The fossil record indicates that there have been five or six large bouts of extinction over the past 500 million years, and most of the organisms going extinct at once were animals. We tend to notice when animals in our vicinity become less numerous, but we are less apt to notice, or perhaps care about, the plants. However, plants are more important from an ecological standpoint because most insect and other animal species depend on plants in one way or another to stay alive. Probably 10% of the world's plant species are now facing extinction, and by the year 2000 and beyond, as many as 25% of the world's plant species can be expected to be threatened with extinction. By the year 2000, up to 700 of the 25,000 native plant species in the United States will become extinct, with at least 3000 of those now classified as endangered.
Questions to consider as you discuss this with your students are: What can be done to slow down the extinction rates of plants? Where are many extinctions occurring? [In the tropics, but examples can be found everywhere.] What will happen to the animals when the plants on which they depend become extinct? What can each of us do to help solve this problem?
What types of plants dominated the landscape in the day of the dinosaur? Illustrate your presentation with drawings of the types of plants present during various geologic periods. Bring in fossil specimens or photographs as available.
During the Silurian period of the Paleozoic era (around 410 million years ago), the low-lying primitive vascular land plants first appeared. Seed ferns evolved during the Devonian period, approximately 390 million years ago. During the Carboniferous period, 286-360 million years ago, the great coal-forming forests dominated the earth. Ferns, horsetails, and club mosses were at their most numerous. Conifers appeared in the fossil record next, some 246-286 million years ago during the Permian period.
At the beginning of the Mezozoic era, during the dawn of the dinosaurs, forests of ferns and gymnosperms covered the landscape, and cycads and gingkos first appeared in the fossil record. When dinosaurs reached immense sizes and numbers in the Jurassic period 150-200 million years ago, cycads and other gymnosperms flourished. From 70 to 150 million years ago, the flowering plants appeared, placental mammals evolved, and dinosaurs went extinct in massive numbers at the close of the Cretaceous period. From then on, the angiosperms and mammals have been the dominant organisms, along with modern insect groups and, most recently, humans.