Three of the
vertebrate classes are fish. The most primitive of these is
Agnatha
. It consists of jawless fish
that do not have scales. These are the lampreys and
hagfish. Fish that have skeletons consisting of hard
rubber-like cartilage rather than bone are
members of the class Chondrichthyes
.
These are the sharks and rays. All of the
bony fish are members of the class Osteichthyes
.
Tuna, bass, salmon, and trout are examples of Osteichthyes.
 |
| Ray (class Chondrichthyes) and
bony fish (class Osteichthyes) |
Animals
in the
class Amphibia
spend part of their lives
under water and part on land. Frogs, toads, and salamanders are amphibians.
Many of these species must keep their skin moist by periodically returning to
wet areas. All of
them must return to water in
order to reproduce because their eggs would dry out
otherwise. They start life with gills, like fish, and later develop lungs to breathe
air.
 |
| Salamander and frog (class
Amphibia) |
The
class Reptilia
includes turtles, snakes,
lizards, alligators, and other large reptiles. All of
them have lungs to breathe on land and skin that does not need to be kept wet.
They produce an amniote
egg which usually has a calcium carbonate rich, leather hard shell that protects the embryo from drying out.
This
is an advantage over fish and amphibians because the amniote egg can be laid on land where it is
usually safer from predators than it would be in lakes,
rivers, and oceans.
 |
|
 |
| Tortoise, snake, and lizard
(class Reptilia) |
| |
|
Amniote egg |
The
class Aves
includes all
the
birds. They also produce amniote eggs but usually give them greater
protection from predators by laying them high off of the ground or in other relatively
inaccessible locations. In the case of both reptiles
and birds, the eggs are fertilized within the reproductive tract of females.
There are other striking similarities between reptiles and birds in their
anatomies and reproductive systems. This is not surprising because
birds are descendents of theropod dinosaurs (two-legged mostly carnivorous
dinosaurs).
 |
Birds (class
Aves) |
Dogs,
cats, bears, humans and most other large animals today
are members of the vertebrate class Mammalia
. All mammals conceive their young within the reproductive
tract of the mother and, after birth, nourish them with milk produced by their
mammary glands
. Mammals are
heterodonts
with strong jaws. That is to say, they have a
variety of specialized teeth (incisors, canines, premolars,
and molars). This allows them to chew their food into small pieces
before swallowing it. Subsequently, they can
eat any size plant or animal.
Many reptiles must swallow their prey whole, which limits them to hunting
smaller game.
 |
| Mammalian heterodontism |
Like
birds, mammals are endothermic
,
or warm blooded. They are able to maintain a
relatively constant body temperature regardless
of external environmental conditions mainly by using internal physiological
mechanisms. In other words, they are
homeothermic,
or stable in core body temperature, as a result of endothermy.
All of the living species of insects, fish, reptiles, and
amphibians are
ectothermic
, or cold blooded. They
keep their body temperature in a
normal range mainly by avoiding exposure to environmental
temperature extremes. For instance, reptiles
usually remain in shaded areas on hot days to prevent fatal overheating.
On cold nights, their lowered body temperature can cause them to become
sluggish and inactive. In contrast, endothermic animals are
able to remain active at night and often in the winter when the air
temperatures are
especially cold. They can
also move about in the heat of very warm days. This ability
most likely provided an advantage for the early small mammals in
surviving alongside dinosaurs and other large reptiles,
which apparently were mostly ectothermic. The downside of endothermy is the need to consume far
more calories relative to body size in order to maintain a constant
core body temperature. Small mammals, such as moles
with their rapid metabolism rates, must eat insects or other high calorie
foods every half hour or so in order to stay alive. By comparison,
cold blooded rattlesnakes usually eat only once every 3-6 weeks and have been known to go without food for as long as
two years.
Aiding in mammal body
temperature control is their insulating hair and sweat glands.
Sweating helps
to dissipate heat by evaporative cooling. Compared to
most other land mammals, humans are relatively hairless, but they have far more
sweat glands. Mammals have four chambered hearts (like
birds), complex nervous systems, and large brains
relative to the size of their bodies. This broad range of
useful features has made mammals highly adaptive and successful. They first appeared
about 200,000,000 years ago, early in the age of dinosaurs, and replaced reptiles as the dominant
class of land animals after 65,000,000 years ago. As
the rapidly changing environment at that time led to the mass extinction of
most large reptiles, it left vast evolutionary possibilities which mammals
took advantage of by rapidly diversifying through adaptive radiation.
Important to
mammalian success is their reproductive system. Their bodies took the amniote egg revolution of reptiles
and birds one step
further. In effect, the uterus
functions as the
protective eggshell. Young mammals spend a long period of their early development
within their mother's uterus. After birth, they are provided with protein and fat
rich milk to eat and are usually
protected until maturity. Pregnancy and milk
production require mothers to significantly increase their
own calorie consumption
in order to provide nutrients for their infants. A nursing human female
normally uses about 30% of her body's energy just to produce milk.
 |
| Mammalian mother and her baby |
Despite their
success, mammals still only make up about .4% of known animal species.
It is humbling to realize that all chordates together are only just over 3.7%
of known animal species. By comparison, well over 1/2 of all animal
species are insects.