Trilobite fossil classification is challenging based on the sheer
numbers of species alone. Saying that trilobites were prolific in their
heyday is rather an understatement. Assuming that there are still many
undiscovered trilobite species lurking below the surface of our planet,
paleobiologists have already organized the class of known trilobites
into nine orders, over 150 families, approximately 5000 genera and more
than 15,000 species! This widely diverse class of arthropods ranged in
size from as small as a millimeter in length to close to 70 centimeters,
but size alone is not worthy of creating these sorts of distinctions.
There are differences in body shape, exoskeleton texture, the sutures
that jointed the exoskeleton plates, and the eyes. The eyes alone have
so many variations that one could make an entire study of just that
aspect of trilobitism!
With so many variations, trilobite fossil classification is truly a complicated affair. In its most simple form, the trilobites are a class of the phylum Arthropoda of the Animal Kingdom. They are on the same classification level as present-day insects, crustaceans, and arachnids.
Separation into particular orders is based, at least in part,
upon the size and shape variations of the three body segments: cephalon,
thorax and pygidium. Below are two examples of body variation.On the left is a redlichiida trilobite and the one one the left is an agnostus trilobite. Easy to see the difference between these two.
But size and shape alone are not the only considerations for trilobite fossil classification. One order, the Lychidia, takes into consideration the quality of the exoskeleton: granulate (granules on the outer exoskeleton) or tuberculate (tubercles on the outer skeleton.) Then there is the pattern the tubercles makes on the body: rows of two, or single, or random…you get the idea, each of these 15,000 species has some unique identifying characteristic. And for that characteristic to be labeled, in a fossil, no less, there would need to be some pretty close scrutiny taking place!
Surprisingly, the greatest variation and largest numbers of
species were present toward the beginning of the geologic time span in
which the trilobites lived. Trilobites came into existence approximately
540 million years ago during the Cambrian Period of the Paleozoic Era.
These geologic time periods are classified in a similar manner to the
classification of trilobites. For an in depth look at this geological
classification system, go to our web pages called The Clock of Eras.
Main trilobite fossil page