Life › The Cambrian explosion › Eyes & the predator–prey arms race
Anomalocaris
c. 520–505 million years ago· Fossil or site· Animals
Well establishedStrong fossil, genetic or geological evidence.
A large swimming Cambrian predator with spiny grasping arms, a ring-shaped mouth and big compound eyes.
See it on the Life map →What happened
Anomalocaris ('strange shrimp') belongs to the radiodonts, early relatives of arthropods. It swam by rippling flaps along its sides, seized prey with two spiny front appendages and had a round mouth ringed with plates. Length estimates vary, from about 40 cm to over half a meter, a giant for its time. Its arms, mouth and body were first described as separate animals, including a shrimp and a jellyfish, until a 1985 study matched them.
Why it matters
It is the classic example of a top predator in the first animal ecosystems, showing that food webs of hunters and hunted existed over 500 million years ago.
The evidence (fossils, genes, rocks)
Many Burgess Shale specimens, related radiodonts from Chengjiang and elsewhere, and fossil compound eyes from Australia's Emu Bay Shale.
Still debated
Whether its arms and mouth were strong enough to crack trilobite shells; recent biomechanical studies suggest it hunted softer prey instead.
Connections (3)
▸Eyes & the predator–prey arms race equipped the predator AnomalocarisWell established
The connection
Anomalocaris and its radiodont relatives had large compound eyes on stalks. Fossil radiodont eyes from Australia, about 515 million years old, held over 16,000 lenses each, rivaling modern dragonflies. Sharp vision let these swimming predators spot and track prey from a distance, a central part of the Cambrian arms race between hunters and hunted.
When and how
Cambrian seas worldwide, about 520–505 million years ago; the Emu Bay Shale eyes were described by John Paterson and colleagues in Nature in 2011.
The evidence
Fossil eyes preserving lens patterns, and Burgess Shale specimens showing stalked eyes on the heads of otherwise complete radiodont bodies.
Debates and open questions
Some Australian eyes were later reassigned to other radiodonts, and how much these predators relied on sight rather than touch is uncertain.
▸Anomalocaris is famous from Burgess Shale (Canada)Well established
The connection
Anomalocaris canadensis was named from fossils in the same Canadian Rockies rocks, and its tangled history played out there. Separate pieces had been described as a shrimp, a jellyfish and other animals; restudy of complete Burgess specimens in 1985 showed they were one predator, a turning point in understanding Cambrian life.
When and how
British Columbia, about 508 million years ago; the arm was named by Joseph Whiteaves in 1892, the whole animal assembled by Harry Whittington and Derek Briggs in 1985.
The evidence
Burgess Shale specimens with appendages, mouth and body still attached, which settled which parts belonged together.
Debates and open questions
Several species once placed in Anomalocaris, from China and Australia, have since been moved to other genera, and the list keeps changing.
▸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.