The Stegosauria was a group of moderately large quadrupedal ornithischians that arose in Middle Jurassic times. The first fossil to be discovered was an incomplete skeleton, described by Richard Owen in 1875. Shortly afterwards, numerous specimens of Stegosaurus (Fig. 71) were excavated from two quarries in North America. Later, stegosaurs were found throughout the northern continents, even extending into China. They were characterised by a double row of tall spikes running down their backs. The possible functions of these have already been discussed (Sects. 7.5.2, 9.2.2) - they may have had a thermo-
regulatory or a defensive function, or both. The number of spikes varied between species. The skulls of these dinosaurs were extremely small in relation to the sizes of their bodies, as were the brains within them. That of Stegosaurus was about the size of a walnut although the animal's body length was up to ca. 9 m. The ratio of brain mass to estimated body mass (encephalisation quotient) is, however, not related to intelligence mainly because it does not take into account increases in the surface area of the brain due to folding.
Like the iguanodontids and hadrosaurids, the stegosaurs probably roamed the countryside in large herds, but they were exclusively quadrupedal. Nevertheless, the massive hind limbs were over twice the length of the forelegs so that the body sloped forwards from its highest point above the hips. The low position of the head was probably an adaptation for feeding on low ground cover - cycads, ferns and so on. At the same time, stegosaurs would undoubtedly have been able to rear up on their hind legs had they wished. Their feeble jaws and teeth merely cropped large amounts of vegetation, which passed into a vast stomach where it fermented at the posterior end of the gut processor continuum (Sects. 10.1, 10.2). Defence against predators consisted of the spines on the body and a massive tail (Sect. 9.2.2).
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