The sauropodomorphs quickly evolved into two major groups, the Prosauropoda and the Sauropoda. Although prosauropods appeared earlier, no known prosauropod could have been the ancestor of the sauropods. The prosauropods were widespread and had at least seven families. They lived until the Early Jurassic. The largest prosauropods, some as long as 40 or more feet, were straight-limbed dinosaurs that resembled the later sauropods in some ways. All prosauropods were plant-eaters.
Family: Thecodontosauridae: The most primitive prosauropod, Thecodontosaurus, was also one of the smallest. It was about six to ten feet long. Like all prosauropods and most sauropods, it had a prominent claw on each front foot and a large claw on each back foot.
Family: Plateosauridae: This is the best-known family of prosauropods, with animals found in Europe, China, and North and South America. They were 25 to 30 feet long with narrow, long snouts, long necks, powerful front and back limbs, and heavy bodies.
The second group of sauropodomorphs, the Sauropoda, probably came from an ancestor much like Thecodontosaurus. This probably happened sometime in the Late Triassic, when sauropods first appeared. All sauropods were giants and four-legged plant-eaters.
Like today's elephants, sauropods had little fear of predators because of their size. Being large also helped them reach food, such as leaves in treetops, that was too high for smaller plant-eaters. Sauropods had many features because of their large size. They lost the grasping function of their front feet, and their legs looked like long, straight columns. Their vertebrae (bones in the spine) had deep hollows to lighten the weight of their backbone. Also, to add strength they had more vertebrae where the pelvis and spine joined.
Sauropod skulls were either blunt (flat) or tapering (came to a point) and the nostrils were back from the tip of the snout. Sauropod heads, which were lightly built and fragile, often broke off after death.
Family: Vulcanodontidae: The earliest true sauropod is Vulcanodon from the Early Jurassic of Zimbabwe. The only skeleton is missing the head, neck, and much of the tail. It had a bulky body and its legs were long and straight. The front limbs were almost as long as the back, and each back foot had five toes.
Family: Barapasauridae: The next most primitive sauropod, Barapasaurus, is known from parts of several skeletons from the Early Jurassic of India. If was up to 60 feet long, with a slender body and a long neck, tail, and limbs.
Family: Euhelopodidae: Most of the sauropods known from the Middle and Late Jurassic of China are now placed in a separate family, the Euhelopodidae. Euhelopodids are one of the more primitive sauropod families, but they include such exotic animals as the extremely long-necked Mamenchisaurus and Omeisaurus.
Family: Cetiosauridae: This family is from the Middle Jurassic, perhaps from an ancestor from the Vulcanodontidae family. The cetiosaurids had expanded and spread into Europe, North and South America, Africa, and Australia by the Middle Jurassic.
Cetiosaurid skulls were blunt and box-like, with nostrils at the side of the snout. The neck was short, usually with 12 vertebrae. They ranged from small to large for sauropods; most were 35 to 60 feet long. The best-known genus is Shunosaurus from the Middle Jurassic of China. It had a small, bony club at the end of its tail. Cetiosaurids lasted until the Late Jurassic.
Family: Brachiosauridae: The front limbs of the brachiosaurids were as long or longer than the back limbs. This gave the body a backward slope from the neck to the tail. The number of neck vertebrae in this family increased to 13 or more. The nostrils were farther back from the tip of the blunt snout and above the eyes in the skull of Brachiosaurus. Most brachiosaurids were larger than the cetiosaurids, 80 or more feet long even though they had shorter tails. They were among the heaviest land animals known. To reduce weight, their huge vertebrae were almost completely hollow. Known worldwide, brachiosaurids appear in the fossil record during the Middle Jurassic, were most numerous in the Late Jurassic, and almost vanished by the end of the Early Cretaceous.
Family: Camarasauridae: In this family the skull was boxlike. They still had 12 neck vertebrae and the front limbs were slightly shorter than the back limbs. One of the last known camarasaurids was Opisthocoelicaudia from Mongolia, a heavy-bodied sauropod with a short tail that probably helped support it when it stood on its hind limbs to reach food.
Family: Diplodocidae: This family includes some of the most well known sauropods, including Apatosaurus and Diplodocus. Diplodocid skulls were long and tapered to a spoon-shaped snout and had nostrils on top of the skull. Their small, rod-shaped teeth were in the front of their snout. Diplodocids had long necks, with up to 15 vertebrae. Their backs were short compared to the length of their back limbs, and their tails ended in a whiplash that was probably used as a weapon. The long necks and tails made some diplodocids the longest animals that ever lived.
Family: Titanosauridae: Almost all southern-hemisphere sauropods from the Late Cretaceous, and many earlier ones, were titanosaurids. Their limbs were stocky. The vertebrae from the front and middle of the tail were unique and are the best feature that distinguishes the family. Not one complete or nearly complete titanosaurid skull has been found. One of the most interesting titanosaurids was Saltasaurus, which was squat and covered with armor similar to the ankylosaurs.
Most titanosaurids were about 40 to 50 feet long, but a few became gigantic. The titanosaurids lived mainly in the southern hemisphere during the Cretaceous Period, surviving there as the northern-hemisphere sauropods became extinct.