The Handy Dinosaur Answer Book. Patricia Barnes-Svarney

Чтение книги онлайн.

      Just like this paddle tail newt, ancient amphibians survived on both land and sea; they were the first animals to survive for extended periods outside the water (iStock).

      Even after all these changes, amphibians still were tied to ponds, lakes, or the edges of the oceans, especially since the eggs still had to be laid and hatched in water. Evolution did not change the amphibians too much—modern amphibians are still tied to water.

      When did reptiles first evolve from amphibians?

      It is thought that during the Carboniferous period, a group of amphibians gave rise to the reptiles. The first reptiles were small, lizard-sized animals, but they had many differences from their amphibian ancestors, including waterproof skin and thick-shelled eggs. This made it unnecessary for the reptiles to stay near water, to keep moist, or to lay their eggs in water. In fact, the evolution of the reptiles, geologically speaking, occurred very rapidly. Within about 40 million years, the reptiles had produced thousands of different species.

      What was one of the most important changes that enabled reptiles to become true land-dwelling animals?

      The development of the amniote egg freed the reptiles completely from life in the water by allowing them to fully reproduce on land. Unlike the young of amphibians, who had to go through a larval stage in the water before metamorphosing into an adult, the amniote egg acted as a sort of “private pond” for the young reptiles.

      The egg itself had a hard shell, which contained numerous small pores. These pores allowed air to enter, but the shell prevented the inside from drying up as long as the surroundings were humid. The eggs were fertilized inside the mother’s body before being laid. There were three very thin bags inside the shell itself, each of which had a specific function. The first bag held the developing young and a liquid (which took the place of the pond or stream); this area was called the amnion, from which the egg gets its name. The next bag contained the yolk, the source of food for the developing embryo. The third bag was in contact with the air diffusing in through the shell. Thus, the young reptiles had food, air, protection from predators, and an aquatic environment in which to grow. The young would eventually hatch into a miniature version of its parents and was able to fend for itself. Because of the egg, the reptiles no longer had to have a source of water to reproduce and could spread out, populating the land as well as hunting for prey well away from water.

      What were some of the earliest known reptiles?

      Two of the earliest known reptiles, the Hylonomus and Paleothyris, both descended from amphibians during the Middle Carboniferous period of the Paleozoic era. The best evidence of the change from amphibian to reptile was the early reptiles’ high skulls—evidence of additional jaw muscles—and thicker egg shells. The Hylonomus still claims the prize (so far) as the oldest-known reptile and lived about 315 million years ago. The Paleothyris evolved about 300 million years ago. The fossils of both these reptiles were found near Nova Scotia, Canada, in ancient tree stumps. Apparently, the animals fell into tree stumps in pursuit of insects or worms. There they were trapped and eventually died.

       What amphibians are living today?

      Names of modern amphibians are familiar to us: frogs, toads, salamanders, newts. They represent the descendants of groups that did not become extinct at the end of the Mesozoic era (when dinosaurs died out). Of the modern amphibians, the newts and salamanders are probably the most similar to the early amphibians, although the modern-day versions are much smaller.

      The vertebrate class Amphibia today includes about 3,500 species in three orders: frogs and toads (order Anura), salamanders and newts (order Caudata), and caecilians (order Gymnophiona). There is, however, a much larger number of extinct species of amphibians: this ancient group of animals were the first vertebrates to begin exploiting terrestrial environments, where they became prey for other species.

      Why did the reptiles dominate during the Mesozoic but not the amphibians?

      Besides the ability to not depend on water as much as amphibians, there are probably two main reasons why reptiles became dominant in the Mesozoic. First, reptiles developed adaptations in their skeletal structure, allowing them to move much quicker than amphibians. Second, during the Permian period the climate became hotter and drier, and many water sources disappeared. The reptiles’ new adaptations—from the development of scales to retain water to eggs that could survive without staying in water—allowed them to thrive where amphibians could not.

      Did some reptiles return to the oceans?

      Yes, as the reptiles spread out over the land, some of them returned to the water. Over a period of time, they evolved and adapted to the water again. Their legs gradually evolved back into fins and flippers; eyes adapted to seeing underwater; and bodies became streamlined for better speed in the water. In addition, they could no longer lay their eggs on land. Thus, they evolved a way of producing living young within their bodies, a process called СКАЧАТЬ