Anomalocaris

Quick facts

Anomalocaris was a Cambrian radiodont and one of the earliest large animals adapted for active swimming and predation. It had no familiar walking legs. Rows of lateral flaps drove the body through the water, paired jointed appendages projected in front of the head, and large compound eyes helped locate mobile prey.

An animal assembled from three supposed organisms

Joseph Whiteaves described an isolated frontal appendage in 1892 and interpreted it as the body of an unusual shrimp, giving rise to the name Anomalocaris, or “unusual shrimp”. The circular mouth was separately named Peytoia, while body impressions were assigned to yet another animal.

Specimens preserving parts together eventually revealed one body plan. Work by Harry Whittington and Derek Briggs in the 1980s established the integrated reconstruction. Even so, not every circular Cambrian mouth belongs to Anomalocaris; radiodont genera differed in oral-cone construction.

Body and swimming

The elongated body of A. canadensis carried broad lateral flaps. Coordinated travelling waves along those flaps produced thrust. Three-dimensional models indicate good stability and manoeuvrability, and a tail fan assisted turning. They do not provide a directly measured top speed.

Two curved segmented appendages with spines extended from the front. They could open around prey and draw it towards the mouth. The oral cone consisted of plates in a three-rayed arrangement with three larger elements. It was neither a modern jaw nor a powerful toothed crusher.

Large faceted eyes contained many lenses. This is direct evidence for sophisticated vision, although fossil lenses cannot disclose exact acuity or colour perception. Eyes, swimming surfaces and grasping appendages together identify an active animal in the water column rather than a slow bottom crawler.

Size and the famous metre-long giant

Complete A. canadensis specimens are several tens of centimetres long. Larger incomplete material permits higher estimates, but the popular metre figure often combines different radiodont species or scales isolated parts with large uncertainty. Calling it large relative to its Cambrian community is more accurate than assigning every individual a record size.

An animal 40–50 centimetres long still exceeded many neighbours. Cambrian scale should be judged within its ecosystem, not against modern marine vertebrates.

Could it crush trilobites?

Older reconstructions frequently show Anomalocaris breaking mineralised trilobite shells. Biomechanical analysis of the frontal appendages of A. canadensis indicates that they tolerated loads associated with grasping flexible mobile prey better than forceful fracture of hard armour. Its oral cone also did not close like a strong serrated jaw.

This does not prove that it never ate a trilobite. Small or freshly moulted individuals and exposed soft tissue could have been vulnerable. A damaged shell without the predator beside it, however, cannot be assigned confidently to Anomalocaris. No unequivocal stomach content fixes the diet.

Relationships and environment

Radiodonts occupy the stem lineage leading towards arthropods. Jointed frontal appendages and compound eyes occur alongside swimming flaps and the absence of a complete modern arthropod exoskeleton. Relationship does not turn Anomalocaris into a shrimp or the direct ancestor of all living arthropods.

It shared the Burgess Shale ecosystem with sponges, worms, trilobites and Hallucigenia. The Palaeozoic Era guide gives the wider time setting, while the catalogue compares the different types of evidence preserved in this fauna.

Evidence, inference and reconstruction