What are reptiles?
Reptiles form a Class of Vertebrates (other Classes include fish, amphibians, dinosaurs
(including birds) and mammals) with over 6,600 species worldwide. Reptiles can be
distinguished from other animals because of their scaly skin and, except for snakes,
true claws. There are a few other characteristics which all living reptiles share:
They are ectotherms (often referred to as cold-blooded). This means that reptiles do not
hold their body temperature constant like mammals, but their internal temperature depends
upon that of the surrounding environment. The exception is some large sea turtles which
have some ability to regulate their body temperature metabolically. Some dinosaurs may also
have been warm-blooded (endothermic).
Reptiles have complex, shelled eggs similar to a bird's, which are laid on land. In some
species, especially those in cooler climates, the female retains the eggs inside
(ovoviviparity) or has a primitive placenta which feeds the embryos directly (viviparity).
In these cases the young are born directly rather than being laid as eggs.
Unlike bird eggs, the shells of reptile eggs are often soft and leathery. In many
species the eggs are laid in moist soil. They absorb some water and may enlarge in size
after being laid.
Unlike amphibians, all reptiles have internal fertilization of the eggs by the male.
How many kinds of reptiles are there?
There are four major groups or Orders of living reptiles. Two of these are found in Canada,
the turtles (Testudines) and the lizards and snakes (Squamata). The crocodiles (Crocodylia)
which include alligators are confined to tropical and subtropical areas. The final Order of
living reptiles are the Sphenodontida represented by two species of Tuataras (Sphenodon
punctatus and S. guntheri). These are restricted to small islands off the coast
of New Zealand.
There are also several groups of reptiles which are now extinct. The most famous of
these are the dinosaurs. Although extinct their descendents are still with us and are
among the most well known group of animals around -- the birds. Another less well known
group of early reptiles, the Synapsida, gave rise to the mammals -- which includes