Here is a question that has frustrated me for quite some time: What was the placement of Stegosaurus's tail spikes? I know that while they have traditionally been restored pointing upwards, this is now considered inaccurate. So what was the correct position? I have heard some places that the spikes were positioned horizontally, but at least one book tells me that the spikes "diverged from one another like a pin cushion." Whatever the arrangement was, would it have been similar for other stegosaurs?

According to Kenneth Carpenter (1998, Modern Geology 23:127-144), the tail spikes of Stegosaurus should project horizontally and postero-laterally (pointing back- and outwards), not upwards. In the traditional reconstruction, upward projections of the spikes would have made sense as an offensive weapon because the tail was reconstructed as drooping down to the level of the ground. However, Carpenter (1998) argues that the tail was probably held straight, horizontally and parallel to the ground - as tail anatomy indicates. Also, articulated stegosaur tails are frequently found straightened. Further, 'the long base of the plates effectively "lock" segments of vertebrae together' (Carpenter 1998, p 139) a condition only possible if the tail was held straight. With a tail held straight parallel to the ground, then horizontally- and posterolaterally-projecting spikes would be much more effective as offensive weapons, if the tail were to be swung from side to side.

Additionally, the tail spikes themselves support the posterolateral reconstruction. If held in the upward position, then the base of the spikes are not evenly spaced with the caudal vertebrae. Articulated specimens also reflect a more postero-lateral arrangement.

Carpenter (1998) reconstructs the proximal pair of tail spikes as projecting more laterally almost perpendicular to the tail, while the distal pair project posterolaterally. So I guess this would be the condition of the 'pin cushion' you describe.

I don't know of tail spike arrangements in other stegosaurs, and as Carpenter (1998) pointed out, it may be something that hasn't been addressed as much. But then, someone else may know more???