Amphibians Essay Research Paper AmphibiansIntroductionAmphibians are unique

Amphibians Essay, Research Paper

AmphibiansIntroductionAmphibians are unique life forms in that there is no single characteristic that describes amphibians, such as hair on mammals and gills on fish. The name, Amphibia means “double life” (Duellman 1). The term Amphibian can be interpreted in two ways, either as an animal spending part of its life in water and then changing to an aquatic adult, or as an animal spending its life in and out of water. Amphibians are surprisingly variable. There are three main types of Amphibians (Cochran 8). They are caecilians, salamanders, and anurans. Caecilians are legless, wormlike amphibians (Vogel 79). They normally live in burrows of tropical regions. All salamanders have scaleless skin, four legs and a tail. Anurans are either frogs or toads. They normally move around in hops. Many amphibians metamorphose, or change, from a gill breathing aquatic larvae to an air breathing terrestrial adults. These amphibians have great abilities. A new born tadpole carries an aquatic life. After it metamorphoses, it becomes terrestrial. It is like being born again. The tadpole learns to travel by means of jumps, as well as to swim with limbs. This variability of amphibians make them one of the most interesting species to study and indeed amphibians have contributed a great deal to science. Studies on amphibian metamorphosis provide scientists with knowledge about the actions of the thyroid and pituitary hormones. Because of the ease of their breeding in laboratories and their relatively simple chromosome complements, important advances in studies of hybridization and speciation have been made. The study of vocals of frogs have provided advances in animal sound communication. The poison from poison-dart frogs shows great medical promise, especially for patients who do not respond well to pain killers made from opium seeds. The poison is far stronger and is two-hundred times more powerful than morphine, in killing pain. Evolution Of AmphibiansAmphibians are the oldest known land vertebrates and accordingly have been ideal subjects to study the process of evolution. The earliest amphibians existed 360 million years ago (Radinsky 87). The early amphibians known as ichthyostegids, had a tail fin and canals on the skull. These two features suggest that they spent most of their time in the water. Between 340 and 250 million years ago, the population of amphibians grew, and several groups emerged. One group of amphibians, known as labyrinthodonts, grew to extraordinary size. They could be up to four feet long. They were short-legged and large headed. Their skulls were deep and massive, and their jaws were lined with small, sharp, conical teeth. Also, there was a second row of teeth on the roof of the mouth. Many labyrinthodonts had a notch on the back of the skull which acted as a large eardrum and it transmitted vibrations to the inner ear. Their bulky skeleton and their short limbs suggest that the majority of the labyrinthodonts were slow, clumsy walkers on land, and they probably spent most of their time in shallow water. The large jaw and sharp teeth suggest that they were predators and they fed on large prey. As some labrythodonts strengthened their limbs as adaptations for life on land, they reduced the size and strength of their limbs. The weaker ones developed long, flexible bodies with weak vertable columns. Scientist interpret this to mean that they had secondarily adapted to full aquatic life. Their heads flatted which suggested that they spent much time lying on the floor of a lake or pond. Some developed long and narrow snouts, which was a good characteristic to have for traveling through water. Another grouping of early amphibians were the lepospondyls. They were mostly small, less than a foot, with long, slender bodies and weak legs. The lepospondyls lacked the notch on the back of the skull. They had small sharp teeth which suggested that they fed on small invertebrates. A later group, known as the nectrideans evolved more long bodies. Their weaker limbs, and less stiffened vertable columns suggested that they had almost become completely aquatic. Like the labrinthodonts, the nectrideans had the notch on the back of there skull which acted as an ear drum. A last group called the diplocaulids, developed large, flattened, triangular skulls, with the dorsal eyes and nostrils that are normally a characteristic of animals that live on the floor of a body of water. The study of these early amphibians provide convincing evidence of evolution. ReproductionMost amphibians go through annual periods of mating (Duellman 19). In anurans, rainfall can cause mating. Amphibians reproduce sexually. Some amphibians lay their eggs and let them develop by themselves. Others, lay eggs and guard them. The mother is usually the one who guards the eggs. A salamander will guard her eggs by wrapping herself around them. Some anurans carry their eggs imbedded in their backs. Eggs and LarvaeAll amphibian eggs are basically the same in that there are layers of semipermeable membranes surrounding the ovum. However, a lot of differences exist in individual eggs, such as size. Eggs laid in water form into large clumps, or they are scattered and deposited at different sites. Normally, the clumps are attached to sticks or vegetation in the water. This serves to maintain the position of the clutch. The terrestrial eggs are sometimes in strands connected by jelly between each of them. Th