What is Class Reptilia ?
Class Reptilia include the vertebrates that have successfully transitioned to a terrestrial lifestyle by evolving protective scales and a "land egg." This group is made up of the familiar turtles, crocodiles and alligators, lizards, and snakes. They adapted to life on dry land by modifying their skin with tough, horny scales to prevent water loss.
Reptiles got around their dependence on water for fertilization of eggs and development of their young by evolving internal fertilization and the land egg. The land egg not only provided a shell to protect the embryo from the elements and predators, but it also provided an internal, self-contained aquatic environment for the embryo. In addition, the land egg had its own nutrient supply in the form of a yolk sac, and it allowed for gas exchange to occur between itself and the environment.
Stronger limbs, not having to rely on water for breathing, and their ability to regulate their temperature by specialized behavior also contributed to the reptile's success on land. In terms of body temperature regulation, reptiles are considered to be cold-blooded, or "ectotherms." Another term for this type of physiology is "poikilotherm." Ectotherms/poikilotherms are able to regulate their body temperature either by basking in the sun to absorb heat from their surroundings, or by hiding in burrows to reduce heat loss.
Today's reptiles are thought to have descended from an early line of ancestral reptiles that appeared about 300 million years ago, when the earth's huge swamps started to dry out. One hundred million years later, these ancestral reptiles gave rise to another wave of evolution that produced the great dinosaurs during the Jurassic Period. This "Age of the Reptiles" came to an apparent abrupt end about 65 million years ago.
The exact cause of the demise of the dinosaur is still unknown, but one popular theory holds that a huge meteorite slammed into the earth, producing "nuclear winter" conditions that blocked out the sun's radiant energy, cooling off the earth. According to the Alvarez Hypothesis, an asteroid estimated to be about 10 kilometers in diameter struck the earth, sending dust and debris up into the atmosphere, where it produced total darkness around the globe for about three months. Three months without radiant heat and light to drive plant photosynthesis would surely have caused the death of many organisms, including the dinosaurs.
There is evidence that suggests such a catastrophic event actually happened. Samples taken from the earth's crust around the world show a significant layer of iridium and platinum that corresponds to the geologic time frame when the dinosaurs died out. The significance of the iridium layer is that iridium occurs in high levels in meteorites and asteroids, but is normally rare on earth.
Another theory holds that the extinction of dinosaurs may have been caused by a global greenhouse effect brought on by increased volcanic activity, which produced excessively high levels of carbon dioxide. High global temperatures are thought to have interfered with the reproduction and the physiology of animals that were not adapted to warmer climates, killing them off.
A more recent theory suggests that space dust from asteroids that broke up during the time of the extinction might have rained down on earth and produced the global cooling that killed off the dinosaurs.