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Trilobites Essay

1006 words - 4 pages

Trilobites


Trilobites are extinct sea creatures that were one of the first forms of life on earth.
They ruled the world before the time of the dinosaurs, from the Cambrian Period to the
Permian Period (between 245 and 600 million years ago). Trilobites are the only extinct
form of Arthropods (invertebrate animals having jointed bodies and legs),and were related
to the lobster and crab. Their closest living relative today is the horseshoe crab.The name
Trilobite refers to it's three lobes, or body sections, consisting of a central axial lobe, and
two pleural lobes on the sides. The three main parts of it's body are called the
Cephalon,(head) the Thorax (body), and the Pygidium (tail). There are 8 Main Orders of
trilobites, and over 15,000 different species, many of which have a stratigraphic value,
especially those found within Cambrian and Ordovician rocks. It is the fact that trilobites
are found world wide which makes some of them serve as excellent index fossils, and it is
their short life periods which allows precise dates and correlation’s to be made about the
rocks in which they are found.
Most trilobites were sea floor dwellers, living in the soft sediments, while others
were free swimming. They appear to have been exclusively marine organisms since the
fossilized remains of trilobites are always found in rock containing fossils of other
salt-water animals, e.g. brachiopods, crinoids, and corals. Trilobites ranged in size from
1/4 of an inch,and smaller, to almost 3 feet long. Trilobites were also the first creatures to
develop eyes and sensory organs. Trilobites had two compound eyes. In some trilobites,
the eyes had densely packed lenses and may have served merely as a light sensitive
warning device to detect movement. In other trilobites, the eyes had fewer and more
complex lenses and may have been capable of forming images and perceiving depth.
Trilobites lived in shelf and slope environments around continental margins and in the
shallow continental seas that covered areas of the earth that today are land masses. Most
trilobites were bottom dwellers, although some may have been swimmers or floaters.
Some that possessed exceptionally large eyes and a large field of vision, such as
Carolinites, are thought to have been swimmers that inhabited surface waters. Others,
with reduced eyes or no eyes at all, preferred deeper, darker waters. Many trilobites, such
as Olenellus, burrowed into the sea bottom for protection and to seek food. Trilobites
employed a variety of feeding strategies. Many plowed through mud at the bottom of
oceans and seas, ingesting the sediment to sort out organic matter. Others were
scavengers or predators. Most trilobites could roll themselves up into a defensive position
so that only the exoskeleton was exposed. Growth of trilobites involved a process of
molting caused by the growth of the body within the exoskeleton becoming too large to be
contained. With the hard...

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