Prosauropods, like the Plateosaurus, roamed the earth from 230 to 190 million years ago, the early ancestors of sauropods, giant plant-eating dinosaurs such as Apatosaurus. The fossil record indicates that members of this group were the first dinosaurs to rely on plants for food. They had small skulls and long, slender necks, were bipedal and browsed on bushes and trees. Yet, despite this simalarity to sauropods, they were much smaller, ranging from a few hundred kikograms to perhaps 1,500 kg (3300 lbs).) All were quadrupedal, but many were capable of bipedal posture and gait.

During the late Triassic and early Jurassic periods, prosauropods were the largest plant-eating dinosaurs.

New fossils were found in Madagascar in 1999. Scientists believe the fossils are about 230 million years old, which would make these dinosaur remains the oldest ever found. The fossils included the fragmentary remains of two plant-eating dinosaurs, prosauropods, about the size of a new-born calf, which are not only the earliest dinosaurs known from the island, but probably the earliest dinosaurs known from anywhere in the world.

Sauropods included Apatosaurus (Brontosaurus), Diplodocus, and Brachiosaurus. They descended from prosauropods. By the middle of the Jurassic Period they far surpassed all other dinosaurs in size and weight, reaching lengths of more than 25 m (more than 82 ft) and weights of about 90 metric tons.

Fossilized trackways indicate that these animals traveled in herds. That sauropod fossils have been found on every continent suggests they were able to adapt to diverse environments.

All sauropods had the same basic structure:

• Large, barrel-like body
• Sturdy, columnar legs
• Long, heavy tails
• A small head at the end of a long and stout neck (hence were of low intelligence)

The dominant herbivores, they were common during only the middle part of the Age of Reptiles. Although they survived until the end, in most places in the Northern hemisphere they were displaced by more advanced ornithischians of much smaller size.

Elephants walk around on tiptoe. A shock-absorbing wedge of spongy tissue beneath each heel lift them off the ground. Sauropods had the same type of spongy wedge beneath their heels.

Sauropods were usually considered semiaquatic or amphibious because their bones were not strong enough to support their massive weight, thus requiring the bouyant effect of water. Their long neck enabled them to reach the surface for air. Footprints show that some sauropods did migrate across land. The long neck might have functioned like that of a giraffe.

In Diplodocus, the neck and tail are both very long; the lower (haemal) arches of the tail vertebrae, at the point where the tail would normally reach them, are canoe-shaped instead of tongue-shaped, and appear to have assisted the animal in the tail as a prop for the hind limbs in a tripod stance as it fed on high vegetation, with its forelimbs raised off the ground.

In Brachiosaurus, the neck is very long but the tail is short, so the tripod stance was impossible. However, its forelimbs were longer than the himb limbs, and this length plus the long neck enabled it to reach high vegetation by different means than Diplodocus used.

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