Many dinosaurs had some kind of protective armour, but the ankylosaurs took it to the extreme. These tank-like four-legged plant-eaters were covered head-to-tail in bony plates, studs, and spikes that fused into a living suit of armour. Gastonia, shown here, was an Early Cretaceous ankylosaur from Utah whose shoulders bristled with massive forward-pointing spikes. A hungry theropod would have had to deal with all of these spikes, plus armoured back plates, before reaching the only unprotected part - the soft belly. Most ankylosaurs would simply crouch down to hide their bellies when threatened. Their armour worked: ankylosaur skeletons rarely show predator bite marks.
Dinosaur armour was made of osteoderms - flat or knobby plates of bone embedded in the skin and often covered with a layer of horn or keratin (the same material as your fingernails). In ankylosaurs like Edmontonia, the osteoderms ranged from small bumps to massive triangular spikes 30 cm (12 in) long projecting from the shoulders. Crocodiles still have osteoderms today - the bumpy scales along their backs are the same kind of bony armour, though much less elaborate. Some dinosaur osteoderms had internal channels for blood vessels, suggesting they may also have helped regulate temperature.
Edmontonia
A nasal boss is a thickened pad of bone on the snout - found in a few dinosaur groups, but most famously in Pachyrhinosaurus ("thick-nosed lizard"). Where most of its horned-dinosaur relatives had a sharp facial horn, Pachyrhinosaurus had a giant, rough, bony knob in the middle of its face - sometimes flattened, sometimes raised. The boss was probably used in head-to-head shoving matches between rival males, much like modern bighorn sheep or musk oxen, to determine dominance and breeding rights. It may also have helped protect the head from predator bites.
Pachyrhinosaurus
Yes - at least in ankylosaurs. Recent studies of well-preserved ankylosaur skin have revealed an extraordinarily tough multi-layered armour. Large osteoderms (called scutes) were fused into a flexible armoured shell across the back, with smaller irregular ossicles filling the gaps between them. The whole structure interlocked in a way that distributed force across a wide area when struck. Modern engineering studies have compared this layout to medieval scale armour or even fibreglass composites. The 2017 discovery of Borealopelta in Canada preserved its armour and skin so completely that scientists could even determine its colour - reddish-brown on top, pale below.
Ankylosaur skin
Pachycephalosaurus - meaning "thick-headed lizard." This Late Cretaceous dinosaur had a dome-shaped skull with a top of solid bone up to 25 cm (10 in) thick - the thickest skull cap of any animal known. Surrounding the dome were knobby bumps and short bony spikes. For decades, palaeontologists pictured Pachycephalosaurus ramming heads with rivals like modern bighorn sheep. Some recent studies have questioned this - the thick dome could have shattered if struck too hard - but evidence of healed injuries to dome skulls suggests these dinosaurs really did fight head-to-head, just perhaps with shoves and side-on blows rather than full-speed collisions.
Pachycephalosaurus
A few sauropods had body armour, despite their giant size. Saltasaurus, an 8-metre titanosaur from Late Cretaceous Argentina, had two kinds of armour: fist-sized bony scutes set into the skin of its back and neck, and a denser layer of tiny pea-sized nodules across its entire body. Several other titanosaurs - Ampelosaurus, Rapetosaurus, Antarctosaurus - also had osteoderms. By this point in the Cretaceous, predators like Carnotaurus had become smaller and faster, perhaps prompting these sauropods to evolve light armour as backup to their size.
Saltasaurus