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.
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| 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 since their eggs would dry out otherwise. They start life with gills, like fish, and later develop lungs to breathe air.
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| 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 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.
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| Tortoise, snake, and lizard (class Reptilia) |
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.
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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. Many reptiles must swallow their prey whole, which limits them to hunting smaller animals.
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| 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 other classes of living animals are ectothermic, or cold blooded. They keep their body temperature in a normal range mainly by avoiding exposure to environmental temperature extremes. On hot days, reptiles usually remain in shaded areas 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. 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 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. The latter function 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. This broad range of useful features has made mammals highly adaptive and successful. They first appeared about 200,000,000 years ago in the age of dinosaurs and replaced reptiles as the dominant class of land animals after 65,000,000 years ago.
Important to mammalian success is their reproductive system. Their bodies took the amniote egg revolution of reptiles 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 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.
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| 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.