Animals of the Past

Trilobites

Timeline: Palaeozoic to Permian

Geographical range, habitat: cosmopolitan; oceans

Body length: 1 mm – 70 cm

Because of their diversity, rapid evolutionary development, and easily fossilised rigid exoskeleton, trilobites are suitable to be considered index fossils of the Cambrian period.
They became extinct at the end of the Permian period.

Species

Because they had an easily fossilised exoskeleton, there are many surviving fossils. Nearly 17,000 trilobite species are known of from the Palaeozoic era..

Trilobites share a common body plan: a head (cephalon), a body (thorax) and a tail (pygidium).

Head (cephalon)

Some species of trilobites had antennas in order to smell and taste, and some species also had eyes.

Fossil

Their remains accumulated
in sedimentary rocks in large quantities, from which we can conclude that many of their species lived in huge numbers in the seas of the Palaeozoic Era.

Extinction

Though the exact cause is unclear, it is likely that they decreased in number as they became the primary food source for early emerging species of fish (for example, sharks).

Compound eyes

In contrast to the eyes of present-day
arthropods, a lens composed of calcite
crystals collected light from the environment.

Thorax

The thorax of trilobites was
composed of segments, the number
of which may have exceeded 100.
The segments allowed these animals, similarly to present-day woodlice,
to curl into a ball to protect
themselves from danger.