• Enlarged nares
• An elongated neck
• Leaf-shaped teeth

The most primitive known sauropodomorph is Saturnalia of the Late Triassic of Brazil. (Some fragments from the Middle Triassic of Madagascar might be VERY early sauropodomorphs, but might instead be from some non-dinosaurian herbivorous archosaurs).

Basal sauropodomorphs are often called "prosauropods". However, there does not appear to be a true clade "Prosauropoda". Instead, basal sauropodomorphs form a paraphyletic grade leading to the Sauropoda; this grade has at its base small bipedal forms such as Saturnalia, Thecodontosaurus, Efraasia, and Plateosauravus, followed by larger-bodied (3-10 m) long facultative bipeds such as Plateosauridae, Riojasauridae, Massospondylidae, and Melanorosaurus.

Basal sauropodomorphs are characterized by:

• Originally small size (1.5-2 m long), but eventually reaching 10 m or more
• Small skull size
• Elongate necks
• Femur longer than tibia, even in small forms
• Originally obligate bipeds, but as size increases they become facultative bipeds
• Retention of a big thumb claw and grasping hands
• Some prosauropods may have a beak, although this is uncertain
• Elongate leaf-shaped teeth (not squat teeth, as in ornithischians) with no occlusion
• Only two or three sacrals: lower than almost all other dinosaur groups
• Basal sauropodomorphs were the most common herbivorous dinosaurs from the Late Triassic to the Early Jurassic; no basal sauropodomorph survived into the Middle Jurassic, though
• Global distribution

These three selective seem to have led to the evolution of the true Sauropoda. Primitive sauropods are known from the Late Triassic, but sauropods do not become common outside of the southern regions of Pangaea (modern South America, southern Africa, Australia) until the Middle Jurassic.

Sauropods are characterized by:

• Extremely large size: all sauropods were at least elephant-sized as adults, and many much, much larger
• Elongated forelimbs (60% or more hindlimb length)
• Obligate quadrupedality

Antetonitrus of the Late Triassic of Africa is among the best known early sauropod. Unfortunately, its skull and most of its neck are not known, so it isn't certain if the following features are present in it, or in the next more advanced group of sauropods:

• Reduced skull size
• Rounded anterior end of snout
• Nares placed at least as high dorsally as the orbits
• Tooth-to-tooth occlusion for precise bites
• Extra cervical vertebrae
• Four or more sacrals
• Reduced number of phalanges on manus

Shunosaurus of the Middle Jurassic of China is a good example of an early sauropod. The giant size of sauropods would allow them to feed even higher in trees, digest more plants, and serve as defense against ever-larger predators. (Some sauropods developed additional defenses: Shunosaurus, for instance, had a tail club).

Different clades of sauropods had different solutions to tree-feeding. Some added extra neck vertebrae; some increased forelimb length; some reared up on hind legs; and many had combinations of these.

The main sauropods of the Late Jurassic and the Cretaceous belong to the clade Neosauropoda. Neosauropods are characterized by:

• Nares even more dorsally placed
• Teeth concentrated at front of jaws
• Column-like metacarpals

There are two main clades of neosauropods: Diplodocoidea and Macronaria.

Diplodocoids:

• Pencil-shaped teeth only at very end of snout
• Forelimbs much shorter than hindlimbs

• The recently-discovered Rebbachisauridae (Cretaceous of South America and Africa), with a dental battery similar to that in hadrosaurids (but only at the front of the jaws)
• The flageillcaudates (whip-tails), including Dicraeosauridae and Diplodocidae, and characterized by:
  • Nares are placed together above the orbits
  • Long sloping skulls (although no complete dicraeosaurid skull is known)
  • Tails ended in very narrow and long caudals

Dicraeosaurids (Late Jurassic-Early Cretaceous of South America and Africa), with relatively short necks and very tall neural spines (forming fins or sails in Amargasaurus of the Early Cretaceous of Argentina) were relatively small, as were rebbachisaurids. However, the Diplodocidae (best known from the Late Jurassic of western North America), included some gigantic forms:

• Very long necks
• May have reared up on hind legs to feed extremely high in trees
• Known from several different genera, most importantly Diplodocus and Apatosaurus (formerly Brontosaurus)

Macronarians ("big noses") are characterized by:

• Nares as large or larger than orbits
• Stout spoon-shaped teeth (although some titanosaurians have pencil-shaped teeth)
• Elongated metacarpals (almost half the length of the radius)

Camarasaurus of the Late Jurassic of western North America, Europe, and Africa is a typical primitive macronarian, as is Jobaria of the Early Cretaceous of Africa. The more advanced macronarians (the Titanosauriformes), and are characterized by expanded snouts and nares placed on top of their skulls. Titanosauriforms include some of the largest dinosaurs (rivalled only by the largest diplodocids), and are divided into:

• Brachiosauridae, with forelimbs longer than hindlimbs
  • Built uphill to reach high into trees without having to rear up
  • Best known example is Brachiosaurus of the Late Jurassic of western North America and Africa; also, Maryland's State Dinosaur Astrodon is a brachiosaurid
• Titanosauria, with extra wide hips, more flexible dorsal columns, and robust ulnae and radii:
  • Were probably more agile than other sauropods, and may have been better at rearing
  • Most common sauropods in the Cretaceous; were the main plant eaters until the end in most of the world
  • Some forms had pencil-shaped teeth (convergent with Diplodocoidea)
  • Some forms had armored backs
  • The largest known dinosaur, Argentinosaurus is a titanosaur

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Last modified: 9 January 2007