A fake spider at the tip of this snake’s tail helps it lure birds. Scientists got their first look inside the “spider.”

Lots of animals use mimicry to survive. Some copy the appearance of threatening predators, and others have evolved to resemble plants or rocks so that they can blend into the background and avoid being attacked. But a viper from Iran has found an even stranger strategy. The tip of this snake’s tail looks like a spider, complete with a bulbous body and long dangling legs. And when a bird swoops in to try to eat this “spider,” the snake springs into action and eats the bird.

The elusive spider-tailed horned viper is a relatively recently-discovered species, and until now, scientists only knew what the outside of this snake’s unusual tail looked like. But in a new study published in the journal The Anatomical Record, researchers used XCT scanners to get a glimpse of the inner workings of this snake’s spidery tail—and found that its tailbones are bizarrely normal.

“Scientists first collected a specimen of this snake in 1968, and when they brought it back to the Field Museum, people thought that it had some sort of deformity at the tip of its tail, like a tumor,” says Sara Ruane, the Associate Curator of Herpetology and Director of the Core Laboratories  at the Field Museum in Chicago and the study’s co-author. “It sat on the shelves of the museum for decades, but it wasn’t until another specimen with a similar tail was found in 2003 that people revisited it.”

In 2006, scientists described the snake as a new species, Pseudocerastes urarachnoides, the spider-tailed horned viper. When the researchers described the species, they used the specimen in the Field Museum’s collections as the holotype, the specimen that serves as a standard-bearer for the physical characteristics of the species.

Soon after, researchers captured a video of the snake using the spider-like appendage at the tip of its tail to lure in birds. “I remember being a PhD student in about 2008 and first seeing the videos of this snake in the wild,” says Ruane. “The appendage at the end of its tail is so outrageous, I remember thinking it was so cool. It’s truly unique among snakes.”

When Ruane began working at the Field Museum in 2021, she was excited to see the holotype of the spider-tailed horned viper in the museum’s collections. “It's my favorite specimen in the museum, I use it in show-and-tells and tours all the time,” says Ruane.

But Ruane isn’t the only one who took notice of the snake specimen. In 2023, she attended a conference with Georgios Georgalis, an associate professor at the Polish Academy of Sciences in Kraków. He mentioned that he’d heard the Field Museum had the holotype of the spider-tailed horned viper, and that he was interested in studying it.

“I am primarily a paleontologist, so I try to use modern snake species to make sense of fossil remains,” says Georgalis, the study’s first author. “In vipers, the vertebral column unfortunately doesn’t give us very many clues about what kind of snake we’re looking at. When we find fossils, we often just find individual bones rather than the entire skeleton, and the individual vertebrae are so different from each other along the snake’s body that it’s hard to use them to identify a species.”

But the spider-tailed horned viper provided a unique opportunity for a researcher studying snake vertebrae. “Viper vertebrae have not been very widely studied, overall, and since this viper in particular has such an unusual tail, we wanted to know more about the bones that make it up,” says Georgalis. He asked Ruane if it would be possible to collaborate on studying the Field’s viper specimen in greater detail.

“It just so happened that the Field Museum was getting a new XCT lab,” says Ruane. In the XCT (X-ray computed tomography) lab, scientists are able to scan objects and take thousands of X-ray images of them, stacked together to produce a 3D rendering of the object’s interior. Effectively, the CT scans enable scientists to see finely detailed structures inside of objects without having to cut them open and risk damaging them—an especially important factor when working with a scientifically valuable specimen like a species holotype.

“With the spider-tailed horned viper, we were particularly interested in seeing what the vertebrae at the tip of the tail look like, because the exterior of this snake’s tail is so unusual, and because it’s used for this unusual bird-luring behavior,” says Georgalis. “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. We wanted to investigate if the spider-tailed horn viper’s tail was similarly strange inside.”

When the museum’s XCT lab began operations in late 2024, Ruane made sure that the spider-tailed viper was one of the very first specimens scanned. Surprisingly, though, the scans revealed that the inside of the viper’s tail looked… normal. “It turned out that the spider tip on the tail is only external morphology—the tail vertebrae themselves were typical of most vipers,” says Georgalis. “There was nothing in the snake’s skeleton that would suggest how extreme its tail looks on the outside.”

For a paleontologist like Georgalis, these findings are important because they underline the fact that fossil skeletons only tell part of the story of what an ancient animal looked like and how it behaved. There’s nothing in the spider-tailed horned viper’s skeleton that would suggest it has such a crazy tail, or that it twitches its tail to trick birds into becoming its dinner. “It's one of the, let's say, frustrations of the life of a paleontologist,” says Georgalis. “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% sure of what the animal actually looked like.”

In addition to the reminder that prehistoric life might be much stranger than we’d guess just by looking at bones, this finding lays the groundwork for scientists studying how tissues like this snake’s tail evolve and develop. And, for Ruane, the project underscores the value of museum collections. “Specimens that were collected 50, 100, 200 years ago are still being used all the time to learn new things,” says Ruane.