CT scan finds ordinary vertebrae inside the spider-tailed viper's fake spider

Viper vertebrae vary so much that a single bone rarely identifies a species, which is part of why a paleontologist went looking at a snake famous for its skin. Georgios Georgalis of the Polish Academy of Sciences in Krakow, the study's first author, works from living animals toward fossils, and he says the viper vertebral column gives few clues about which snake you are holding: fossils usually arrive as single bones, and vertebrae differ so much along one animal's body that matching them to a species is hard ^[3][4]. Viper vertebrae have not been widely studied in any case ^[12]. A tail ending in a spider looked like the place where bone might finally say something.

The scan did not deliver that. Phys.org's account of the work describes the tail bones as bizarrely normal ^[2]. Set that against the exterior, a bulbous body with long dangling legs, and the shape has to be carried by something other than the skeleton: the scales and the soft tissue beneath them ^[16]. The inference rests on the absence of skeletal novelty, not on a measurement of the scales. The account does not say which tissues do the work, or how the tail is held and waved while a bird is deciding.

The reason this needed a scanner rather than a scalpel is that the specimen is the holotype, the physical standard-bearer for the species ^[8], and no curator opens one to satisfy curiosity. Ruane, the Field Museum's associate curator of herpetology ^[5], says the timing was lucky: the museum was getting a new XCT lab when Georgalis raised the idea at a 2023 conference ^[10][13].

The specimen's own history is an argument for scanning old material. It was described as Pseudocerastes urarachnoides in 2006, 38 years after it reached Chicago ^[7][14], and until this study the tail was known only from the outside ^[18]. Before 2003, when a second snake with the same sort of tail surfaced, the first one was read as a deformity, something like a tumour ^[6]. Ruane says she first saw field video of the luring behaviour around 2008, as a Ph.D. student ^[11].

The scan does not show whether normal means normal to the eye or normal against measurements of other viper tails, and with a single animal on the stage there is no denominator for variation between individuals, sexes, or growth stages ^[17]. For anyone reading bones, the finding is that the most theatrical tail in the family leaves its signature where paleontologists cannot usually look ^[16][4].