The term trilobite refers to an extinct class of arthropods. Tri-lobe-ite alludes to the three-part, longitudinal division of their exoskeleton. These animals first appeared at the beginning of the Cambrian Period approximately 570 million years ago. They flourished throughout the early Paleozoic Era and then decreased in diversity until their extinction in the late Permian Period about 240 million years ago. These marine invertebrates were primarily bottom dwellers. Fossilized remains of trilobites have been found on every continent.
The trilobite is the earliest animal known to possess vision. The remarkable construction and preservation of trilobite eyes enable us to study the development of a sensory organ which is only rarely preserved in other organisms. At least two types of eyes are distinguishable in the trilobites. The schizochroal or aggregate eye may contain from 2 to more than 750 separated lenses.
The holochroal or compound eye consists of touching hexagonal calcite lenses which may number from 100 to more than 15,000. Two eyes, roughly crescentic in shape, allowed most trilobites continuous 360 degree vision along the ocean floor. Some trilobites such as the Agnostids, apparently did not have eyes.
In addition to the chitinous exoskeleton typical of all arthropods, trilobites typically possessed a mineralized body covering divided into a cephalon (head), segmented thorax (body) and pygidium (tail). The mineralized dorsal exoskeleton and ventral hypostome probably consisted of a primary layer of calcite with additional thin lamellae of chitin and perhaps apatite. This provided protection for the soft parts of the body. Trilobite exoskeletons may be found intact or, more frequently, as disarticulated parts..
Molting & Enrollment
The segmented and "jointed" construction of the mineralized exoskeleton allowed many types of trilobites to enroll (roll up in a ball) when facing danger, providing additional protection for the appendages and soft parts. The presence of a hard exoskeleton made it mandatory for the trilobite to molt, or shed its body covering, in order to grow. This molting usually involved some "disassembly" of the mineralized exoskeleton. Thus one trilobite, over its lifetime, may have left many parts to fossilize, but a "complete" articulated specimen always represents the death of that individual.
Trilobites arose and diversified over a span of more than 300 million years. During that time more than 10,000 species developed. Each species possessed a dorsal exoskeleton with unique morphological characteristics. Trilobites came in many different sizes, from the miniscule adult Shumardia which measured less than 5mm long, to the gigantic Uralichas, more than 700mm in length. Trilobites also came in many different shapes. Some trilobites, like Peronopsis, were so conservatively smooth and streamlined in form that it is difficult to tell head from tail. Others, like Olenoides, developed spiny ornamentation which provided additional protection against predators.
Trilobites have been admired by man and have been considered as objects of value for thousands of years. They have been found in archaeological context both in Europe and North America. La Grotte du Trilobite, a 15,000 year old site in France, produced a drilled trilobite which originated more than 2000 kilometers away. In Dudley, England, the "Dudley locust," an Ordovician trilobite of the genus Calymene, has been collected and sold in shops since the 17th century. Today trilobites are prized by collectors and museums throughout the world as objects of beauty and curiosity. Scientists use trilobites as stratigraphic index fossils which help to determine the age of rock formations. The true value of trilobites, however, lies in the story which the fossils tell about the history of life on this planet.
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