Trilobite
The
class Trilobita (commonly called
trilobites) is an
extinct group of
Arthropods appearing in the
Cambrian era and flourishing throughout the lower
Palaeozoic before slowly declining to extinction. The last of the trilobites disappeared in the
mass extinction at the end of the
Permian. Trilobites are well-known, possibly the second most famous fossil group after the
dinosaurs, although they are by no means as awe-inspiring. Most were small marine animals that filtered mud to obtain food. Their charm is of a different sort.
The bodies of trilobites are divided into three parts (
tagmata): a cephalon (head), composed of the two preoral and first four postoral segments completely fused together; a
thorax composed of freely articulating segments; and a
pygidium (tail) composed of the last few segments fused together with the telson. In the most primitive trilobites the pygidia are still fairly rudimentary. The thorax is fairly flexible—trilobites are often found curled up like modern sow bugs do for protection when disturbed.
Trilobites had a single pair of preoral
antennaee and otherwise undifferentiated biramous limbs. Each leg had 8 segments (?), analogous to those of other arthropods, with no knee joint. The first segment also bore a featherlike epipodite, or
gill branch, which was used for respiration and swimming. The limbs were covered by lateral projections called
pleural lobes, extending outward from a central
axial lobe; it is this tripartite division that gives trilobites their name.
Trilobites were only armored on top, but still had a fairly heavy
exoskeleton. In molting it generally split between the head and thorax, which is why so many trilobite fossils are missing one or the other: many trilobite fossils are actually molts, rather than dead individuals. In most groups there were two facial sutures on the cheeks to make shedding easier. The cheeks usually also supported a pair of crescent-shaped compound eyes, which were surprisingly advanced in some species.
Trilobites had unique eyes, which were made of
calcite (
calcium carbonate, CaCO
3). Pure forms of calcite are transparent, and the trilobites used clear calcite crystals to form the lenses of their eyes. In this, they differ from most other arthropods, which have soft eyes. The trilobite eyes were compound, with each lens being an elongated prism. The number of lenses in such an eye varies, however: some trilobites had only one, and some had thousands of lenses in one eye. In these compound eyes, the lenses are arranged hexagonally (nature's way of close packing). Of course there's never a rule without an exception, and to prove that, there were blind trilobites as well.
An egg hatched to give a tiny larva called a
protaspid, in which only the fused segments of the cephalon are present. Subsequent thoracic segments were added behind the cephalon in successive molts in intermediate stages called
meraspids, until finally the adult number of segments was reached, at which point the animal is called a
holaspid. Trilobite larvae are reasonably well known and provide an important aid in their classification.
Trilobites 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 such as brachiopods, crinoids, and coral, and they are found in a range of environments from extremely shallow water to very deep-water. The tracks left behind by trilobites crawling on the sea floor are preserved occasionally as trace fossils.
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).
Trilobites range in length from one millimetre to one metre (1/25 inch to 39 inches), with an average size of two to seven centimetres (1 to 3 1/2 inches).
The exact 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, the trilobites possibly providing a rich source of food for these new arrivals.
Trilobite fossils are found worldwide, with thousands of known species. Because they evolved rapidly trilobites serve as excellent zone (or
index) fossils, enabling geologists to date the age the rocks in which they are found. They were among the first fossils to attract widespread attention, and new species are being discovered every year. The native American Indians had a name for trilobite which means "little water bug in the rocks" - a name which demonstrates extraordinarily zoological acuity.
A famous locality for trilobite fossils in the United Kingdom is Wren's Nest, Dudley in the West Midlands where
Calymene blumenbachi is found in the
Silurian Wenlock Limestone formation. This trilobite is featured on the town's coat of arms and was named the 'Dudley locust' or 'Dudley bug' by quarrymen who once worked many of the now abandoned
limestone quarries. The other trilobites found here include
Dalmanites,
Trimerus and
Bumastus.
Further reading
\n* Trilobite! - Richard Fortey (ISBN 0002570122)\n* Riccardo Levi-Setti. Trilobites. University of Chicago Press, 1993.
