Cambrian's fiercest hunter defanged

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Computer modelling hints that Anomalocaris didn't have the chops to chew up trilobites.

Anomalocaris may not have been quite as fearsome as was thought. Credit: Wikimedia Commons

A creature thought to be the fiercest predator of its day might not have been quite so fierce after all, according to palaeontologist Whitey Hagadorn of Amherst College, Massachusetts. Anomalocaris, a strange shrimp-like animal that lived around half a billion years ago and could grow up to a metre in length, is often portrayed as the Tyrannosaurus rex of the Cambrian, hunting and eating hard-shelled prey such as trilobites. But Hagadorn's models suggest that Anomalocaris's mouth was incapable of such attacks.

"Everyone shows it grabbing trilobites and munching them. Like a cookie monster. Not possible," Hagadorn says. He presented his ideas on 5 August to the Burgess Shale centenary meeting on the Cambrian explosion, taking place in Banff, Canada.

“Everyone shows it grabbing trilobites and munching them. Like a cookie monster. Not possible. Whitey Hagadorn , Amherst College”

Hagadorn first suspected something was wrong with the Anomalocaris image when, after examining hundreds of their fossilized mouths, he had not seen a single broken plate-like 'tooth'. How could these 'monsters' have been cracking through the hard shells of trilobites without occasionally damaging their own mouthparts?

Anomalocaris mouths are relatively common in the Burgess Shale, a bed of rock in the Canadian Rockies that has preserved both the hard and soft parts of the animals that lived in the seas some 505 million years ago. The mouths are round, with plates pointing inwards (the fossils look a bit like pineapple rings). Chemical evidence from the fossils indicates that the mouths were probably made of a material similar to the cuticle that covers modern arthropods.

Hagadorn and colleagues tested the physical strength of different types of modern arthropod cuticle — from the soft shells of shrimps to the hard shells of lobsters. They then plugged the numbers into a computer model of an Anomalocaris mouth as it faced up to various types of trilobites — big and small, thick and thin, tough and weak.

Jellyfish diet?

A circular mouth couldn't close down on the side of a trilobite, Hagadorn says, so Anomalocaris would probably have had to half-swallow a trilobite and constrict its mouth around it. In order to crack the strong, corrugated trilobite shell, the flat plate-like 'teeth' would have to be very hard. But the team found that even if the Anomalocaris plates were as strong as lobster shell, and the trilobite had just moulted so that it was covered by a relatively thin shrimp-like coat, it couldn't be cracked.

Hagadorn suspects instead that the group Anomalocaris belongs to — the Dinocarids (which means 'terrible crabs') — ate soft worms and jellyfish, or maybe just filtered plankton. Something else left the bite marks observed in trilobite fossils, or perhaps, he says, they aren't bite marks at all.

"I think he could be quite right that the mouthparts are not so robust. But it's got claws as well," says Desmond Collins of the Royal Ontario Museum, the palaeontologist who found the first complete Anomalocaris fossil. Is it still a fierce predator? "Sure, sure," he says. He notes that many Anomalocaris have a second set of teeth inside their mouths, suggesting that they were chewing on something substantial.

Hagadorn hasn't modelled the claws, in part because they are more variable from one beast to the next, but he says that they also seem to be soft — fossils show them draped over trilobites, for example.

Dave Rudkin of the Royal Ontario Museum, which holds many specimens collected from the Burgess Shale, says that Hagadorn's work can't yet rule out the idea that Anomalocaris preyed on hard-shelled animals. Trilobites have a soft underbelly, he points out. "What we need is to find Anomalocaris poop," he says, to pin down what they were eating.

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In the Field: Burgess Shale Centenary

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Walcott 2009 Cambrian explosion conference

Burgess Shale

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Jones, N. Cambrian's fiercest hunter defanged. Nature (2009) doi:10.1038/news.2009.811

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