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  Trilobites are a well known, and easily recognizable, extinct group of highly developed marine arthropods.  They once flourished in the ancient seas from the  
beginning of the Cambrian period, some 570 million years ago, becoming extinct at the end of the Permian period.  Trilobites were the first arthropods to appear, inhabiting the earth during the Paleozoic era.  
Trilobites are extinct arthropods.  Arthropods have segmented body appendages and are covered by a chitinous exoskeleton.  Examples of living arthropods include insects, crabs, centipedes, spiders and lobsters.  The name trilobite comes from the three longitudinal lobes the body is divided into.  They are the raised central lobe or ridge, called the axis, with two lower lobes on either side.  They are also divided into a head called the cephalon, a central thorax of between 2 and 44 segments, and a tail shield called the glabella.  The number of segments is fixed within each species.  The axial region of the head shield, or glabella, has cheeks on either side.  Many trilobites have two well developed compound eyes, one on each cheek.  Other trilobites however, such as agnostids, were totally blind.  

Each segment of the animals body carried a pair of jointed limbs on the underside. These limbs are very rarely preserved in fossils, although x-ray techniques have been used to reveal the legs, antennae, featherlike gills and other appendages of some completely preserved trilobite fossils from the Ordovician Utica Shale of New York state, and the Devonian Hunsrück Slate of Germany.  

How the fossils of trilobites are found has a large part to do with trilobites having an articulate body encased in a hard external shell, called an exoskeleton, which was divided up into segments.  Because of these segments, some species were able to roll up as their means of defense.  In fact, trilobites are sometimes found preserved in an enrolled state.  Most of the post-Cambrian trilobites could roll themselves up into a ball when distressed.  As with different living organisms today, the trilobites shed or molted their exoskeleton periodically as the animal increased in size.  A new and larger outer shell was grown in place of the original.  The disarticulated remains of these molted exoskeletons are often found individually, or sometimes they accumulated in an area, possibly caused by the current actions of the water they inhabited.  Many have facial sutures across the head shield, where it would split so the animal could crawl out of the exoskeleton, once it had outgrown it.  Many trilobite fossils are shed exoskeletons and lack part of the head shield. A specimen containing the entire head shield, and the remainder of the specimen along with it, represents the death of that trilobite. 

Most trilobites were bottom dwellers, walking on the sea floor.  The tracks left behind by trilobites crawling on the sea floor are preserved occasionally as trace fossils.  Some were burrowers living in the soft sediments of the sea floor, while others were free swimming. Rusophycus is a horizontally oriented bilobate, ovoid shaped burrow that has parallel to subparallel scratch marks laterally extending from a central bisecting plane.  An example of one can be seen in the image above.  It is believed to be a fossil impression of where a trilobite, or another arthropod, once rested.  Trilobites appear to have been exclusively marine organisms, since their fossilized remains are always found in rock containing fossils of other salt-water animals, such as brachiopods, crinoids, and coral.  To learn more about these other marine organisms, just click on the appropriate icons below.
There are about 10,000 known species of trilobites ranging in size from 5mm to over 70cm (2 feet).  Though they became extinct over 240 million years ago, their fossil remains have been found on every continent of the world.  The reason for the extinction of the trilobites is not clear, although it would seem to be no coincidence that their numbers began to decrease with the arrival of the first sharks, and other early fishes in the Silurian and Devonian periods.  Perhaps the trilobites were possibly a rich source of food for these new arrivals!  Trilobite fossils are found worldwide, and some of them serve as excellent zone or index fossils, which enables geologists to date the age of the rocks in which they are found.  Some trilobites evolved into elaborate spiny forms, particularly during the Devonian period.  Such specimens are found in the Devonian Hamar Laghdad formation of Alnif in Morocco.  To see images of trilobites of the more commonly recognized forms, or the spiny forms, just click on the appropriate icon below
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