The Tip of This Snake's Tail Looks Like a Spider. But Inside, Its Bones Are 'Bizarrely Normal'

The spider-tailed horned viper has a special trick to lure birds—at the tip of its tail, elongated scales form a bulbous lobe and sprout outward like spindly legs. As the snake lies in wait, it wiggles this strange-looking appendage on the ground in a way that resembles the motion of an arachnid. If the reptile is lucky, a bird might come investigate the makeshift spider, and that’s when the predator goes in for the kill.

This strange tail “is unlike any other studied [in] vipers and actually seems to be unique among all living reptiles,” Georgios Georgalis, a paleontologist at the Polish Academy of Sciences, tells Smithsonian magazine. Despite the strange external appearance of the tail, the animal’s underlying bones, Georgalis and others reveal in a new study, are “bizarrely normal,” per a statement from the Field Museum. The new work, which created 3D images of the snake’s skeleton, was published today in The Anatomical Record and offers the first detailed look at the vertebrae of the spider-tailed horned viper (Pseudocerastes urarachnoides).

Well-known for their venom, vipers are among the world’s most iconic snakes, but scientists have not deeply examined their vertebral anatomy. In the few viper species with their spinal columns mapped out, the work has typically been limited to a few bones. The function of some of these structures, which may be unique to certain species, he adds, “deserves much greater attention.” Endemic to Iran, the spider-tailed horned viper is “one of the most fascinating viper species in the world,” Georgalis says.

Its tail offered an especially interesting opportunity to study a snake’s skeleton more closely. He wanted to know: Did the extreme structure outside the tail show up in the vertebrae? When the museum’s scientists collected the first specimen of the viper in 1968, they “thought that it had some sort of deformity,” she adds, “like a tumor.” It wasn’t until 2003 that researchers discovered another snake of the species with similar tail anatomy.

Scientists formally described the viper three years later, and subsequent research revealed how the creature hunted using the tip of its tail. Other snakes hunt with this strategy, known as caudal luring, but their tails look a little more standard—they might be a distinct color from the rest of the body, for instance, without the elaborate appendages seen in the spider-tailed horned viper. While some snakes use tail movement to lure in potential prey, these motions might also serve to attract mates or signal possible threats.

Snakes with tail rattles, made with hollow keratin segments, use them to ward off predators—and some scientists have suggested this behavior might have evolved from caudal luring. In 2023, Georgalis approached Ruane with the idea of collaborating to study this species using the museum’s original specimen. They decided to image the viper using the Field Museum’s new X-ray computed tomography lab, which allowed them to take thousands of X-ray images of the skeleton and stack them together into a 3D model.