Researchers using CT scans have found that the spider-shaped tip on the tail of Iran's spider-tailed horned viper is made of modified scales rather than bone, according to a study published Thursday in the journal The Anatomical Record.
The snake, known scientifically as Pseudocerastes urarachnoides, lives in the arid Zagros Mountains of western Iran. It uses the fleshy tail appendage to lure prey: a bird flying overhead spots what looks like a plump spider crawling over the rocks, and when the bird swoops down to seize it, the rough surface nearby turns out to be the viper itself, which strikes and eats the bird.
Sara Ruane, associate curator of herpetology and director of the core laboratories at the Field Museum's Negaunee Integrative Research Center in Chicago, said that everyone who looks at the tail thinks the snake has a tumor, some kind of growth or a parasite, and that it must be an anomaly. She said it was only after a second individual was collected in 2003 that anyone considered that the structure might simply be normal for the species.
The first specimen, collected in 1968, remains in the Field Museum's collections as the holotype, the reference individual scientists use to establish a species' physical characteristics, Ruane said. Researchers used that same holotype in 2006 to formally describe the snake as a newly identified species.
A Snake Living on the Edge
Ruane first saw a striking video of the viper waving its unusual tail to attract birds in 2008, while she was a doctoral student. Other vipers are known to wiggle their tails, particularly young snakes with bright green or neon-yellow tail tips that can resemble a wriggling worm to an unsuspecting frog. But Ruane said the apparatus on the spider-tailed viper's tail operates on a completely different level.
The viper is elusive, using its muscles to survive on towering rock faces, and its extreme habitat makes it difficult to study or collect in the wild. Ruane said some videos show the snakes wedged into tiny crevices where it seems impossible any animal could grip so well, let alone a limbless one, and that when the snake grabs a bird, sometimes the entire snake flies off the cliff along with its prey.
Inside the Tail
Ruane met Georgios Georgalis, an associate professor at the Institute of Systematics and Evolution of Animals at the Polish Academy of Sciences in Krakow, at a conference in 2023, and he expressed interest in studying the viper's holotype. As a paleontologist, Georgalis studies modern snakes to help interpret fragmented fossil remains of long-extinct species.
Ruane recalled that the timing was ideal, since the Field Museum was installing a new X-ray CT scanning lab. The technology scans objects to produce thousands of X-rays, generating 3D renderings that reveal the interior of structures without damaging the specimen, such as the inside of a snake's tail.
Kurt Schwenk, professor emeritus in the department of ecology and evolutionary biology at the University of Connecticut in Storrs, who was not involved in the study, said snake skeletons contain hundreds of vertebrae, which is why snake spinal columns have historically been understudied.
The holotype was one of the first specimens scanned when the lab began operating in late 2024. The scans showed that the tail vertebrae were normal and typical of most vipers, Georgalis said. Instead, he said, the fleshy appendage is made up of elongated, modified scales, with no bony element present at all.
Georgalis wrote in an email that if these remains existed only as fossils today, scientists would have no way of reconstructing or imagining the tail's spider-like shape. He called it a painful reminder that fossil remains in paleontology do not offer the absolute truth about how remarkable extinct reptiles and other vanished animals once were.
Schwenk, a reptile morphologist, said he had not expected the lure to include anything beyond soft tissue. He wrote that scale modifications forming spines, horns, bumps and similar structures are fairly common in lizards and snakes, but that outside the head, they rarely or never involve the underlying skeleton. He also noted that although snake vertebrae are typically shorter toward the tail, the spider-tailed viper's tail vertebrae appeared exceptionally thin, which he said would make the tail especially flexible and better able to make the tight, sinuous curves that create the lure's realistic, spider-like movements.
A 'Spectacular' Adaptation
Questions remain about the spider-tailed viper, including how it evolved this unusual tail-tip structure, known as a caudal appendage. Ruane pointed to the viper's two closest relatives, the Persian horned viper and the Field's horned viper, saying both have oddly shaped tails that look smaller than expected and heavily keratinized, and that in the specimens she has seen, the tail is a different color, black against the snake's sandy brown or gray body.
Many snakes wiggle their tails to scare off predators or lure prey, but Ruane said the spider-tailed viper may be an extreme example, similar to rattlesnakes, of how features evolve over time from certain behaviors. She added that future research could examine which genetic mechanisms may have switched on or off to produce the spider-like appendage.
Schwenk said he believes the tail-waving behavior was likely already present in the viper's ancestors, and that the spider-tailed viper took that ancestral behavior and modified it into something truly spectacular. He said the spider-like structure, combined with the tail's sinuous movements, creates the impression of a moving spider so convincingly that the first time he saw a video of the behavior, he was completely fooled, calling it a wonderful example of the power of natural selection in shaping animal form and function.
