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Chapter 22 Answers History of the Vertebrates Visual Understanding Figure 22.17 What advantages do reptiles have over amphibians by covering the reptile egg with a leathery outer shell? Think about frogs’ eggs. They are jelly-like masses, looking like clusters of tiny grapes, attached to a twig or a bit of grass in the water. These eggs cannot be hidden since they must have free exchange of gases with the water around them. Birds, fish, other amphibians, reptiles and mammals all eat the amphibian eggs which are laid by the female and then abandoned. If the pond floods, this can cause the eggs to break loose and be buried or be damaged by hitting against rocks. If the pond dries up, the eggs are exposed to the air and quickly dry out. Once the tadpoles hatch out of the eggs they can swim from predators, but they are limited to the water where they are. If the pond dries up during the next several weeks as the tadpoles mature, they will die. Now consider the reptile egg. The leathery shell has pores in it so it still allows for gas exchange with the environment, but makes it much less likely that the egg will dehydrate before hatching. Because they do not have to be in the water, the eggs can be hidden under leaves or in a shallow hole. This means that fewer eggs need to be laid, since there is less likelihood that they will be found and eaten. There is no longer a worry about the pond flooding or the pond drying up, since the eggs and the newly hatched reptiles do not need to be in water. The babies are also able to disperse and hide, making it less likely that they will be eaten. Overall, most reptiles lay fewer eggs than most amphibians because the individual eggs are more likely to hatch and grow into adults. It is more energetically expensive for a female to lay a reptile egg than to lay an amphibian egg, but she needs to lay fewer eggs to have successful offspring. Figure 22.23 How can the teeth of these skulls tell you about the animal’s diet? There are four basic groups of teeth, incisors, canines, premolars and molars. Not all animals have all four groups, and they can be dramatically modified for different purposes. Incisors are like wide, flat knives, like a butcher knife, used for cutting things neatly. Canines Deer and horses and cows have incisors located at the front of their mouths for nipping off leaves and grasses. Beaver have huge incisors, and squirrels have smaller ones, that are almost like chisels, and are used for gnawing at hard things like wood and nuts. In carnivores they are present but may be very small. Canine teeth are more pointed and sharp, for stabbing and ripping at meat or meatlike fruits and vegetables (e.g. squashes, fleshy fruits like mangoes). Carnivores such as dogs and cats have well-developed canines for stabbing and holding prey. Herbivores like deer and beaver have no canines at all. Omnivores usually have them but they are smaller than the large canines in carnivores. Premolars and molars are usually flatter on top, and used for grinding and mashing things. In carnivores they may have pointed tops to help tear through tough meats. In herbivores these teeth are usually large and flattened on top, used for grinding vegetation, including stems and seeds. By looking at the different kinds of teeth in a skull, and their size, a lot can be learned about the animal’s food and habits. Challenge Questions Overview of Vertebrate Evolution Your friend Ahmed tells you that his father can’t understand why, since there have been five mass extinctions already on our planet, people get so upset about the possibility of a few species dying out because of global warming or rainforest destruction or acid rain. What do you tell Ahmed? The previous waves of extinctions were caused by natural events such as volcanoes and asteroids. In some cases nearly all the different kinds of organisms alive were extinguished. Once such an event began, there was no way to predict which species might survive, but the larger species were often among the ones that died out. As humans cause changes in the climate, and chemical contamination of the air, soil, and water, some species are dying off. They are the first indication that there is a problem. Can we predict which other species will die or survive? Can we be sure that we will survive as other species become extinct? The Parade of Vertebrates Use the changes in the heart and circulatory system to trace the evolutionary ascent of the birds and mammals. Fish have a simple circulatory system with a heart pumping blood to the gills to get oxygen. The blood travels from the gills to the rest of the body, then back to the heart. It is called a singleloop system. Amphibians have a two-chambered heart but the two sides are not completely separated. Blood is pumped from the heart to the lungs to obtain oxygen, and then returns to the heart for a second push to increase its flow to the body. The incomplete separation of the two sides means that the blood rich with oxygen mingles with the oxygen-poor blood on the other side, so the system is not as efficient as it could be. Reptiles still do not have complete separation of the two sides of the heart, though it is improved. Birds and mammals have complete separation of the two sides of the heart so that the oxygenated blood that comes from the lungs does not mix with the blood coming from the body. There are four full chambers; two on the right side for the blood coming from the body and bound for the lungs, two on the left side for the blood coming back from the lungs rich with oxygen and bound for the body. This complete separation of the blood makes circulation much more efficient, allowing for the maximum amount of oxygen to be delivered to the cells throughout the body as rapidly as possible.