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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.