In 1877 O.C. Marsh described and named Stegosaurus ungulatus from bones collected in the Morrison Formation at Como Bluff, Wyoming, some of which is included in the mounted skeleton on display in the Yale Peabody Museum’s Great Hall. This skeleton is reconstructed from the fossil bones of at least five individuals, possibly including more than one species and different sexes.
Marsh believed the animal had 8 spikes on its tail, arranged in pairs. Recent detailed research on the specimens Marsh used for his description, together with a thorough analysis of the original collector’s field journals, show no evidence for 8 tail spikes for this species. Instead, it is likely there were only 4, arranged in 2 pairs.
The orientation of the spikes is also subject to debate: some paleontologists believe they pointed up, as shown in the Peabody's nount, while others believe the spikes were more horizontal and probably pointed backwards. There is little doubt, however, that Stegosaurus's tail was a formidable weapon against attack by a hungry Allosaurus or Ceratosaurus.