Narrative

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.