Cambrian Radiodont Was Speedy, But Not Strong Enough to Crack Trilobite Shells

Anomalocaris canadensis — the iconic 60-cm radiodont from the 508-million-year-old Burgess Shale of Canada — is one of the largest Cambrian animals and is often considered the apex predator of its time. This animal is commonly interpreted as a demersal hunter, responsible for inflicting injuries seen in trilobites. A new analysis of the animal’s arachnid-like front ‘legs’ demonstrates that, despite being a raptorial predator, Anomalocaris canadensis was incapable of crushing biomineralized prey with these feeding structures. The results suggest that Anomalocaris canadensis targeted mobile soft-bodied prey within a well-lit water column.

First discovered in the 1800s, Anomalocaris canadensis — which means ‘weird shrimp from Canada’ in Latin — has long been thought to be responsible for some of the scarred and crushed trilobite exoskeletons.

“That didn’t sit right with me, because trilobites have a very strong exoskeleton, which they essentially make out of rock, while this animal would have mostly been soft and squishy,” said Dr. Russell Bicknell, a postdoctoral researcher at the American Museum of Natural History.

Recent research on the armor-plated, ring-shaped mouthparts of Anomalocaris canadensis lays doubt on the animal’s ability to process hard food.

The latest study set out to investigate whether the predator’s long, spiny front appendages could do the job instead.

The authors produced a 3D reconstruction Anomalocaris canadensis from the extraordinarily well-preserved fossils of the animal that have been found in the Burgess Shale.

Using modern whip scorpions and whip spiders as analogues, they were able to show that the predator’s segmented appendages were able to grab prey and could both stretch out and flex.

A modeling technique called finite element analysis was used to show the stress and strain points on this grasping behavior of Anomalocaris canadensisillustrating that its appendages would have been damaged while grabbing hard prey like trilobites.

The researchers used computational fluid dynamics to place the 3D model of the predator in a virtual current to predict what body position it would likely use while swimming.

The combination of these biomechanical modeling techniques painted a different picture of Anomalocaris canadensis than was previously assumed.

The animal was likely a speedy swimmer, zooming after soft prey in the water column with its front appendages outstretched.

“Previous conceptions were that these animals would have seen the Burgess Shale fauna as a smorgasbord, going after anything they wanted to, but we’re finding that the dynamics of the Cambrian food webs were likely much more complex than we once thought,” Dr. Bicknell said.

The findings were published in the journal Proceedings of the Royal Society B.

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Russell D.C. Bicknell et al. 2023. Raptorial appendages of the Cambrian apex predator Anomalocaris canadensis are built for soft prey and speed. Proc. R. Soc. B 290 (2002) 20230638; doi: 10.1098/rspb.2023.0638