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Dinosaurs Construction. Look at how the bone structure is configured.
As in all four-legged animals, the dinosaurian pelvis was a paired structure consisting of three separate bones on each side that attached to the sacrum of the backbone. The ilium, above (attached to the spine), and the pubis and ischium, below, formed a robust bony plate at the centre of which was a deep cup the hip socket, or acetabulum. The hip socket faced laterally and was pierced or open at its centre for the articulation of the medially projecting proximal head of the thighbone. The combined saurischian pelvic bones presented a triangular outline as seen from the side, the pubis extending down and forward and the ischium projecting down and backward from the hip socket. The massive ilium formed a deep vertical plate of bone to which the muscles of the pelvis, hind leg, and tail were attached. The pubis had a stout shaft, commonly terminating in a pronounced expansion or bootlike structure (presumably for muscle attachment), that joined its opposite mate in a solid symphysis. The ischium was slightly less robust than the pubis, but it too joined its mate in a midline symphysis. There were minor variations in this structure between different saurischian genera and families. The ornithischian pelvis was constructed of the same three bones on each side of the sacral vertebrae, to which they attached by coossification. The lateral profile of the pelvis was quite different from that of the saurischians, with a long but low iliac blade above the hip socket and a modified ischium-pubis structure below. Here, the long, thin ischium extended backward and slightly downward from the hip socket. The pubis had a short to moderately long anterior blade, but posteriorly it stretched out into a long, thin postpubic process lying beneath and closely parallel to the ischium. The resulting configuration resembled that of birds, whose pubis is a thin process extending backward beneath the larger ischium.
 Notice the difference in the bone structure.
Saurischia The order Saurischia is known from specimens ranging from the Middle Triassic to the latest part of the Cretaceous in geologic time and recovered from every continent on the Earth. Two distinctly different suborders are traditionally included in the order the Sauropodomorpha (herbivorous sauropods and prosauropods) and the Theropoda (carnivorous dinosaurs). These groups are placed together only because both have the saurischian type of pelvis along with a few other primitive archosaurian features in common. No common ancestor has been widely recognized, and they could just as well be placed in separate orders. A little-known group, the Staurikosauria, is also classified in the order. Included in this group as infraorders are the well-known sauropods, or ³brontosaur² types, and their probable ancestral group, the prosauropods. All were plant eaters. Prosauropoda Most primitive of the Sauropodomorpha were the early (Triassic) saurischians known as prosauropods or plateosaurs. Found in Late Triassic and Early Jurassic rocks (230 to 187 million years old), their remains are probably the most ubiquitous of all Triassic dinosaurs. They have been found in Europe (Germany), North America (New England, Arizona, New Mexico), South America (Argentina), Africa (South Africa, Lesotho, Zimbabwe), China (Yunnan), and Antarctica. The best-known examples are Plateosaurus of Germany and Massospondylus of South Africa. Prosauropods were not large, as Dinosaur go, ranging from less than 2 metres (7 feet) in length up to about 7 metres (23 feet) and about 1 ton in maximum weight. Because their forelimbs were conspicuously shorter than their hind limbs, these animals (known from very complete skeletons) usually have been reconstructed poised on their two hind legs in a bipedal stance. Their anatomy, however, clearly indicates that some of them could assume a quadrupedal (four-footed) position. Footprints generally attributed to prosauropods appear to substantiate a quadrupedal form of locomotion. Prosauropods have long been seen as including the first direct ancestors of the giant sauropods, probably among
the family of melanorosaurids. That view still prevails, largely because of their distinctly primitive sauropod-like
appearance and also because of their Late TriassicEarly Jurassic occurrence. No better candidate has been
discovered so far. In general body form they were rather stocky, with a long, moderately flexible neck
(containing surprisingly long and flexible cervical ribs) and a head that was small in comparison with the body.
The jaw was long and contained rows of thin, leaflike teeth suited for chopping up (but not grinding or crushing)
plant tissues, although there is an indication of direct tooth-on-tooth occlusion.
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