Trilobites formed the class Trilobita and are well known marine and extinct arthropods whose fossils have been recovered from different places. The first fossil record suggests that they were present in the Early Cambrian Period but studies indicate that they flourished well in the lower Paleozoic era and become extinct with the beginning of the Devonian Period. They underwent mass extinction at the end of Permian about 250 million years ago. They were the most successful species that were present on the earth for about 270 million years. Their fossil record suggests that they were already dispersed in different locations. As their exoskeleton was perfect to undergo fossilization about 17,000 species have been found out which belong to the Paleozoic time. These fossil records have contributed a lot towards biostratigraphy, paleontology, evolutionary biology and plate tectonics. They are often placed under the subphylum Schizoramia within the superclass Arachnomorpha although different taxonomists place them differently.
Trilobites have switched over to a wide variety of habitats like some moved over the sea-bed as predators, scavengers or filter feeders and some swam, feeding on plankton. Some have even developed a symbiotic relationship with a sulphur eating bacteria. Their role in parasitism is still debatable. They show their characters resembling with Crustacea, Chelicerata and Mandibulata. Only their exoskeleton has been found in the fossilized form and only traces of their soft parts have been found out. They may vary in size from 1 mm to 72 cm but the typical size may reach 3-10 cm. the largest fossil of trilobite was found in 1998 by the Canadian scientists in the Ordovician rocks on the shores of Hudson Bay belonged to the genus Isotelus rex. The exoskeleton is made up of calcite and calcium phosphate entangled in the protein matrix of chitin that covers the upper surface and curled on the lower side in the form of fringe that forms a structure known as doublure. The body is divided into three segments namely head, thorax and pygidium. The thorax is composed of a number of segments ranging from 2-61 but in general they may be 2-16. The segments lie in between head and the pygidium. Each segment is made up of a central axial ring and outer pleura that protects the limbs and the gills. The pleurae may be sometimes prolonged into spines. Apodemes are bulbous projections present on the ventral surface to which the leg muscles are attached.
The pygidium is also formed of a number of segments and the telson is fused together. The segments resemble the thoracic segments but are not articulated together. Depending upon the position of pygidium trilobites may be classified as micropygous meaning pygidium smaller than head, isopygous where is pygidium similar in size to that of head and macropygous where pygidium is larger in size as compared to that of head. The exoskeleton consists of a large number of scale-like structures collectively known as prosopon. It includes finer scale features like ribbing, domes, pustules, pitting, ridging and perforations. Its exact role is not known but it is believed that it is involved in structural strengthening, sensory pits or hairs, preventing predator attacks and maintaining aeration while enrolled. Some have developed horns over their heads that resemble like that of the modern beetles. The soft pars of the body have been recovered from only 21 species. They have a single pair of pre-oral antennae and undifferentiated biramous appendages. Each exopodite has 6-7 segments. Exopodites also bear coxa which also bears gill branch used for swimming or respiration. The mouth is situated at the edge of hypostome and lacks teeth. The mouth is coupled with the glabella which is an adaptation for carnivorous feeding habit. The mouth is attached to the stomach through the oesophagous. The intestine in turn is finally attached to the pygidium. The presence of heart, liver and other internal organs is simply predicted but no direct evidence is available in this regard.
Many trilobites had complex eyes although those dwelling at the bottom of the sea lacked them. The eyes were typically compound with lenses in the form of elongated prism. The number of lenses may vary from one to thousands in a single eye. The lenses are arranged in a hexagonal fashion. The lenses are made up of calcite. The prosopon are of several types which help on the collection of chemicals as well as vibrations. They undergo moulting and the successive growth stages are known as instars. During moulting each segment grows in size and new segments get added up in the sub-terminal growth zone.