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 exoskeleton being shed periodically and a larger new one being
developed. After each molting the trilobite was only covered by a flexible, chitinous shell
which gradually mineralized and hardened.
It has been recognized that some Trilobites evolved elaborate spiny forms during
the Devonian era. It was thought at one time that this spinuosity was a development for
planktonic life, in an aid to prevent the animal from sinking deeper in the water column but
most trilobites were too large for these spines to function in that way, so it is more likely
that they were used for protection. The majority of trilobites had a smoother exterior early
in the Cambrian System, but during the Silurian/Devonian several species developed spines
and bumps. In the image to the left is an example of spine development, here the elongated
pleural spines of Kettneraspis willanisi can be seen clearly (It is 3/4" in length). Another
function of the spike development could have been to spread their weight, preventing
them sinking into the sediment of the sea floor. In trilobites, especially from the Cambrian,
long genal spines were developed. These supported the weight of the whole exoskeleton,
the thorax and pygidium being carried higher than the rest of the body prevented them
touching the sea floor and kept total bodily contact to a minimum. This higher carriage of
the thorax and the pygidium is found primarily in the primitive trilobites only.
Trilobites? jointed legs identify them as members of the phylum: Arthropoda, one
of the most successful of all animal groups. They were also among the first animals to
possess the segmented outer skeletons. The combination of these physical features gave
trilobites distinct advantages in mobility, feeding, and self-protection. For any species of
trilobite its survival depended on obtaining a sufficient amount of food, on protecting itself
from predators and other dangers in its environment, and successively completing molting
cycles and reaching maturity to enable it to produce young.
In most instances, both sexes of trilobites occur separately. Males commonly pass
sperm to females in sealed packets called spermatophores. The males lay these packets on
the ground, and the females later pick them up, or the male deposits them into the female?s
genital opening. The fertilized eggs hatch after days, weeks, months, and even years. The
larvae have little or no physical resemblance to adults. Larvae may also inhabit different
environments and eat different foods than their parents. The life spans of trilobites range
from a few weeks to several decades. The trilobite population sharply declined ca. 510 Ma
and finally became extinct ca. 250 Ma. The reason for their extinction is not clear,
although it would seem that their decline may be related to the first arrival of sharks and
other primitive fish in the Silurian and Devonian periods which may have found them a
good source of food. The Trilobites fought back for a time, growing spines during the
Devonian period but these were no match for the larger predators.
The fossilized remains of trilobites are useful because they help scientists develop
relative time scales for the ancient marine environment. Because trilobites evolved quickly
and were widely distributed, comparing the trilobite fossils found in rock layers in different
regions of the earth can indicate which rock layer is older than the other. Trilobite fossils
are particularly helpful in developing time scales for the early Paleozoic era. Trilobites
were highly successful bottom dwelling sea organisms with over 10,000 species. They
thrived for 300 million years as the most abundant and successful ocean animal through
the Cambrian period until they bit the cosmic dust in the Permian era.

Trilobites 7.1 of 10 on the basis of 1028 Review.