Across tens of millions of years, sauropods and theropods seem to have been locked in an evolutionary size race. As the theropods grew larger and more deadly, the sauropods grew bigger still, until plant-eaters like Camarasaurus dwarfed Jurassic predators like Allosaurus. By the time of Tyrannosaurus rex, the giant sauropods of North America had largely disappeared - but their southern cousins like Argentinosaurus remained gigantic, outweighing even the largest predators. However, being enormous came with serious trade-offs: slow speed, huge food requirements, and tricky temperature regulation. Many smaller plant-eaters thrived by going in the opposite direction - growing armour, sharp horns, or running fast.
Not at all - many were quite small. Gargoyleosaurus, one of the earliest known ankylosaurs, was only 3-4 metres (10-13 feet) long, smaller than a horse. Yet despite its modest size, Gargoyleosaurus was far from defenceless: its shoulders were lined with sharp spikes, its back was covered with bony plates, and its skin was studded with small armoured nodules. Even a hungry Allosaurus would have struggled to make a meal of it. Other small plant-eaters, like the bipedal Hypsilophodon, took the opposite approach and relied on speed to escape danger.
Gargoyleosaurus
Almost certainly. To pump blood up a 12-metre neck to a head held high in the trees, a giant sauropod like Brachiosaurus would have needed extremely high blood pressure - several times higher than a human's. Many scientists believe these animals had a four-chambered heart, like modern birds and mammals, rather than the simpler three-chambered heart of most reptiles. Some researchers have suggested sauropods may have held their necks more horizontally most of the time to reduce the strain on the heart, only raising them up to reach high branches. The exact answer is still debated - because hearts, being soft tissue, never fossilize.
Heart of Brachiosaurus
Fossil footprints, known as trackways, are some of the most exciting evidence we have of dinosaur behaviour. They reveal how fast a dinosaur was moving, whether it walked alone or in a group, and how it placed its feet. Palaeontologists calculate speed by measuring the stride length between footprints and applying formulas borrowed from modern animal biomechanics. Brontopodus tracks - left by big sauropods - tell us these animals walked at around 4-10 kmh (2.5-6 mph), with a leisurely lumbering gait. Running sauropod tracks are extremely rare, suggesting these giants almost never moved quickly. A trackway in Texas shows what appears to be a sauropod being followed by an Allosaurus-like predator.
Sauropod footprints (Brontopodus tracks)
Older books claimed that sauropods must have lived in water because they would have collapsed under their own weight on dry land. We now know this is wrong - sauropods were perfectly built for land life. Their legs were pillar-like and held vertical, just like an elephant's, to transmit their weight directly to the ground. Their feet were broad, with toes that splayed outwards on a fleshy heel pad - much like a modern elephant's foot - to spread the load. The vertebrae of the spine were hollow and lightened by air sacs, reducing the total weight the legs had to support.
Diplodocus forefoot
Yes - many plant-eaters avoided becoming dinner by being fast. The ornithopod group included slender, bird-like bipedal plant-eaters such as Dryosaurus (a Jurassic dinosaur 3-4 metres long) and Hypsilophodon (only 2 metres long). Both had long, slim legs built for sprinting and a stiff tail held off the ground for balance. Gallimimus, an ostrich-like omnivore, may have reached speeds of 60 kmh (37 mph). For dinosaurs with no horns, plates, or spikes, speed was the difference between life and death.
Dryosaurus
Some probably could. The skeletons of Saltasaurus, Diplodocus, and a few other sauropods have features suggesting they could rock backwards onto their hind legs to feed from very high foliage. Some palaeontologists also believe sauropods could rear up briefly when defending themselves from a predator, or perhaps during mating displays.
Saltasaurus