Thyreophoran posterior defenses

The PNSO Pinacosaurus is reputed to be the most accurate ankylosaur replica to date. After obtaining one, I was struck by how prickly the creature was, especially posteriorly, where the caudal and pelvic areas are thoroughly guarded by spikes (see above). 

Pinacosaurus had a tail club, albeit of modest size. It obviously wasn't sufficient to defend the rear of the dinosaur by itself, or the array of spikes wouldn't have been selected for. Evidently, all ankylosaurs needed caudal osteoderms, because of a weakness in the club defense. 

The tail was swung at a predator, but if unhurt the attacker had an opportunity to strike every time the club was swung back, away from it (before it could be swung at it again). By timing its move carefully, a theropod could bite the proximal part of the tail, practically stopping its oscillation and, if sufficient damage was done, effectively disarming the prey. In fact, a big predator could render the club virtually useless just by blocking oscillations with its body, as it moved in from the side/rear. Tactics such as these must have succeeded early in ankylosaur history. Certainly their impact was felt by the Campanian-Maastrichtian, when caudal osteoderms appeared in Pinacosaurus,  Dyoplosaurus, Zuul, Saichania and Tarchia.  A probable specimen of the latter, ZPAL Mgd I/113, is a good example. No doubt other ankylosaurs, known from less complete material, also had caudal and pelvic spikes.

But what about stegosaurs? They also relied on distal caudal weaponry, but lacked the extensive posterior osteoderms--at least ones capable of stopping an attack, which vertically oriented plates or spines could not do.

Evidently, stegosaurs relied on a caudal defense which was more effective in guarding the rear areas, with little or no assistance from spines. The key difference was the flexibility of the stegosaur tail, distally as well as proximately. Whereas the distal half or more of the ankylosaur tail consisted of a rigid handle, the stegosaur tail could bend at its tip. The ability to bend increased the reach of the thagomizer, as did the length of the spikes, in contrast to a club (see below). Stegosaur weaponry could hit an attacker striking the base of the tail. (In addition, caudal plates prevented an efficient grip on the tail, since a theropod's  upper jaw could not bite down on its dorsal surface).

Effectively guarding the rear of the body by itself, the thagomizer proved to be a very economical weapon, obviating any need for extensive, additional osteoderms.