Dinosaurs, crocodiles, flying pterosaurs, and thecodonts dominated the Mesozoic era, which began 248 million years ago and ended 65 million years ago. For more than 160 million years, during the Triassic, Jurassic, and Cretaceous periods, these awesome creatures ruled the land, sea, and air. Most disappeared at the end of the Cretaceous in one of the best-known mass extinctions of the prehistoric world.

Dinosaurs: Divided at the Hip
Dinosaurs are broadly classified into two orders (groups) based on their pelvic structure. They either had hips resembling modern-day birds (Ornithischia) or lizards (Saurischia). Ornithischians were vegetarians; Saurischians could be either vegetarians or carnivores. Both groups had members that walked on two legs (bipeds) or four legs (quadrupeds).

Ornithischians
These dinosaurs can be divided into two groups--the bipedal ornithopods ("bird feet") and the quadrupedal "armored" dinosaurs. They had leaf-shaped teeth well suited to eating foliage.

Ornithopods were bipeds and a real dinosaur success story: common and varied. The best known of the ornithopods were the hadrosaurs ("duckbills"). Duckbill skulls varied from the primitive flat-topped hadrosaurs (hadrosaurine duckbills) to the more sophisticated crested hadrosaurs (lambeosaurine duckbills), which are distinguished by tubes and crests of various shapes and sizes on the tops of their skulls. There has been much speculation amongst paleontologists about the function of the crests. All hadrosaurs had longer hind limbs than forelimbs--the forelimbs ranged from half to three-quarters the length of the hind limbs. Hadrosaurids are now considered part-time bipeds, using a quadrupedal (four-footed) stance during slow locomotion or when standing still, and a bipedal stance for fast locomotion. Other families in the Ornithopoda suborder include the pachycephalosauridae ("boneheads"), and hypsilophodontidae (herd-living, gazelle-like bipeds that lived everywhere except Asia). While the ornithopods could use speed to escape predators, the quadrupedal ornithischians took another approach. They were armored.

Ankylosaurs and stegosaurs were quadrupeds with small heads, massive bodies, short and massive forelegs, and stout hind limbs. Stegosaurs had heavy tails armed with pairs of spikes and two rows of vertical, bony plates that extended down either side of their backbone. Some paleontologists believe the plates to be armor, while others believe the plates are poorly positioned and couldn't have provided much protection. The plates do exhibit a high degree of vascularization, which would allow them to serve as solar panels and/or heat radiators. The backs of ankylosaurs were heavily armored with thick plates and spikes. Their heads were protected by plates and the tips of their tails were armed with a large ball of fused bone that could be swung like a club.

The ceratopsians, or horned dinosaurs, were also armored dinosaurs. These, too, were quadrupeds and had large, beaked skulls. They were often horned (three-horned Triceratops, five-horned Pentaceratops, single-horned Centrosaurus). The head was backed by a frill that looked like--and may have acted as--a shield. Other theories for the frills exist: They could have served as temperature regulators (the frills were highly vascularized, containing numerous canals and blood vessels) or even sex or species identifiers.

Saurischians
Saurischians are the more primitive of the two dinosaur orders and are comprised of two suborders - the theropods and the sauropods. Theropods include all the meat-eating dinosaurs. These carnivores walked upright on two legs and had teeth well adapted to their eating habits: large, curved, compressed, and serrated --like steak knives! Their front legs were small, clawed, armlike, and did not support locomotion. That was left to the dinosaur's powerful hind legs; their tracks show narrow three-digited feet - just the thing for a fleet-footed predator. (Many actually had five toes, but only three showed in their tracks.)

Less swift, but impressive by way of size and bulk were the sauropods--gigantic herbivores, and the largest land animals that ever lived. They weighed as much as 50 metric tons and some attained body lengths of more than 40 m. These creatures walked on all fours (quadrupeds) and had a small head atop an extra-long neck; an enormous body (to accommodate an enormous gut); thick, pillar-like legs to support their great weight; and a long, thick tail that tapered to a whiplash. There is evidence that sauropods traveled in herds and were egg-layers. Two other interesting features of these dinosaurs were great cavities in the vertebrae (which helped lighten the load of the animal while at the same time retaining the skeleton's structural strength), and a backbone that was fused to a massive hip girdle by four and sometimes five sacral vertebrae. This helped support the weight of the heavy body and tail.

Suggested Activities
As discussed above, scientists classify dinosaurs into two major groups: those with reptilelike hips and those with birdlike hips. Ask students to classify the following as reptile-hipped or bird-hipped: Apatosaurus, Hadrosaurus, Tyrannosaurus rex, and Triceratops. (All of the meat-eaters and the sauropods (long necks) were reptile-hipped. The rest of the plant eaters were bird-hipped.) Set dinosaur models on tables and let students use them to draw a reptile-hipped and a bird-hipped dinosaur.

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