Anomalocaris: The Strange Apex Predator of the Cambrian Seas

Anomalocaris was a large predatory animal that swam in the seas of the Cambrian Period, between roughly 520 and 499 million years ago. With a pair of spiny grasping limbs, large compound eyes and a body edged with swimming flaps, it was one of the biggest animals of its time. Its fossils were found in pieces and misidentified as a shrimp, a jellyfish and a sea cucumber before scientists realized in the 1980s that all three belonged to a single creature.

Background: a world of small animals

Anomalocaris lived during the Cambrian Explosion, when most major animal groups first appear in the fossil record. Most Cambrian animals were small, often only a few centimetres long, and many lived on or in the sea floor.

Against that background, Anomalocaris stood out. It was an active swimmer, and its size and equipment suggest that it was among the top predators of its ecosystems. It belongs to a group called the radiodonts, named for the radiating plates of its mouth, which flourished in Cambrian seas around the world.

A puzzle in pieces

The “abnormal shrimp”

The story began in 1892, when the Canadian palaeontologist Joseph Frederick Whiteaves described a strange fossil from the Mount Stephen trilobite beds near Field, British Columbia. It was curved and segmented, with spines along one edge. Whiteaves interpreted it as the body of a shrimp-like crustacean and named it Anomalocaris canadensis, meaning roughly “unlike other shrimp”.

In fact the fossil was a frontal appendage: one of the two grasping limbs that the animal held in front of its head.

A jellyfish and a sea cucumber

After discovering the Burgess Shale in 1909, the American palaeontologist Charles Doolittle Walcott described more fossils from the area. In 1911 he named a round, ring-shaped fossil with a square central opening Peytoia nathorsti and interpreted it as a jellyfish. He described a flattened body fossil as Laggania cambria and treated it as a sea cucumber.

Over the following decades, other researchers noticed odd associations, such as “shrimp” appendages found close to the ring-shaped fossils, but the parts were still classified as different animals.

Putting the animal together

The breakthrough came during a major restudy of the Burgess Shale fossils led by Harry Whittington of the University of Cambridge. In the late 1970s, Simon Conway Morris noted that a Laggania specimen had a Peytoia-like structure where its mouth should be. Whittington and Derek Briggs then examined specimens in which the appendages, mouth and body were found together.

In 1985 they published their reconstruction in the Philosophical Transactions of the Royal Society, under the title “The largest Cambrian animal, Anomalocaris, Burgess Shale, British Columbia”. The “shrimp” was the grasping limb, the “jellyfish” was the circular mouth and the “sea cucumber” was the body.

Later work refined this picture. Studies by Allison Daley and colleagues around 2010 to 2012 showed that Burgess Shale radiodonts included more than one animal. The Peytoia mouth and Laggania body belong to a separate radiodont, now called Peytoia, while Anomalocaris canadensis had a somewhat different, more irregular mouth.

Anatomy and size

Anomalocaris had a flattened body with a series of flaps along each side, which it probably moved in waves to swim, much like a modern ray or cuttlefish. Its tail ended in a fan of three pairs of broad blades that may have helped with steering.

At the front, two spiny frontal appendages curled down towards a circular mouth on the underside of the head. The mouth was surrounded by hard plates that could probably open and close. A pair of large compound eyes sat on short stalks.

Estimates of its size vary. The largest individuals of Anomalocaris canadensis are usually reconstructed at roughly 30 to 50 centimetres long, and older claims of a metre or more were based on isolated appendages. Even at the lower end, it was a giant among Cambrian animals.

How it hunted

For many years Anomalocaris was pictured as a trilobite-crusher. Some trilobites from Cambrian rocks show healed bite marks and injuries, and large coprolites (fossil droppings) containing trilobite fragments have been found, and these were often linked to Anomalocaris.

Later studies questioned this. Analyses of its mouth plates found little sign of the wear expected from crushing hard shells. In 2023 a team led by Russell Bicknell used computer models of the frontal appendages and concluded that they were suited to grabbing soft, fast-moving prey in open water, rather than breaking trilobite armour. Other large predators may have caused the injuries seen on trilobites.

Its eyes support the picture of an active hunter. In 2011 John Paterson and colleagues described fossil compound eyes from the Emu Bay Shale on Kangaroo Island, South Australia, about 515 million years old. Attributed to Anomalocaris or a close relative, each eye had more than 16,000 lenses, comparable to the eyes of some modern insects.

Place among the radiodonts

Anomalocaris is the best-known member of the radiodonts, a group that also includes Peytoia, Hurdia and Amplectobelua. Not all were hunters: Tamisiocaris from Greenland and the giant Aegirocassis from Morocco, about 480 million years old and around two metres long, had appendages adapted for filtering small food from the water.

Radiodonts are now generally placed near the base of the arthropod family tree, as relatives of the ancestors of arthropods. Animals such as Opabinia and the “gilled lobopodians” help link them to worm-like ancestors with stubby legs. Radiodonts survived beyond the Cambrian, with the Devonian Schinderhannes from Germany, about 400 million years old, among the last known.

Species assigned to Anomalocaris or closely related genera have been reported from Canada, the United States, Australia and the Chengjiang biota of China, showing that radiodonts were widespread.

Myths and facts

Myth: Anomalocaris was a giant monster several metres long. Popular images sometimes exaggerate its size. The best estimates put it at well under a metre, though that still made it huge compared with its prey.

Myth: it was the ancestor of shrimps or lobsters. Its original name reflects an early mistake. Anomalocaris was a side branch on the early arthropod family tree, not a crustacean, and it left no living descendants.

Fact: it became a symbol of Cambrian strangeness. Stephen Jay Gould’s 1989 book Wonderful Life made Anomalocaris famous as one of the “weird wonders” of the Burgess Shale, and it is now one of the most recognisable prehistoric animals in museums and documentaries.

Timeline

• c. 520 million years ago: Anomalocaris-like radiodonts appear in early Cambrian seas.
• c. 515 million years ago: Radiodonts with highly developed compound eyes live in the Emu Bay Shale area of Australia.
• c. 508 million years ago: Anomalocaris canadensis is preserved in the Burgess Shale of British Columbia.
• c. 499 million years ago: The youngest Anomalocaris fossils date from around this time.
• 1892: Joseph Frederick Whiteaves names Anomalocaris canadensis from an isolated appendage.
• 1911: Charles Doolittle Walcott describes Peytoia as a jellyfish and Laggania as a sea cucumber.
• 1985: Harry Whittington and Derek Briggs publish the first reconstruction of the whole animal.
• 2011: Fossil compound eyes with more than 16,000 lenses are described from South Australia.

Why it matters

Anomalocaris shows that large, active predators with sophisticated eyes existed early in animal evolution, shaping Cambrian ecosystems through predation. Its tangled history of misidentification is also a classic lesson in palaeontology: fragments can mislead, and new specimens and fresh study can transform our understanding of an animal.