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Trilobites
c. 521–252 million years ago· Organism or group· Animals
Well establishedStrong fossil, genetic or geological evidence.
Armored, three-lobed sea arthropods that appeared about 521 million years ago and lasted nearly 270 million years.
See it on the Life map →What happened
Trilobites are named for the three lengthwise lobes of their bodies. Their calcite exoskeletons fossilized well, and because they molted repeatedly, one animal could leave many fossils. More than 20,000 species are known, from about 1 mm to over 70 cm long. Many could roll into a ball like a pill bug, and most had compound eyes with mineral lenses. They declined after the Devonian extinctions and vanished in the Great Dying, 252 million years ago.
Why it matters
As abundant, fast-changing fossils, they are used to date Cambrian to Permian rocks and are a key record of early arthropod life.
The evidence (fossils, genes, rocks)
Huge numbers of specimens worldwide, from Morocco to Utah to China, preserved in sequence in dated rock layers; some even preserve legs and antennae.
Still debated
Exactly how they relate to other arthropods, and why they slowly declined long before their final extinction.
Connections (6)
▸Cambrian explosion gave rise to TrilobitesWell established
The connection
Trilobites were among the arthropods that emerged in the Cambrian explosion. They appear about 521 million years ago, already varied and widespread, with hard exoskeletons and compound eyes, and they became the most common fossil animals of the Cambrian.
When and how
Early Cambrian seas worldwide, about 521 million years ago; Siberia, Morocco and China have some of the oldest.
The evidence
Trilobites appear in dated rock layers soon after the explosion begins, with arthropod-style trace fossils just before them.
Debates and open questions
Their sudden, already-varied appearance suggests soft-bodied ancestors lived earlier but left few fossils.
▸Trilobites were early, sharp-eyed players in Eyes & the predator–prey arms raceWell established
The connection
Trilobites had compound eyes with lenses made of calcite, a clear mineral, so the lenses themselves fossilize. They are among the oldest well-preserved eyes, letting scientists reconstruct what early animals could see. Trilobites both hunted and were hunted, so their eyes and armor are a key record of the predator–prey arms race.
When and how
Seas worldwide from about 521 million years ago; Euan Clarkson and Riccardo Levi-Setti analyzed trilobite lens optics in the 1970s.
The evidence
The lenses are preserved directly in thousands of fossils, and their optics have been modeled in detail.
Debates and open questions
How sharp their vision really was, and whether trilobite eyes evolved before or after the first large visual predators.
▸Hallucigenia is a distant cousin of TrilobitesStrong evidence
The connection
Hallucigenia belongs to the lobopodians, legged worms near the base of the group that includes arthropods and velvet worms. Trilobites are true arthropods with jointed legs and hard exoskeletons. Both descend from a common ancestor that molted its outer skin, so lobopodians hint at what arthropod ancestors looked like before legs became jointed.
When and how
Both lived in Cambrian seas around 520–508 million years ago; Hallucigenia at the Burgess Shale and Chengjiang, trilobites worldwide.
How sure are we?
Shared features such as claws, molting and leg structure, plus family-tree studies; a 2014 claw study placed Hallucigenia close to velvet worms.
Debates and open questions
Lobopodians are a mixed group, and exactly how they relate to each other and to true arthropods is still debated.
▸Anomalocaris may have hunted TrilobitesDebated
The connection
As one of the largest Cambrian predators, Anomalocaris was long assumed to eat trilobites. Some trilobites carry healed, bite-shaped injuries, and some fossil droppings contain trilobite fragments. But studies of its mouth and grasping arms suggest they were too weak to crack hard shells, pointing to softer prey.
When and how
Cambrian seas, about 520–505 million years ago; injured trilobites from several sites; biomechanical studies since the late 2000s, including Russell Bicknell's team in 2023.
How sure are we?
Indirect only: injured trilobites and fossil dung (coprolites) with shell fragments, but no fossil caught in the act, and other predators could be responsible.
Debates and open questions
Whether Anomalocaris crunched trilobites, ate only freshly molted soft ones, or targeted soft-bodied swimmers instead is unresolved.
▸Great Ordovician Biodiversification Event diversified TrilobitesWell established
The connection
Trilobites appeared in the Cambrian, but in the Ordovician new trilobite groups spread into new habitats and lifestyles: open-water swimmers, deep-water forms, burrowers, and species with huge eyes or long spines. Meanwhile older Cambrian-style groups declined, before heavy losses in the Late Ordovician extinction.
When and how
Shallow and deep seas worldwide, about 485–445 million years ago, around the continents of Laurentia, Baltica and Gondwana.
The evidence
Fossil databases record many new trilobite families and genera in Ordovician rocks, with new lifestyles visible in their anatomy.
Debates and open questions
Why some trilobite groups thrived in the radiation while older ones faded: competition, habitat change or chance.
▸Trilobites were wiped out by The Great Dying (end-Permian extinction)Well established
The connection
Trilobites had lived for nearly 270 million years but were already in long decline; by the late Permian only one order, the Proetida, remained. The Great Dying finished them off, ending a group that had been among the commonest animals of the Cambrian seas.
When and how
Late Permian seas; the last trilobites disappear at or just before the boundary about 252 million years ago.
The evidence
No trilobite fossil has ever been found in Triassic or younger rocks, despite more than two centuries of intensive collecting.
Debates and open questions
Whether the last trilobites died in the boundary crisis itself or slightly earlier is unclear, because late Permian trilobite fossils are rare.