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- Ichthyosaurs
- At various times throughout the Late Paleozoic and into the Mesozoic Eras some reptiles left the land and adapted to full-time life in the sea. Some of the most successful of these species were the Ichthyosaurs. They looked very much like modern dolphins, and like dolphins, they breathed air. Some Ichthyosaurs grew to huge sizes, up to 60 feet long. They had long, thin snouts filled with sharply pointed teeth that would have made a quick meal of fish and smaller ammonites. Most were close to the size of modern dolphins and all of them died out in the great extinction at the end of the Cretaceous. It is interesting to note that, although many mammals have gone back to the sea (dolphins, whales, seals, manatees, etc.) some reptiles, such as marine iguanas, are beginning to adapt to permanent life in the water.
- Ichthyostega
- The first part of this creature's name means fish for a very good reason. This very early amphibian was one of the first creatures to leave the water and live part-time on dry land. Its ancestors were fish, and it would have been born in the water and lived there full-time until its legs developed, just like a modern tadpole. It was most likely omnivorous - eating fish, insects, other amphibians and soft plants. Ichthyostega grew quite large for an early amphibian, up to three feet (1 m), and scientists wonder if, even though it is one of the earliest ever found, there might not have been earlier, smaller amphibians. These early amphibians were called tetrapods and were the link between fish and reptiles. Once a creature laid the first egg with a shell, instead of the jelly-like amphibian eggs, the link to the water had been broken and land animals had been born.
- Iguanodonts
- Iguanodonts were a group of dinosaurs that lived in the late Jurassic through the late Cretaceous. An Iguanodon (the dinosaur from which the family takes its name) was one of the first dinosaurs discovered and named. Members of this family were all herbivores, and had the ability to walk on either two or four legs - its design allowed for a bipedal stance. Adults could be as small as four feet (1.2 m) or as large as almost 40 feet (12 m). They have been found in large numbers on every continent and were most likely herding animals.
- Inostrancevia
- This was a carnivorous Mammal-Like Reptile about the size of a large tiger - almost 10 feet (3 cm) long. Its skull was 18 inches (45 cm) long and filled with the advanced dentary (teeth) of the later Mammal-Like Reptiles. At this stage of the evolution of this family of animals, the limbs were more tucked under the body than they are in modern mammals. This is in contrast to more primitive reptiles that had limbs that projected outward as opposed to the more downward direction of this creature. Inostrancevia lived in the late Permian period of the Paleozoic era about 250 million years ago. It was named after A. Inostrantzev, a famous Russian geologist. Inostrancevia fossils were found in Northern Russia. It is a Therapsid and a member of the Gorgonopidae family.
- Insectivorous
- Animals that eat only insects are called insectivorous.
- Insects
- The first non-plant creatures to live out of the water were insects. About the time the first amphibians crawled out of the seas, insects had a burst of diversification. Dragonflies were among the earliest insects to colonize land, over 400 million years ago. 360 million years ago saw new families such as the grasshoppers, mantids (praying mantis) and cockroaches. It was at this time that complex wings developed in insects. All these bugs made for a good food source for the newly emerging land animals.
- Instinct
- Instinct refers to behavior that is not learned, but is normal behavior born into an animal. For example, fish are born with the ability to swim and do not need to learn this behavior.
- Invertebrate
- Any animal without a backbone, such as a clam or an insect, is an invertebrate.
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