SUMMARY

The tail of Kentrosaurus probably had a large motion arc covering the entire half-circle behind the animal (Mallison 2010a). Modeling results indicate that it could swing across most of this arc with sufficient speed to cause serious injury. Muscle force estimates at the unrealistic lower end of the spectrum reported here result in moderate impact speeds and impact forces. Such hits certainly smarted, comparable to the hits large extant monitors distribute with their tails (e.g., Holland 1915). More muscular and realistic models of Kentrosaurus achieve speeds at which impacts could seriously harm predators. Without whiplash motion, speeds of approximately 5-14 m/s could likely be achieved for the tail tip, depending on the specific tension of the musculature assumed. In the case of spike tip hits with an intact horn cover, as envisaged by Carpenter et al. (2005) for Stegosaurusand with the impact angle close to 90°, at these speeds the tail spike tips could very likely penetrate soft tissues deeply, and fracture thin bones such as ribs or facial bones. Collisions with larger contact areas, e.g. the side of a spike, at any significant speed would result at least in the typical injuries resulting from blunt trauma with moderate energy, such as concussions, large hematomas, and crush injuries. Simulations of simple tail swings using the largest musculature reconstruction, the 'croc' model', suggest that a hit squarely on a predator's skull may well have been sufficient to maim or kill even without whiplash motions.

Whiplash motion models indicate high tail tip velocities and thus impact forces for relatively low accelerating and decelerating torques. Large arcs could be covered at speeds greater than 20 m/s, and top speeds of ~ 40 m/s appear realistic. The risk of the tail damaging itself when hitting osteological stops seems minimal due to the motion geometry. Whiplash strike simulations predict impact forces easily sufficient to cause critical injuries on predators of all sizes. Within the arc covered by the whiplash swing of the tail tip at speed greater 40 m/s, which is small compared to the total motion range of the tail but may have amounted one quarter of the animal's aspect, strikes probably could cause lethal deep penetrating trauma to the head, neck, and torso of even large predators. For small- to medium-sized (< 200 kg) theropods even impact speeds of 20 m/s or less were potentially lethal, due to the large inertia of the tail tip. Blunt trauma of the skull was likely incapacitating, while internal organs may have been less affected, although a strike against the ribcage would likely have resulted in multiple rib fractures.

An aimed blow required exact timing of the impact in both space and time. At slow swing speeds (2 s), a near miss would invite a predator to step into the arc, exposing the tail base and pelvic region to bites. The swing speeds and tail tip speeds sustained across large arcs, however, suggest that such an attack strategy was risky, because realistic models predict swing times much lower than 1 s, and times to accelerate the tail tip to speeds sufficient to cause serious injury at below 0.5 s. Return strikes may have been possible in less than 2-4 s. Overall, modeling results suggest that Kentrosaurus was capable of defending itself effectively against any single threat, so that coordinated attacks by two or more predators may have been required to endanger the animal.