The arthropods are one of the more advanced groups of invertebrates, and
they are known from the Cambrian to the Recent (Pl. 34). Modern
representatives of this group include the crabs, shrimp, crayfish,
insects, and spiders. Arthropods vary greatly in size and shape and are
among the most abundant of all animals. They have become successfully
adapted to a wide variety of environments and live on land, in water,
and in the air. The typical arthropod has a segmented body which is
usually covered by a chitinous exoskeleton which, in some forms,
contains additions of calcium carbonate. They are highly specialized and
well-developed animals in which locomotion is by means of paired jointed
appendages.
Although the arthropods are of great importance in nature today, only a
few groups are of importance to the paleontologist. Only two of these,
the trilobites and the ostracodes, are discussed herein.