What Is a Trilobite? An Animal That Ran for 270 Million Years

The name refers to a division running lengthways rather than across, so the body is split into a raised central axis and two flatter side regions, which is visible in nearly every specimen and is the feature the group is named for. Crossways, the animal divides into a head shield, a segmented thorax and a tail piece, and the number and arrangement of thoracic segments is one of the main characteristics used to classify them. They are arthropods, related to crustaceans, insects and arachnids, with a jointed external skeleton that they moulted periodically as they grew, which matters enormously for the fossil record because a single animal leaves many shed exoskeletons over its life, so most trilobite fossils are moults rather than dead animals. The exoskeleton was reinforced with calcite, which is why it preserves so readily, and this is a large part of why trilobites are abundant while soft-bodied contemporaries are known only from a handful of exceptional sites.

Trilobite eyes are among the most remarkable structures in the fossil record and are unique in an important respect:

• •The lenses were made of calcite, a mineral, rather than of protein, so the optical elements themselves are preserved
• •Calcite is birefringent, splitting light into two rays, except along one specific crystal axis, and trilobite lenses are oriented precisely along that axis so the problem does not arise
• •Holochroal eyes have many small lenses packed together under a single covering, resembling the compound eyes of modern insects
• •Schizochroal eyes, found in one major group, have fewer and larger lenses each with its own covering and an internal doublet structure that corrects for spherical aberration, a design independently described by seventeenth-century optical theorists
• •Some species living in deep or muddy water lost their eyes entirely, and blind forms evolved repeatedly
• •Eye position and size support inferences about lifestyle, with upward-facing eyes suggesting burrowing and stalked eyes suggesting life on soft sediment

Trilobites are classic index fossils, meaning species that were widespread geographically, abundant, easily recognised and short-lived in geological terms, which is exactly the combination needed to date rock. A layer containing a particular trilobite species can be correlated with layers containing the same species on another continent, and the subdivision of the Cambrian and Ordovician periods rests substantially on trilobite zones established before radiometric dating existed. Their distribution also contributed to the acceptance of continental drift, since distinct trilobite faunas found on opposite sides of the modern Atlantic indicated that the ocean did not exist when they lived and that a different ocean separated regions now joined. They document evolutionary patterns clearly because the record is so dense, including well-studied cases of gradual change in segment counts through successive layers that featured prominently in debates about whether evolution proceeds steadily or in bursts.

The group occupied an enormous range of lifestyles. Most were bottom dwellers, crawling or burrowing on the sea floor and feeding on sediment, detritus or small organisms, and their feeding traces are preserved as distinctive trace fossils. Some were predators with robust head shields and spiny limbs. Some swam or floated, with streamlined shapes and large eyes. Many could roll up completely, enclosing the vulnerable underside within the armoured shield in the manner of a woodlouse, and fossils are frequently found in that defensive posture. Spines of extraordinary elaboration appear in several lineages, variously interpreted as defence, as flotation aids and as sensory structures. Diversity peaked early and declined through repeated extinction events, with a severe loss in the late Devonian reducing the group to a fraction of its former variety, and the survivors persisted in reduced form until the end-Permian extinction removed them entirely, which is the same event that eliminated most marine life and cleared the way for what followed.

A raised central axis and two flanking regions give the animal its name, and a calcite-reinforced exoskeleton is why they fossilise so readily, though most specimens are shed moults rather than dead animals. Their lenses were calcite oriented along the one axis that avoids splitting light, and one group used a corrected doublet. Widespread short-lived species make them the standard tool for dating Cambrian and Ordovician rock.