Order: Saurischia The name of this order means "lizard-hipped," and refers to dinosaurs with a characteristic, lizard-like pelvic structure. Saurischians were also distinguished by their long necks and asymmetrical fingers.
Suborder: Theropoda These "beast-footed" dinosaurs included some of the most familiar predators roaming the landscapes of the Jurassic and Cretaceous periods. Theropods never went extinct; today they're represented by the vertebrate class "aves"--that is, birds.
Infraorder: Herrerasauria The herrerasaurs comprise only four dinosaurs, the most well-known of which are Staurikosaurus and Herrerasaurus. Among the first dinosaurs, herrerasaurs are characterized by weird anatomical traits, such as only two sacral vertebrae and a more primitive hand structure than later theropods (some paleontologists even dispute whether herrerasaurs were dinosaurs at all!). Herrerasaurs became extinct at the end of the Triassic period (208 million years ago), well before the better-known dinosaurs of the Jurassic and Cretaceous.
Infraorder: Ceratosauria Unlike the case with the more primitive herrerasaurs, it's generally accepted that ceratosaurs were true dinosaurs. These creatures were characterized by their hollow bones, S-shaped necks, and unique jaw structures, and are the first dinosaurs to show any sort of resemblance to birds (which evolved tens of millions of years later). The most well-known ceratosaurs are Ceratosaurus, Dilophososaurus and Coelophysis].
Infraorder: Coelurosauria Technically, what sets coelurosaurians apart from other theropods is that they're more closely related to birds than to their sister infraorder, the carnosauria (described below). One problem with this classification--the membership of which is far from set in stone--is that it includes such a wide array of members, ranging from Velociraptor to Ornithomimus to Tyrannosaurus Rex. Coelurosaurs are distinguished by the structure of their sacrums, tibias and ulnas, among other skeletal features.
Infraorder: Carnosauria You might expect an infraorder named carnosauria to include such terrifying meat-eaters as Tyrannosaurus Rex, but that isn't the case. Besides their carnivorous diet, carnosaurs were distinguished by the comparative lengths of their femurs and tibias, the size of their eye sockets and the shapes of their skulls, among other anatomical features. They also had fairly large front arms, which is why T. Rex didn't make the cut. Examples of carnosaurs include Allosaurus and Spinosaurus.
- Infraorder: Therizinosauria This infraorder was once known as segnosauria, and it's hopped back and forth all over the evolutionary map: the latest trend is to consider therizinosaurs as closely related to birds, hence their inclusion under theropods. These dinosaurs (some of which may have been herbivorous or omnivorous) were characterized by their extremely long claws, backward-facing pubic bones (similar to birds), four-toed feet, and (mostly) large sizes. Not many dinosaurs are included in this infraorder; the most prominent examples are Therizinosaurus and Segnosaurus.
Suborder: Sauropodomorpha The description of this suborder pretty much matches that of the infraorder sauropoda, described below. These four-footed, none-too-bright herbivorous dinosaurs often reached astonishing sizes.
Infraorder: Prosauropoda As you might guess from their name, the prosauropods ("before the sauropods")--small- to medium-sized herbivorous dinosaurs with long necks and small heads--were once thought to be ancestral to big, lumbering sauropods like Brachiosaurus and Apatosaurus. However, paleontologists now believe that these Triassic- and Jurassic-era dinosaurs weren't the direct ancestors of the sauropods, but more like their great, great, etc. uncles. A classic example of a prosauropod is Plateosaurus.
- Infraorder: Sauropoda Sauropods were the true giants of the dinosaur age, including such lumbering beasts as Diplodocus, Argentinosaurus and Apatosaurus. These four-legged, long-necked herbivores were characterized by their erect limbs (similar to those of modern elephants), long necks and tails, and relatively small heads with tiny brains. They were especially numerous toward the end of the Jurassic period, though many species (the titanosaurs) survived up to the K/T Extinction.
Next page: Ornithischian Dinosaurs