The great, car-sized predatory "shrimp" that was master of Earth's seas a half billion years ago may have been unable to eat anything harder than baby food.
Several lines of evidence along with a new 3-D model of the way the mouth parts of Anomalocaris worked show that the infamous carnivore could not crack the hard shells of trilobites and other protected critters of the Cambrian seas.
"The mouth parts appear soft and bendable in (fossil) specimens," said paleontologist James "Whitey" Hagadorn of the Denver Museum of Nature & Science. "That struck me as kind of funny."
Soft mouth parts don't jive with the now standard interpretation of Anomalocaris being a predator that wolfed down trilobites and anything else it could catch. That got Hagadorn wondering: "Is it possible that Anomalocaris didn't eat trilobites?" he asked.
To find out, Hagadorn started looking at all the available fossilized feces from the Cambrian time when Anomalocaris lived. None containing mangled bits of the hard, indigestible exoskeletons of trilobites could be found that could be traced to Anomalocaris.
What's more, there is no evidence of trilobite parts in the gullets of any known Anomalocaris fossil.
For that matter, no gut contents have ever been found for any of the dozen or so known species of Anomalocaris, he said. That's especially strange considering that hundreds of other fossil animals found nearby have guts filled with all sorts of interesting stuff.
To get a better idea just what Anomalocaris might be capable of eating, Hagadorn and his colleagues devised a 3-D, finite element analysis model of the Anomalocaris's whorl of mouth parts. The model let them virtually operate the mouth to see what it could do and how much force it could generate.
The answer is that Anomalocaris couldn't close its mouth and could not create forces strong enough to break a modern lobster or shrimp shell -- the analogues they used for trilobite shells.
Yet another sign that the Anomalocaris's mouth was not for hard foods is that out of more than 400 mouth specimens Hagadorn inspected, none showed any chipped or damaged parts. That would be difficult to manage if it spent a lot of time crunching on hard, mineralized shells of trilobites organisms.
Hagadorn is scheduled to present his findings on Nov. 1 at the annual meeting of the Geological Society of America in Denver.
What the new work does not explain are all the damaged and apparently bitten trilobites that are also in the fossil record.
"I'm a bit skeptical about Whitey Hagadorn's conclusions regarding Anomalocaris," said paleontologist Mark McMenamin of Mount Holyoke College in Massachusetts.
"The damage to those Cambrian trilobites is real, and if it was not Anomalocaris's doing, who then was the predator? If Hagadorn is right and Anomalocaris could only gum its prey, then the search is on for the actual durophagous (that is, shell-cracking) Cambrian predator."