The Dinosaur Era
Dinosaurs lived during the Mesozoic Era (248-65 mya), which is often called the Dinosaur Era. The Mesozoic is divided into three periods:

• Triassic, named after three (trias) kinds of rocks in Germany;
• Jurassic, named after the Jura Mountains in France;
• Cretaceous, named after large deposits of chalk bordering the English Channel.

Although dinosaurs lived during all three periods of the Mesozoic, the species present in each period were quite different. Point out to your students that movies and collections of toy dinosaurs often mix species together that lived millions of years apart. For example, Stegosaurus and Tyrannosaurus rex never met because the former lived during the Jurassic, the latter during the Cretaceous. Similarly, despite movies and cartoons, dinosaurs and human beings missed each other by about 60 million years.

The Triassic Period
The Triassic, which lasted from about 248 mya to 213 mya, was a period of transition. At its beginning all of Earth's major land masses were joined in a supercontinent called Pangaea ("all-Earth"). The Appalachian Mountains existed at the beginning of the Triassic, but instead of the eroded and subdued mountains we know today, the Appalachians were the largest mountains on Earth. They were comparable in height and ruggedness to today's Himalayas. However, by the Triassic, the Appalachians had stopped growing, and were beginning to wear down. By the end of the Triassic, tectonic forces were beginning to tear Pangaea apart into Laurasia and Gondwanaland, the Atlantic Ocean had begun to open, and the stage was set for the formation of the Rockies and Sierra Nevadas in western North America.

Earth's climate was changing from the cold glacial climate that existed at the end of the Permian to the tropical climate of the middle Mesozoic. This change was in part because of the breakup of Pangaea and its mountain structures that had strongly affected Paleozoic weather. As a consequence of the warmer weather, the ice caps melted and sea levels rose again, creating extensive shallow seas across continental lowlands that lasted all through the Mesozoic.

Life at the beginning of the Triassic was recovering from the mass extinction episode--the largest on record--at the end of the Permian. New life forms radiated quickly to fill the many available ecological niches. Dominant land plants included the palm-like cycads and ginkgo trees. By the end of the Triassic, true conifers had appeared. The amphibians that had survived extinction began to lose land dominance to the proliferating reptiles, which included the mammal-like reptiles and the archosaurs. The archosaurs were the ancestor of modern-day turtles, crocodiles, flying reptiles, and the thecodonts--the direct ancestors of the dinosaurs.

The Jurassic Period
In the early part of the Jurassic Period, which lasted from 213 mya to 144 mya, dinosaurs began to grow very large. This was to become the "age of giants," although many small dinosaurs coexisted with their larger cousins.

Make your students aware that becoming larger can be a very big problem in itself. The giant sauropods had to support their great weight, find sufficient food, and be able to move. Some paleontologists suspect that the large sauropods had to wander great distances in search of food, and they are envisioned as having traveled in herds. Evidence for this comes from a quarry in China, called Dashanpu, where 10 skeletons of the sauropod Shunosaurus were discovered with many other dinosaurs that were killed in a catastrophic flood.

The very large sauropods like Brachiosaurus would dwarf an elephant. Brachiosaurus was at least 19 meters (62 feet) long and stood at least 9 meters (29 feet) high. Diplodocus was 27 meters (88 feet) long. The tails of these animals probably acted as a counterbalance to the long necks. The sauropod Mamenchisaurus, for example, had a neck that was 10 meters (32.5 feet) long. You might have the students measure the lengths of these animals in the hallway or on the school grounds. But even these great sizes are relative. Supersaurus, Seismosaurus, and Ultrasaurus would have dwarfed Diplodocus. Supersaurus might have been 40 meters (130 feet) long. In Morocco a set of footprints from an unknown sauropod measured 115 cm, or 46 inches in width--we don't know this sauropod's exact length. You might have the children draw a footprint 46 inches in width on the chalkboard and then hold up a student's shoe for contrast.

During the Jurassic today's continents were joined in two supercontinents, Laurasia in the north and Gondwanaland in the south. A tropical sea called the Tethys separated them. During the Jurassic the Tethys grew in its east-west dimension, opening today's North Atlantic. During the Late Jurassic the western states were covered by the shallow Sundance Sea. This sea was gradually filled in by sediments washed from the growing ancestral Rockies. The Jurassic ended without a mass extinction, but as climates began to change worldwide, the large sauropods began to vanish.

Plants in the Jurassic were conifers and ferns (no flowering plants had evolved yet). The ginkgo family, to which the modern day ginkgo tree belongs, grew in a forest of tree-ferns and ferns (there were no grasses). If there is a ginkgo tree in the area, you might want to get some of its fan-shaped leaves to compare with the leaves of maples, oaks, and other trees.

The Cretaceous Period During the last dinosaur period, the Cretaceous, which ended 65 mya, the world was a very warm place, even warmer than now. Apparently, even the high latitudes, where we would expect to find cold conditions, had relatively moderate temperatures.

Cretaceous plants included the first flowering plants (angiosperms). Relatives of oaks, horsetails, sycamores, and magnolias also grew.

Paleogeographic reconstructions show that during the Cretaceous Period Pangaea, the supercontinent composed of many of today's separate continents, continued to break up. The tropical Tethys Sea was joined from the north by the Mowry Seaway, which effectively cut North America in two from north to south. This breakup and the inundation of the land cut off pathways for dinosaur exchanges. Isolation of many families of dinosaurs might have reduced their chances for survival. If something catastrophic happened in a small area occupied by a group, then the entire group would have been at risk.

An important point to address with students is the relationship between space and organism. How much space is needed in a classroom for each child. How much acreage is needed to feed each child? How much land is devoted to housing each child? Would the dinosaurs have the same space problems of as humans?