Faced with predators armed to the teeth, plant-eating dinosaurs evolved a remarkable arsenal of defensive weapons. The ankylosaurs - the tank-like armoured dinosaurs - developed bony tail clubs that could break a predator's leg in a single swing. The stegosaurs grew deadly tail spikes, sometimes called thagomizers. Sauropods relied on their colossal size and sweeping tails. Smaller herbivores used spikes, horns, and even thumb daggers to deter attackers. Euoplocephalus, shown here, was a heavily armoured Late Cretaceous ankylosaur whose tail club could weigh up to 30 kg (66 lb) - a wrecking ball at the end of a 6-metre body.
The stegosaurs are best known for their tail spikes. Stegosaurus itself had four tail spikes, each up to 90 cm (3 ft) long - a weapon palaeontologists nicknamed the thagomizer (after a 1982 Far Side cartoon by Gary Larson, of all things). Huayangosaurus, an early stegosaur from Jurassic China, was already armed with tail spikes about 170 million years ago, around 20 million years before its larger American cousin appeared. The spikes were used like a giant club - whipped sideways into a predator's flank, they could punch holes deep into bone. Healed wounds matching Stegosaurus spikes have been found on Allosaurus bones.
Huayangosaurus
Yes - sheer body weight was the ultimate sauropod weapon. A full-grown Barosaurus weighed around 20-25 tonnes - enough to flatten any predator unlucky enough to be caught beneath its feet. Even a 3-tonne Allosaurus, the leading Jurassic predator, would have been smashed by an angry sauropod throwing its weight around. Sauropods may also have used their long tails as whips. A 14-metre Diplodocus tail, swung with full force, could crack like a bullwhip and deliver a stinging, possibly bone-breaking blow. Adult sauropods were essentially un-killable - which is why predators usually targeted the young.
Barosaurus and Allosaurus
Therizinosaurus's claws were among the most spectacular weapons of the dinosaur world - up to 1 metre (3.3 feet) long, curved like sickles, and growing from a 5-metre-long arm. For a long time their use was mysterious. Some palaeontologists argued the claws were too blunt at the tip to be effective weapons and were probably tools for raking down foliage or ripping open termite mounds. Others have countered that the claws had sharp edges along their sides, making them capable of slicing into an attacker. The most likely answer is both: they were primarily feeding tools, but a desperate Therizinosaurus could certainly have used them in self-defence.
Therizinosaurus
Kentrosaurus - whose name means "spiked lizard" - is a strong contender for spikiest dinosaur. This 5-metre (16-foot) stegosaur from Late Jurassic Tanzania had small bony plates along its neck and back, then six pairs of long, narrow spikes running down its lower back and tail. Each spike was up to 60 cm (2 feet) long. Kentrosaurus also had a spike pointing outward from each shoulder, giving any predator that came too close a nasty surprise. Gastonia and several other ankylosaurs were also famously spiky, with shoulder spikes long enough to gore a predator's leg.
Kentrosaurus
Iguanodon - the second dinosaur ever to be scientifically named (in 1825). At first, palaeontologists thought its conical thumb spike was a horn that sat on the nose. When more complete skeletons were found, the spike turned out to be on the hand - a deadly, dagger-shaped weapon up to 25 cm (10 in) long. Iguanodon probably used its thumb spike to stab attacking predators, perhaps in the throat or chest, while warding them off with its bulky body. Its hand was otherwise remarkably flexible: it had three middle weight-bearing fingers, a thumb spike, and a small grasping fifth finger.
Iguanodon