WHO WERE TRILOBITES?

The word ‘trilobite’ refers to this side-to-side partition. Trilobites rank among the most important early animals. Geologists know that they were marine animals because of the rocks in which they are found and the other types of fossils associated with them.

For support and protection, the soft parts of the animal were covered by an exoskeleton. Usually only the dorsal part of the exoskeleton, covering the animal’s back, was fossilized. The ventral (underside) part of the animal may have been covered by a soft membrane or other material that could not be fossilized.

The exoskeleton is divided from anterior (front) to posterior (back) into:

• cephalon (head-shield)
• thorax
• pygidium (tail-shield)

It is also divided from side-to-side into a central axis, with two side regions (lobes) called pleurae. The cephalon, thorax and pygidium are all divided into segments. In life, the segments of the cephalon and pygidium were fused, but those of the thorax were not, enabling the animal to roll into a ball to protect its relatively vulnerable ventral side in times of danger.

The central region of the cephalon is termed the glabella. The cheeks on either side of the glabella are usually traversed by a facial suture that separates them into fixed cheeks (adjoining the glabella) and free cheeks on the outer edge of the cephalon. The suture helped the animal to moult during growth. When moulting, the trilobite’s cast-off exoskeleton often fell to pieces, so their fossilised remains are usually found as fragments.

EMERGENCE AND EXTINCTION

• They appeared abruptly in the early part of the Cambrian Period and came to dominate the Cambrian and early Ordovician seas
• A prolonged decline then set in before they finally became extinct at the end of the Permian Period, about 250 million years ago.
• In Britain, trilobites occur in the rocks of Cambrian, in Devonian rocks of south-west England and in Carboniferous rocks in Lancashire. An inconsistent molting style, coupled with inefficient physiology contributed to the demise of their species.

EYES AND OTHER SENSORY ORGANS

1. Most trilobites had eyes, although blind forms are known. The eyes are situated on the inner edge of the free cheek, adjacent to the fixed cheek.
2. Trilobites had compound eyes, consisting of a number of separate lenses. The number of lenses and the complexity of the eye structure varied enormously. Some trilobites had large, convex compound eyes (like a fly’s) with a large number of lenses, giving them a wide field of view forwards, backwards, sideways, upwards and even downwards, depending on the actual curvature of the eye.
3. Other trilobites had much smaller eyes, with fewer lenses, giving them a more restricted view.
4. Many trilobite eyes consisted simply of closely packed prisms of calcite, but in some later forms, for example the Silurian–Devonian genus Phacops, the eyes had more complex lenses. Using these, Phacops may have been able to see an object clearly and even estimate how far away it was.
5. Other sensory organs possessed by trilobites included pits, canals, tubercles and spines on the surface of the exoskeleton.

SOFT PARTS

1. Soft parts, preserved under special circumstances in sedimentary deposits known as Lagerstätten (singular Lagerstätte), are known for a few trilobite species. From these, geologists know that trilobites had a pair of jointed antennae protruding forwards from beneath the cephalon and rows of jointed limbs on each side of the body.
2. There were three pairs of limbs beneath the cephalon and a single pair of limbs beneath each segment of the thorax and pygidium. Each limb had two branches: a lower branch used for walking and an upper branch bearing a large number of fine filaments, which may have been used for respiration.

ENVIRONMENT

1. There is some evidence that particular trilobites lived at certain depths in the sea, so they can be used as indicators of palaeoenvironment. Neseuretus is an example of a genus that is associated with very shallow water deposits, whereas trinucleid trilobites, such as Whittardolithus, are thought to have lived on a deep sea bottom.
2. The great variety of body shapes and sizes indicate that trilobites occupied a variety of ecological niches.
3. Blind trilobites may have burrowed and scavenged in mud on the sea floor, or lived at great depth in the sea where there was no light.
4. Other trilobites are associated with trails on bedding planes (e.g. Cruziana), which show that they moved about on the sea-floor, perhaps scavenging or preying on other animals. These trilobites were part of the benthos (sea floor dwelling lifeforms).
5. Yet more trilobites, such as Cyclopyge, had large eyes that enabled them to see downwards as well as in other directions. This, together with their widespread distribution, has led geologists to think that they swam or floated above the sea floor. Many trilobites were 3–6 cm long, but some such as Paradoxides were giants, up to 60 cm or more, while others like the tiny, blind agnostid trilobites were no more than a few millimetres long.

FUN FACTS

1. In South Wales, around Carmarthen, the tails of some early Ordovician trilobites are so conspicuous that they were referred to locally as 'petrified butterflies'.
2. They were associated with legends concerning the magician Merlin.