Trilobites (Class Trilobita) are an extinct class of Arthropods. These animals had two longitudinal groves down their backs that divide their body into three divisions. They are among the earliest known fossil animals being found in the Lower Cambrian and were almost extinct by the Permian.

Although most trilobites were less than three inches in length, fossil species up to almost thirty inches are known.

Trilobites were marine animals and had a multiple segmented body. Some species could roll up into a ball. Trilobites distant relatives of the modern Class Crustacea.

Following is a summary of the various time zones with the earliest zones listed first. This table is based upon a time chart published by Encyclopaedia Britannica with modifications taken from The World of Dinosaurs by Banner Press and other sources.

The Pre-Cambrian era had very few fossils.

Paleozoic Era
  Cambrian      Trilobites           500 million years ago
  Ordovician    First Fish           400 million years ago
  Silurian      Plants and Animals   350 million years ago
  Devonian      First Amphibians     320 million years ago
  Carboniferous First Reptiles       285 million years ago
  Permian       Advanced Reptiles    235 million years ago

Messianic Era
  Triassic      First Dinosaurs      190 to 225 million years ago
  Jurassic      Advanced Dinosaurs
                and First Birds      136 to 190 million years ago
  Cretaceous    Dinosaur Extinction  64 to 136 million years ago

Cenozoic Era
  Tertiary     First Mammals and
               Flowering Plants       64 million years ago

Jointed Legged Animals (Phylum Arthropoda) make up the largest phylum. There are probably more than one million different species of arthropods known to science. It is also the most successful animal phylum in terms of the total number of living organisms.

Butterflies, beetles, grasshoppers, various insects, spiders, and crabs are well-known arthropods.

The phylum is usually broken into the following five main classes:

   Arachnida:      Spiders and Scorpions
   Crustacea:      Crabs and Crayfish
   Chilopoda:      Centipedes
   Diplopoda:      Millipedes
   Insecta:        Insects

There are several other "rare" classes in the arthropods that should be mentioned. A more formal list is as follows:

   Sub Phylum Chelicerata

     C. Arachnida:      Spiders and scorpions
     C. Pycnogonida:    Sea spiders (500 species)
     C. Merostomata:    Mostly fossil species

   Sub Phylum Mandibulata

     C. Crustacea:      Crabs and crayfish
      
   Myriapod Group

     C. Chilopoda:      Centipedes
     C. Diplopoda:      Millipedes
     C. Pauropoda:      Tiny millipede-like
     C. Symphyla:       Garden centipedes

   Insect Group

     C. Insecta:        Insects

The above list does not include some extinct classes of Arthropods such as the Trilobites.

Animal Kingdom contains numerous organisms that feed on other animals or plants. Included in the animal kingdom are the lower marine invertebrates such as sponges and corals, the jointed legged animals such as insects and spiders, and the backboned animals such as fish, amphibians, reptiles, birds, and mammals.