Arthropods include millions of species that are alive today including insects,arahcnids or spiders and scorpions, millipedes or diplopodams, centipedes, orchilopdans, and crabs, shrimps and other crustraceans, trilobites.The uniting features of the artropods is their hard outer casing and usuallynumerous jointed limbs.
The name means “joint-foot”almost all rilobites had a similar body – a cephaon (head end), thorax (middle section)and pygidium (tail).However, there were exceptions (like Naraoia).
The name Trilobites does not come from this three section head to tail structure.It is derived from the three lobes seen from side to side, being the left, centraland right lobes, which were formed by two furrows or division running from head to tail.
Trilobites appeared in Cambrian times as some of the earliest of the complex, hard casedcreatures. They were adapted to a wide range of habitats, from shallow reefs to deepocean floors.
More than 5,000 genera and 15,000 species have been identified, with more discoveredevery year. Their abundance and distinctive features, which changed through time,make them useful indicator fossils for dating rock layers.