hips”) are so named because the bones in the pelvis
are arranged in the same way as those of other reptiles. In
addition, saurischians were characterized by an asymmetrical
hand that had an enlarged thumb, often bearing a big claw.
The vertebrae in the back also had extra joints. Early saurischians
are more common than early ornithischians; in fact, the former
group dominated the dinosaur faunas of the Late Triassic.
About 600 species are known.
Nearly all herbivorous saurischians were sauropodomorphs. The first large dinosaurian herbivores were early sauropodomorphs, the prosauropods (“before lizard feet”). Prosauropods had a well-developed thumb claw, a long neck, and numerous leaf-shaped teeth. Most could walk on either two legs or four. Some forms (Thecodontosaurus, Anchisaurus) were one or two meters long, but several species (Plateosaurus, Riojasaurus) reached three times this size. Prosauropods were common in the Late Triassic and Early Jurassic, but were extinct by the Middle Jurassic.
sauropods ("lizard feet") were the largest known
terrestrial animals, and are among the most recognizable dinosaurs.
These animals were characterized by long tails, small heads, and long
necks (with extra vertebrae as well as very long vertebrae). Walking
on four columnar limbs, they reached 30 meters in length, and weighed
perhaps 50 tons. The earliest sauropods are from the Late Triassic,
but they did not become common until the Jurassic, and lasted until
the end of the Cretaceous.
Theropods ("beast feet") include some of the most famous and popular dinosaurs.
All carnivorous dinosaurs are theropods, and as such they retain the
pointed, serrated teeth of their ancestors. Theropods also evolved
new features, including a joint in the lower jaw to act as a shock
absorber (or perhaps for increased mobility). Some theropods developed
additional openings in the skull, fewer fingers, and feathers. Ranging
in size from a few kilograms to several tons, and from one to 15 meters
in length, theropods lived from the Late (or perhaps Middle) Triassic
through to the end of the Cretaceous. Theropods include the oldest
known dinosaurs (Herrerasaurus and Eoraptor) and
the largest bipeds (Tyrannosaurus, Giganotosaurus,and Carcharodontosaurus).
One very specialized group of theropods—birds—became endothermic
and developed the ability to fly.