Everybody knows that Stegosaurus had four tail spikes. The formidable weapons this odd dinosaur sported were some of its most prominent features. Yet, when Stegosaurus was new to science, it seemed as if this dinosaur bristled with even more spikes.

In 1891, the first full skeletal drawing of Stegosaurus ungulatus was created under the direction of Yale paleontologist Othniel Charles Marsh. The creature was subtly different from Stegosaurus as we know it today, but there was one feature that definitely stuck out. Along the tail were four pairs of spikes. We now know that Marsh, as well as other paleontologists, were wrong about the spikes, but why did they make this mistake?

Paleontologists Kenneth Carpenter and Peter Galton traced Marsh’s scientific steps in a paper included in The Armored Dinosaurs. It turns out the eight-spiked Stegosaurus was born via a mash-up of several different specimens. Marsh recognized this and meant for the reconstruction to represent only the general archetype of the dinosaur.

Still, the eight-spiked form was a closer approximation of the animal than some of the naturalist’s earlier interpretations. When Marsh initially described Stegosaurus in 1877, he thought the dinosaur looked like a giant turtle. Marsh envisioned the large triangular plates as part of a large shell that created a bony “roof” over the animal’s back (hence the name Stegosaurus, meaning “roof lizard”). The Yale paleontologist later discarded this view, but then there was the problem of the spikes. Marsh was not sure about where they should go, and at one point in 1880 proposed that the weapons were wrist spikes that could have been used for defense if Stegosaurus reared back on its tail.

Not long after he proposed the wrist spike hypothesis, Marsh received a quarry sketch from one of his collectors, William Reed, that showed spikes in close association with the tip of the tail. Several other Stegosaurus spikes were found like this, confirming that the spikes belonged at the end of the tail. Marsh changed his ideas accordingly by 1887.

After he recognized the placement of the tail spikes, Marsh thought that the various specimens sent to him represented distinct species of Stegosaurus with different numbers of tail spikes. Depending on the species, a Stegosaurus might have between one and four pairs of spikes—Marsh did not consider the possibility that spikes might have been lost in some specimens or that extra spikes might be found with others. This kind of splitting was common during the height of the “Bone Wars” era. If a bone or specimen looked different enough from what was already known, then it deserved to be separated as a new species. (The practice created persistent headaches for generations of paleontologists after Marsh.)

Frustratingly, Marsh did not provide details about why he thought each species had differing numbers of spikes. It seems that he simply took what was found in the field at face value, even though several specimens with only four tail spikes were known to him by the time of his 1891 reconstruction. The eight-spiked Stegosaurus may have simply been a product of confusion and standard practices at the time, but there is no evidence that this dinosaur had any more or less than four spikes. An eight-spiked Stegosaurus would have certainly looked imposing, but even the correct, four-spiked model is impressive enough.