Trilobites were the most successful single order of Arthropods of all time. They inhabited all of the worlds oceans from 550-250 Million Years Ago - some 300 million years of time on this planet (almost twice that of non-avian Dinosaurs) They were tiny-smallish animals; the largest species known being Isotelus rex, at 4 feet in length, and the smallest types being less than 5 mm long (some were 0.5 mm)
There was a staggering array of different types, suited to many different lifestyles. Some were burrowers, others were bottom feeders, some were blind marine cave dwellers, others were comparitely huge shallow water generalists. Some were spiny, and others were streamlined and bullet shaped. Some were bizarre and moved like underwater butterflies, flexing their bodies whilst horizontal (such as the enigmatic and tiny species, Agnostus) The list goes on - it seems Trilobites had filled almost every niche for animals of their size to do so possible. In fact, even with an inevitably incomplete fossil record (as for all prehistoric animals) the Trilobites have a relatively very detailed and continuous fossil record. This means that a single species can be used as a very precise relative dating tool to date the rocks within which it is found (in the same way that if you find a Tyrannosaurus rex in the Cretaceous Period, you must then instantly know that it can only be of an age 68-65.5 Million Years Ago) Little wonder then, that they are very useful indeed for Palaeontologists - not least in part because a staggering 15,000-20,000 species are known to science.
They first appeared in the Early Cambrian Period, though newer finds suggest that they originated in their most primitive forms in the previous Ediacaran Period, (635-542 Million Years Ago) perhaps going back 550-590 Million Years Ago all told. The last Trilobites finally became extinct during the Permo-Triassic Mass Extinction Event, which also killed 98% of all other life on Earth, around 250 Million Years Ago. Trilobites were phenominally successful during virtually the entire span of the Palaeozoic Era - through the Cambrian, Ordovician, Silurian, Devonian, Carboniferous and Permian Periods. At one point, experts in the 50's and 60's assumed that these animals were 'Kings of the Cambrian', being top predators and so on. However, we have more recently discovered a vast number of different animals that would have eaten Trilobites on a daily basis, such as huge Sea Scorpions, Giant Orthocones, and later Sharks and other fish. In truth, Trilobites were low in the food chain, and possibly they were the staple food of many Palaeozoic marine predators. This made little impact on the Trilobites though - they were 'as common and numerous as muck', and they would have vastly outnumbered their predators in any given period. There may well have been hundreds of trillions of them alive across the globe at any one time in their heyday. With their armoured bodies, they were hunted by creatures that could break through the armour. As a last defence, many could roll up their segmented bodies into a spiny ball.
Trilobites can well be described as, 'the Palaeozoic Era's great success story'. Nevertheless, as fish dominated in the Devonian Period, it is known that Trilobites as a whole declined heavily. By the Carboniferous, they were far less common but still numerous enough to be considered 'off the threatened list'. Sadly for them, the Great Permian Dying spelt the end of a 300 million year success story.