What Were Trilobites? The Animals That Ruled the Seas for 270 Million Years

A trilobite was an arthropod, a relative of crabs, insects and spiders, with a body in three lobes running lengthways, a central axis and two sides, which is what the name means. From front to back it had a head shield with eyes and antennae, a thorax of jointed segments that let it curl into a ball like a woodlouse, and a tail plate. The armour, of calcite mineral in a protein matrix, was shed as the animal grew, so most trilobite fossils are discarded moults rather than corpses, which is one reason they are so common. Beneath the shell, known from a few sites of exceptional preservation, were pairs of legs on every segment, each with a walking branch and a gill branch, and a mouth on the underside that pointed backwards.

They ranged from a millimetre to over seventy centimetres, though most were the size of a thumb. Some ploughed the seabed for organic matter, some hunted worms, some swam, some floated in the open ocean with eyes wrapped round their heads, and a few lost their eyes altogether and burrowed. Their walking tracks are found in rocks alongside them.

Trilobite eyes are the oldest well-preserved visual organs known, and they are unique in being made of the mineral calcite, the same as the shell, in clear crystalline lenses that survive in the fossil. Most had compound eyes of hundreds or thousands of tiny lenses packed in a honeycomb, like a modern insect. One group had a few dozen large lenses, each of which, it was discovered in the 1970s, had an internal shape that corrected for the distortion of a thick lens, a design René Descartes and Christiaan Huygens worked out on paper in the seventeenth century. Trilobites had it 400 million years earlier. What the fossils show about their sight:

• •Lenses of oriented calcite crystal, transparent along one axis, which the animal's growth aligned with the direction of sight
• •Eyes that ranged from a few lenses to fifteen thousand, and from narrow strips to towers standing up from the head
• •Fields of view that in some species covered every direction at once, including upwards, suggesting predators from above
• •Lineages that lost their eyes when they moved into deep water or burrowed, and never regained them

Trilobites were among the first animals to appear in the Cambrian explosion, already diverse when they show up, and for the next fifty million years they were the dominant creatures in the sea, so that the Cambrian and Ordovician periods are divided into zones by which trilobite species are found in them. Their peak came in the Ordovician, with the greatest number of families and the widest range of ways of living. From the end of the Ordovician, 445 million years ago, they declined through each of the mass extinctions that followed, losing families every time and never fully recovering, as fish with jaws and then armoured predators spread through the seas they had had to themselves.

By the Carboniferous only a single order remained, the Proetida, small and conservative, which held on for another hundred million years. The last of them died in the Permian extinction, 252 million years ago, when the seas lost their oxygen and turned acid, along with ninety-six percent of all marine species.

Trilobites are the standard by which the early history of animals is measured. Because they were abundant, widespread, hard-shelled and evolving fast, their species mark thin slices of time, and a geologist can date a Cambrian or Ordovician rock anywhere in the world by which trilobites it holds. Their moulting, growth stages and limb structure are known well enough to test ideas about how arthropods develop, and their eyes are the earliest evidence of an animal able to form an image, which is thought to have set off the arms race of the Cambrian between things that could see and things that could be seen.

They are also simply beloved. Trilobites have been found in prehistoric graves, drilled and worn as amulets; the Ute people of Utah called them little water bugs in stone; and a collector in the twenty-first century can buy a Moroccan trilobite prepared under a microscope with its spines standing free, ten thousand times the size of the lens that made its eye.

Trilobites were armoured marine arthropods with three-lobed bodies, jointed legs and calcite eyes, the first complex eyes preserved in the record, that appeared 521 million years ago, dominated the Cambrian and Ordovician seas, and declined through successive extinctions until the last died in the Permian catastrophe 252 million years ago. Their abundance and rapid evolution make them the ruler by which the Palaeozoic is measured.