just ahead of the tail, the scales are heavily ridged and spiky, which gives the snake a stone-like texture that helps it blend into rock. Toward the tip, the scales stretch into long, flexible filaments, and those are the spider's legs.

What looks like the spider's body is one enlarged oval structure on top of the tail, built from several scales that fused together.

Plenty of snakes wave their tails to draw in prey, from rattlesnakes to boas and pythons. Almost all of them do it with a plain tail, and the most a luring tail usually has is a brighter color than the rest of the snake.

No other living reptile has anything like these filament scales, the authors wrote. The viper built one of the most elaborate lures of all with almost no change to the bones underneath.

Fossils would miss the fake spider

Georgalis is mostly a paleontologist, and that's why the result stings. Fossil snakes almost always survive as scattered single vertebrae, so those bones are what scientists use to name extinct species.

Telling a fossil viper's vertebra from a non-viper's is fairly easy, the authors wrote, but telling one viper genus from another is hard. "It's one of the, let's say, frustrations of the life of a paleontologist," Georgalis said.

"We work with beautiful animals, but most of the time, we're trying to put together puzzles with some of the pieces missing, and in the end, we can't be 100 percent sure of what the animal actually looked like."

The spider-tailed viper is now a living test case. A fossil skeleton of it would show a normal viper, and one of the most elaborate cases of mimicry among snakes would be gone.

"If all we had was the skeleton of this snake, there would be essentially nothing to tell us that it had this incredible spider-like tail, much less that it used that tail to lure birds," Ruane told Earth.com.

Scales, coloring and behavior leave little or no trace in the fossil record, she said. So the animal behind a set of bones may have been much stranger than the bones suggest.

What remains unknown

The other two species in the genus have never had their skeletons examined, so nobody knows which of this snake's bony details its closest relatives share. One candidate is a forked structure on the underside of the tail vertebrae, which the team also found in two distantly related vipers.

How the filament scales grow is still an open question. The authors wrote that higher-resolution scans or an electron microscope would be needed before anyone can say what tissue the "legs" are made of.

Ruane said she doesn't have a specific follow-up planned, but she hopes to work with Georgalis again on this snake and other species.

"New technologies allow us to go back to these specimens and ask completely new questions without damaging them," she said. The 1968 snake went back to its shelf intact.

The scans are now public on MorphoSource, so anyone with the next question can study the skeleton without touching the specimen.

The full study was published in the journal The Anatomical Record.

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