Stegosaurus is an iconic dinosaur that lived for about 10 million years during the Late Jurassic period, from about 155 to 145 million years ago. Stegosaurus was a large armored quadriped herbivore with distinctive kite-shaped plates atop a heavily arched back and a spiked tail. The plates and spikes have been the subject of much speculation. It is thought that the plates helped with thermoregulation while the spiked tail was for defense. Altogether, Stegosaurus had 17 plates and four tail spikes.
Stegosaurus was the largest of the stegosaurids, the clade named after it, with an approximate length of 9 m (30 ft) and height of 4 m (14 ft), though some specimens are as long as 12 m (42 ft). Stegosaurus is thought to have weighed about 5 tonnes, but its brain was tiny, smaller than a dog's, with an estimated mass of just 80 grams. This has helped contribute to the notion that dinosaurs were completely unintelligent, but the contemporary view among scientists is that dinosaurs in general were smarter than popular opinion believes.
Analysis of Stegosaurus teeth shows that they lacked the ability to chew, instead severing plant material and swallowing it whole. Their digestion was assisted by stomach stones called gastroliths, which would have mechanically broken down plant material as they were shuffled around by stomach muscles. Gastroliths have also been found in contemporaneous sauropods, even larger herbivores of an entirely different dinosaur group. It is unknown if Stegosaurus was able to stand on its hind legs, but if it could, it would have had prime access to elevated foliage.
Like many other dinosaurs, Stegosaurus benefited from the thermoregulatory properties of its large size. It is still uncertain how fast the metabolism of dinosaurs was, but it is likely that they were somewhere in between "warm-blooded" mammals and the "cold-blooded" reptiles of today. Accordingly, they would have benefited from large bodies that helped retain heat. This strategy was pursued to the extreme in sauropods, herbivores much larger than Stegosaurus.
Stegosaurus would have had to defend itself from canivorous theropod dinosaurs such as Allosaurus, and indeed, Allosaurus fossils have been found with skeletal punctures that perfectly match the shape of Stegosaurus tail spikes. Stegosaurids in general went extinct not long after the extinction of Stegosaurus itself, leaving the rest of the Age of Dinosaurs without these fascinating armored beasts.