“Weird shrimp” from Canada preferred squishy prey

Biomechanical studies on the arachnid-like front “legs” of an extinct apex predator show that the marine animal was much weaker than once assumed. Anomalocaris canadensis, one of the largest animals to live during the Cambrian (with a body length of about two feet), was probably agile and fast, darting after soft prey in the open water rather than hard-shelled creatures on the ocean floor.

First discovered in the late 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 palaeontologists have found in the fossil record.

‘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 lead author Russell Bicknell, a postdoctoral researcher in the American Museum of Natural History, who conducted the work while at the University of New England in Australia.

Recent research on the armour-plated, ring-shaped mouthparts of A. canadensis throws doubt on the animal’s ability to process hard food. The latest study set out to investigate whether the predator’s long, spiny pair of front appendages could do the job instead.

The first step for the research team, which included scientists from The Natural History Museum London, Germany, China, Switzerland and Australia, was to build a 3D reconstruction of A. canadensis from the extraordinarily well-preserved, but flattened, fossils of the animal that have been found in Canada’s 508 million year-old Burgess Shale.  This was based on study of the most completely known specimens, which were loaned to the Natural History Museum, London for analysis by Drs Allison Daley, then a postdoctoral fellow, and NHM Researcher Greg Edgecombe.  

‘Having access to specimens with the entire body preserved in the fossils allowed us to understand the anatomy of the appendages in the context of the rest of the head and the trunk. We were able to get a better picture of Anomalocaris as a living organism’, Edgecombe remarked.

Using modern predatory whip scorpions and whip spiders as an analogue, the team was able to show that the predator’s segmented appendages were able to grab prey and could both stretch out and flex. A modelling technique called finite element analysis was used to show the stress and strain points on this grasping behaviour of A. canadensis, illustrating that its appendages would have been damaged while grabbing hard prey like trilobites. Finally, 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 use while swimming.

Natural History Museum, London, Researcher Dr Imran Rahman, who conducted the computational fluid dynamics simulations, said, ‘This study emphasizes the great potential of modern computer modelling methods in palaeontology. By employing techniques more commonly used in other disciplines like engineering, we can test ideas about long-extinct animals like Anomalocaris’.

The combination of these biomechanical modelling techniques, the first time they’ve all been used together in a scientific paper, paint a different picture of A. canadensis than was previously assumed. The animal was likely to be a speedy swimmer, zooming after soft prey in the water column with its front appendages outstretched and forward-facing.

‘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 probably much more complex than we once thought,’ Bicknell said.

The study, Raptorial appendages of the Cambrian apex predator Anomalocaris canadensis

are built for soft prey and speed is published in the Journal Proceedings of the Royal Society B.

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