The Mesozoic and Cenozoic Eras
Dinosaurs became extinct more than 66 million years ago. Yet their power to fascinate us has not diminished with time. Dinosaurs were the rulers of the Mesozoic land—all were terrestrial. There
were no marine dinosaurs or flying dinosaurs. Creatures such as plesiosaurs and pterosaurs are not technically considered dinosaurs. Although dinosaurs came in all sizes and shapes, they can be categorized
into two major groups based on their hip structure: Ornithischia and Saurischia.
Ornithischia
Two types of ornithischian dinosaurs, ornithopods and pachycephalosaurs, were primarily bipedal, meaning that they walked on their hind limbs. Ornithopods had well-developed forelimbs so that they
could also walk on all four limbs. The ornithopods included the crested, duck-billed hadrosaurids that thrived during the Cretaceous. The crests of these dinosaurs were connected to their sinus passages.
Paleontologists propose that the duck-billed dinosaurs used the crests as resonating chambers to amplify bellowing, or to attract mates.
The pachycephalosaurs had thick, domed heads. Some paleontologists hypothesize that these bipedal dinosaurs engaged in head-butting competitions for mates, like bighorn sheep do today. Other paleontologists
challenge this idea, however. They point out that although the bone was thick over the dome-shaped head, it would not have been able to withstand the full force of a head-butt with a competing male. Fossils
of pachycephalosaur skulls show no evidence of such collisions.
There is a very good fossil record of a third type of ornithischian dinosaur: the ceratopsian, or horned dinosaur. The most well-known ceratopsian is Triceratops, which lived during the Late
Cretaceous. Triceratops is typical of the later ceratopsians in that it had a very large head with an impressive bony frill and huge horns. The large frill likely made Triceratops appear
quite menacing to predators. The frill would have been of little use for protection, however, because it was basically a bony frame with skin stretched over it.
The familiar genus Stegosaurus is typical of the fourth type of ornithischian dinosaur: the stegosaur. This group of dinosaurs was quadrupedal—that is, they walked on all four legs. They had
bony, spike tails that were used for protection and plates along their backs. These plates had intricate chambers through which blood flowed much like water flows through the chambers in the radiator
of a car. Paleontologists theorize that the bony plates helped regulate the body temperature of the giant herbivores. When the stegosaurs grazed in the hot sun, they likely became quite warm. As blood
was pumped into the bony plates on their backs, the blood would cool, then returned to the stegosaurs’ bodies to cool the animals.
The last group of ornithischians was the armored ankylosaurs. These dinosaurs were the tanks of their day. They had bony armor that covered them from head to tail. The end of an ankylosaur’s tail was
a large, club-shaped hunk of bone that was used for protection. Any predator that tried to crack the shell of this tough plant-eater would probably have gotten quite a blow from the powerful tail of the
ankylosaur.
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Saurischia
There were two major groups of saurischian dinosaurs: sauropods and theropods. The sauropods included Apatosaurus, previously called Brontosaurus; Diplodocus; and Brachiosaurus,
which may have weighed as much as 55 metric tons. There is evidence from partial skeletal remains that some sauropods were even larger than Brachiosaurus. The names Amphicoelias, Giganotosaurus,
Supersaurus, Seismosaurus, and Ultrasaurus have been given to these partial remains. According to one estimate, Amphicoelias was up to 60 m long and weighed more than 150 metric tons.
In contrast, adult African elephants weigh 5 to 7.5 metric tons. Fossil trackways provide evidence that sauropods moved in large herds. Their tremendous size and tendency to travel in large herds were
likely good protection from their meat-eating cousins, the theropods.
All theropods were bipedal carnivores. They ranged in size from Compsognathus, which was roughly 3 kg, to the king of the dinosaurs, Tyrannosaurus rex, which weighed in at 3 to 5 metric
tons. Until recently T. rex was thought to be the largest of the theropods. Discoveries in Africa and Argentina, however, suggest that some theropods reached astounding weights of up to 8 tons.
Movies such as Jurassic Park and The Lost World have popularized two fascinating theropods, Velociraptor and Deinonychus. Paleontologists theorize that both were quick and
agile predators. They had stiffened tails that they used to balance themselves as they ran, as well as large heads full of serrated teeth. When they hunted, they pounced on their prey and slashed at them
with the sickle-shaped claws on each hind foot.
Activity
Use the illustrations and information in this article to develop a multi-media presentation about dinosaurs. For example, you could model the dinosaurs in their natural environment using clay and
papier mâché, or a computer graphics program. Then, you could record a description of the characteristics of each dinosaur on a tape recorder, or present the facts in a poster.
