What's a Reptile? What's an Amphibian? What's the Difference? All amphibians and reptiles have a backbone. Animals with backbones are called "vertebrates" . Reptiles and amphibians are also cold-blooded (or ectothermic , ec-toe-ther-mic). Cold-blooded animals are really neat because their body temperature depends on the temperature of their environment. If it is cold outside, reptiles and amphibians will also be cold. In fact, amphibians, like wood frogs, can freeze completely solid during the winter. They produce an antifreeze-like substance in their bodies that prevents ice crystals from forming and damaging their cells while frozen. To warm their bodies up, you might find amphibians and reptiles, like turtles, basking in the sun. Many amphibians and reptiles also experience a period of very low movement. In colder areas, amphibians and reptiles enter brumation (broo-may-shun) during the winter. This is a period of low activity over extended periods, but when it's warmer, they may become active for a short period of time. (This is also called overwintering). In warmer areas, this reduction in movement is called aestivation (ess-tuh-VAY-shuhn). While reptiles and amphibians are related, there are some notable differences between them. The meaning of the word amphibian provides a clue. It comes from the Greek word "Amphibios," which means to have "two lives." While there are some exceptions, most amphibians lay their eggs in the water and live in water when they are young (tadpoles, as an example) and then move to land when they are older. For example, many frog species like bullfrogs lay eggs in water; these hatch into tadpoles that use gills to breathe underwater. Tadpoles (also called larvae, lar-vee) then undergo a wonderful transformation called metamorphosis . During metamorphosis, tadpoles will lose their tail and gills, replacing them with legs and lungs, which allow them to move and breathe on land. While most amphibians complete the dual water-land life cycle, some amphibians always stay on land or always stay in the water. For instance, Red-back salamanders lay their eggs on land in moist areas (under rocks or leaf litter) and their young hatch into mini-salamanders (there is no larval stage). In contrast, mudpuppies (also called waterdogs) never lose their gills and never leave the water! There are some other general differences: Amphibians don't have scales or claws, they lay eggs that don't have shells, and amphibians can breathe through their skin (though they also have lungs). Reptiles lay eggs that have shells, though there are some exceptions. For instance, some snakes give birth to live young. Reptiles also have scales and dry skin, and most (but not all) have toes and claws. Many, but certainly not all, reptile species are terrestrial, which means they live on land. Others, such as alligators or sea turtles, spend much, if not most, of their time in bodies of water. A Simplified Timeline of Amphibians and Reptiles Unlike humans, which are a relatively new species on Earth, amphibians and reptiles have lived on Earth for a long time--millions upon millions of years. Amphibians are one of the oldest vertebrates (or animals with backbones) to live on land. The earliest amphibian relatives evolved from fish and first appeared in the Devonian period (419-355 million years ago). This was millions of years before the first dinosaurs. The early amphibians did not look like the amphibians we can find today. Some of the earliest amphibians, like the dawn tadpole (Pholiderpeton or also Eogyrinus), had scaly skin and could grow to about 15 feet! That is really big! Today's amphibians are thought--there is still a lot of debate on this--to have evolved from a group called "Lissamphibians" sometime between the Permian and Triassic periods. Within amphibians, frogs evolved 200 million years ago, followed by salamanders, which evolved 170 million years ago. Reptiles evolved a little bit after amphibians (around 330 million years ago). When it comes to modern reptiles, snakes are the oldest and evolved about 38 million years ago, followed by turtles and lizards, which appeared 25 million years ago. We know this because paleontologists--scientists who study fossils--have discovered fossils that have helped us piece together the history of herpetology, one fossil find at a time. But finding fossils of reptiles and amphibians can be difficult. Animals with hard shells, such as turtles, are more likely to fossilize than soft-bodied animals, such as frogs. So the fossil record for reptiles and amphibians isn't at all complete, and we're learning more every year. With that said, we have a very rough idea of when the first reptiles and amphibians appeared, and you can see the basics on the timeline on page 14. Keep in mind, this timeline may soon be out of date, but that's good news, as it means we're learning more! Excerpted from Reptiles and Amphibians for Kids: An Introduction to Herpetology by Jessica Hua All rights reserved by the original copyright owners. Excerpts are provided for display purposes only and may not be reproduced, reprinted or distributed without the written permission of the publisher.
