Reptiles and Amphibians Through the Ages

Evolution and Extinction

The earliest amphibians appear in fossils that are over 370 million years old, whereas reptiles date back around 320 million years. Interestingly, over a geologic time scale, reptiles such as crocodilians and turtles have not changed very much in their appearance or habits. These ancient animals wandered the planet with the dinosaurs and have survived them by millions of years.

Many other amphibians and reptiles, however, have undergone dramatic changes over time. The earliest amphibians looked more like fish with legs. From these early semiaquatic creatures evolved a wide variety of life forms that barely resembled their primitive ancestors. Over hundreds of millions of years, many different amphibians evolved and disappeared, leaving the familiar salamanders and frogs of today.

The early amphibians were such a successful group of animals that the basic four-legged body plan continued to prosper and evolve into many different forms. The first radically different animals to arise from this lineage were the reptiles, which quickly spread to become the dominant life form on land. Over tens of millions of years, these animals flourished, taking on all sorts of shapes and modes of life. Included in this group were the huge dinosaurs, flying and swimming reptiles, and present-day snakes and lizards. From that one original amphibian lineage also stemmed modern-day birds and mammals.

This intricate and ancient history of evolution and family relationships among amphibians and reptiles has been painstakingly reconstructed by an untold number of paleontologists working around the world for several hundred years. Paleontologists are like detectives who piece together the evolutionary record using fossils of organisms that existed long ago. They examine closely the shape, size, and arrangement of bones in fossil skeletons, as well as the position and abundance of the bones compared to the age and other features of the surrounding rock. Using these fragmented records, paleontologists are able to theorize about the ancestry and relationships of species and rates of evolutionary change and extinction.

They also have an incredible ability to cope with variable and sometimes harsh environments. They can go prolonged periods without food, they can withstand a wide range of temperatures, and many can live in freshwater and saltwater environments. If a wet area dries up, an alligator or crocodile can walk great distances over land to another pond. In this way, it is believed that crocodilians have hung on, while many other species disappeared.

In some instances in the past, environmental conditions appear to have changed so much that only the most resilient animals persisted. Paleontologists have demonstrated that the global fossil record tells of a great catastrophe around 65 million years ago. Dinosaurs, as well as many invertebrates and other organisms, disappeared at an astonishing rate at this time. What happened? Scientists have proposed many different theories to explain the abrupt disappearances.

Recent explanations suggest that either an asteroid collision with Earth, or global volcanic eruptions, forced clouds of debris and dust into the atmosphere. The subsequent blocking of the sun's warming light resulted in cooling of the global air temperatures by several degrees. Apparently the many species that died off were incapable of withstanding such major changes in their environment. Many of these organisms had become highly specialized to certain modes of life and were not able to adapt to the new conditions.

It is clear when looking at animals of today, or in the fossil records from millions of years ago, that a wide variety of life forms and a vast array of strategies can be successful. Some strategies work well under constant environmental conditions, whereas other strategies work well under variable or changing conditions. Animals that are specialists often prosper when conditions to which they have adapted remain unchanged. They often have evolved features that allow them to take advantage of unusual foods or very specialized habitats.

For example, spring salamanders survive well in the highly specialized environments of hillside springs and creeks along steep slopes, where conditions are dark, wet, and cold and few other vertebrates survive. However, specialists often are threatened when their unique resources are taken away. Spring salamanders generally do not tolerate increases in water temperature, reduction in oxygen, or the introduction of predatory fish. In contrast, generalists such as crocodiles and turtles tend to continue along during times of change, withstanding and persisting.

Within this framework of evolution, adaptation, and environmental change, it is expected that the extinction of certain species would be inevitable over the immense scale of time that life has existed on Earth. Thus, extinction would appear to be a natural process. However, in the past 40 years scientists and nature enthusiasts have witnessed a dramatic decline in many species of amphibians and reptiles.

In order to sort out the normally occurring process of extinction from some modern-day disturbances, we must draw once again on the expertise of paleontologists. By reconstructing the past history of plants and animals through the fossil record, they have determined that the rates of extinction we are experiencing today far exceed those that occurred before humans dominated the Earth. The causes of global declines of amphibians and reptiles are complex, but there is little doubt that much of the trouble is caused by disturbances to the environment brought about by an ever-growing human population.

This background information can be used with the Dinosaur Detectives lesson plan.

Excerpted from Hands-On Herpetology.

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