119 8 How to Get Big in the Mesozoic: The Evolution of the Sauropodomorph Body Plan OLIVER W. M. RAUHUT, REGINA FECHNER, KRISTIAN REMES, AND KATRIN REIS sauropod (or, more correctly, eusauropod) dino- saurs are highly distinctive, not only in their overall body form, but also in respect to many details of their anatomy. In comparison with basal dinosaurs, typical sauropods are characterized by small skulls, elongate necks, massive bodies, and an obligatory quadrupedal stance with elongate forelimbs and straight limbs in gen- eral. Tracing the anatomical changes that led to this dis- tinctive body plan through sauropodomorph evolution is problematic as a result of the incompleteness of many basal taxa and phylogenetic uncertainty at the base of the clade. The decrease in skull size in sauropodomorphs seems to be abrupt at the base of the clade, but it is even more pronounced toward sauropods. Major changes in the sauropod skull are a relative shortening and broaden- ing of the snout and an enlargement and retraction of the nares. Although the ultimate causes for these evolu- tionary changes are certainly manifold, most if not all of them seem to be related to the ecological and biome- chanical requirements of the transition from a carnivo- rous to an herbivorous lifestyle, in which the skull is mainly used as a cropping device. A relatively elongate neck seems to be ancestral for sauropodomorphs, but the neck is further elongated on the lineage toward sauro- pods, especially by incorporation of two additional ver- tebrae at the base of Sauropoda. The relatively simple structure of the cervical vertebrae in basal sauropodo- morphs might be a secondary reduction relative to basal saurischians as a result of changes in neck biomechanics in connection with the reduction of the size of the skull. Thus, the more complicated structure of sauropod cervi- cals probably reflects changing biomechanical require- ments in connection with an elongation of the neck and an increase in body size, as does the opisthocoelous structure of the cervical vertebral centra. Limb evolution in sauropodomorphs is dominated by adaptations to- ward increasing body size and thus graviportality, with the limbs getting straighter and the distal limb segments relatively shorter. Body size increase in sauropodo- morphs seems to have been rapid but even-paced, with the ancestral body size of the clade being in the 0–10 kg category, and the ancestral body size for sauropods prob- ably being in the 1,000–10,000 kg category. Introduction Sauropod dinosaurs are certainly among the most recogniz- able fossil vertebrates known. With their massive bodies, tiny heads, long necks, and long tails, they are the stereotypical dinosaur and are found everywhere in popular culture, from cartoons to the advertising billboards of a big oil company. However, sauropods are not only popular with the general public. They represent the largest terrestrial animals in the his- tory of life on earth, and hence they are of great scientific