Trilobites: The Iconic Arthropods of the Paleozoic Seas

Trilobites were armoured marine arthropods that lived in the world’s seas for about 270 million years, from roughly 521 to 252 million years ago. With more than 20,000 described species, they are among the most abundant and recognisable fossils of the Paleozoic Era. Their hard calcite shells, compound eyes and rapid evolution make them invaluable for understanding early animal life and for dating ancient rocks.

Origins in the Cambrian

Trilobites appear suddenly in the fossil record about 521 million years ago, during the Cambrian Explosion. The earliest known forms, such as the redlichiids and olenellids, were already fully formed arthropods with segmented bodies, legs and eyes. Their ancestors were probably soft-bodied and left few fossils.

Within a few million years trilobites had spread across the shallow seas of the world. They became the most diverse and abundant animals with hard parts in many Cambrian deposits, which is why the Cambrian is sometimes informally called the “Age of Trilobites”.

Exceptional fossil sites such as the Burgess Shale in Canada and Chengjiang in China preserve trilobites with their legs and antennae, showing what the whole animal looked like rather than just its shell.

Anatomy and behaviour

The body

The name trilobite, meaning “three lobes”, refers to the body’s lengthwise division into a raised central axis and two side lobes, called pleurae. Across its length, the body was also divided into three parts:

• Cephalon: the head shield, which held the eyes, the mouth area and the base of the antennae.
• Thorax: a series of movable segments, from two to more than 40 depending on the species.
• Pygidium: a tail shield made of fused segments.

The upper surface was covered by a hard exoskeleton strengthened with calcite, a form of calcium carbonate. The underside was mostly soft, with a mineralized plate called the hypostome near the mouth.

Legs and gills

Rare fossils show that under each segment trilobites had a pair of branched limbs. One branch was a jointed walking leg; the other carried fine filaments that were probably used for breathing, and perhaps for swimming. A pair of long antennae projected from the head.

Some of the best evidence comes from Beecher’s Trilobite Bed near Rome, New York, where specimens of Triarthrus eatoni were preserved in pyrite, or “fool’s gold”. The Yale palaeontologist Charles Emerson Beecher described their limbs in detail in the 1890s.

Eyes of stone

Trilobites had some of the oldest complex eyes known. Their lenses were made of calcite crystals, oriented so that light passed along the crystal axis without being split. This is why their eyes often survive in fossils when other soft structures do not.

Most trilobites had holochroal eyes, with many small lenses packed closely together under a single covering. The phacopids, from the Ordovician onward, had schizochroal eyes with fewer, larger lenses, each separated by skeleton. Some had eyes raised on stalks, while others living in deep or dark waters were blind.

Growth, moulting and enrolment

Like all arthropods, trilobites had to moult their exoskeleton to grow. The shell split along lines called facial sutures on the head, allowing the animal to wriggle free. Many trilobite fossils are therefore moults rather than dead animals, and a single individual could leave many fossils during its life.

Their young passed through several growth stages, from tiny larvae a fraction of a millimetre across to adults. Fossil series of these stages are among the best records of development of any extinct animal.

Many trilobites could roll up, a behaviour known as enrolment, much like modern woodlice or pill bugs, protecting their soft underside from predators. Fossils of enrolled trilobites are common.

Trilobites also left trace fossils. Furrows made while ploughing through sediment are called Cruziana, resting hollows are called Rusophycus, and trackways of small leg marks are found in many Paleozoic rocks.

Diversity and uses in geology

Ways of life

Trilobites ranged from around 1 millimetre to about 72 centimetres in length. The largest known complete specimen, Isotelus rex, was found in Ordovician rocks near Churchill, Manitoba, and described in 2003. A related species, Isotelus maximus, is the state fossil of Ohio.

They lived in many ways. Some crawled on the sea floor eating sediment or scavenging; some were probably predators that dug up worms; others, with large eyes and streamlined bodies, swam in open water. The tiny agnostids, often just a few millimetres long, may have drifted in the plankton, and some researchers question whether they were true trilobites at all.

Many Devonian trilobites, such as those found in Morocco, bore elaborate spines, horns and forks on their heads and bodies, perhaps for defence, display or stability.

Palaeontologists usually group trilobites into around ten orders, such as the early Redlichiida, the widespread Ptychopariida, the large smooth-shelled Asaphida, the spiny Lichida and Odontopleurida, and the Phacopida with their distinctive eyes. Ordovician seas held the greatest variety of forms, from burrowers to open-water swimmers such as Carolinites, which had very large, bulging eyes.

Time-keepers and map-makers

Because trilobite species evolved quickly, spread widely and fossilized well, they are among the most useful index fossils for the Cambrian and Ordovician. Geologists divide these periods into zones, each defined by the trilobites it contains, so that rocks in different parts of the world can be matched and dated relative to one another.

Trilobites also helped reveal the movement of continents. In the 19th century geologists noticed that Cambrian trilobites in Scotland and north-west Newfoundland resembled those of North America, while species found only a short distance away in England, Wales and south-east Newfoundland resembled those of Scandinavia and Bohemia. In 1966 the Canadian geologist J. Tuzo Wilson used such contrasts to argue that an ancient ocean, later named Iapetus, had once separated these regions before closing and later reopening as the Atlantic.

Decline and extinction

Trilobite diversity peaked in the Cambrian and Ordovician. They were hit hard by the Late Ordovician mass extinction about 445 million years ago, and again by the extinction pulses of the Late Devonian between about 372 and 359 million years ago. After the Devonian, only one order, the Proetida, survived.

In the Carboniferous and Permian, trilobites were relatively rare members of marine communities. The last of them disappeared in the Permian–Triassic extinction about 252 million years ago, ending a history of some 270 million years.

Myths and facts

Myth: trilobites were the ancestors of woodlice or horseshoe crabs. Woodlice are crustaceans and horseshoe crabs are related to spiders and scorpions. Trilobites form their own extinct branch of arthropods and left no living descendants.

Myth: trilobites were primitive. They had complex eyes, jointed limbs, a sophisticated moulting system and a wide range of lifestyles. Their long survival through several mass extinctions reflects real adaptability.

Fact: many fossils are empty shells. Because trilobites shed their exoskeleton many times, a large proportion of fossils are moults. Palaeontologists can often tell them apart from carcasses by the way the head shield is displaced.

History of discovery

People have collected trilobites for thousands of years. A trilobite drilled to be worn as a pendant was found in a cave at Arcy-sur-Cure in France, which is now called the Grotte du Trilobite after it.

In 1698 the Welsh naturalist Edward Lhuyd described a trilobite from Wales in the Philosophical Transactions of the Royal Society, though he thought it might be a kind of flatfish. The German naturalist Johann Ernst Immanuel Walch introduced the name “trilobite” in 1771.

In the 19th century, trilobites became central to the study of early life. The French-born scientist Joachim Barrande spent decades collecting in Bohemia and published a major monograph on its trilobites in 1852. In England, the Silurian species Calymene blumenbachii, found in large numbers near Dudley, became known as the “Dudley locust” and appears on the town’s coat of arms.

Today trilobites are popular with collectors, and large quantities are sold from Morocco and Utah, where Elrathia kingii is especially common. Because some commercial specimens are partly or entirely fake, museums and serious collectors check them carefully.

Timeline

• c. 521 million years ago: The first trilobites appear in the early Cambrian.
• c. 508 million years ago: Trilobites with preserved legs and antennae are buried in the Burgess Shale.
• c. 485–445 million years ago: Trilobites reach high diversity in the Ordovician.
• c. 445 million years ago: The Late Ordovician extinction removes many trilobite groups.
• c. 372–359 million years ago: Late Devonian extinctions leave only the order Proetida.
• c. 252 million years ago: The last trilobites die out in the Permian–Triassic extinction.
• 1698: Edward Lhuyd publishes a description of a trilobite from Wales.
• 1771: Johann Walch coins the name “trilobite”.
• 1852: Joachim Barrande publishes his monograph on the trilobites of Bohemia.

Why it matters

Trilobites are one of the best-documented groups of extinct animals. Because species changed quickly and spread widely, geologists use them to date and correlate Paleozoic rocks across continents. Their eyes, growth stages and long history also make them a vivid window on how early animals lived, evolved and finally vanished. Their story, spanning almost half of the history of animal life in the fossil record, is also a reminder that even the most successful groups can disappear when the planet changes quickly.