If anything lurking in shallow Cambrian seas looked like a monster, Anomalocaris canadensis was it. The 3-foot-long, lobe-winged, shrimp-like creature came equipped with two barbed feelers and an armor-plated mouth – parts paleobiologists once thought were ideal for finding and crunching tasty trilobites.
But a new 3-D model of the creature's mouth parts, presented Nov. 1 at the Geological Society of America's annual meeting in Denver, Colorado, may restrict the ancient predator’s diet only to mushy meals.
"We found that it's extremely unlikely Anomalocaris could eat most trilobites," said James Whitey Hagadorn, the research team's leader and a paleontologist at the Denver Museum of Nature and Science. "It couldn’t close its mouth all of the way, its mouth was too soft to crush trilobite shells."
Jean Vannier, a paleontologist at Université Lyon 1 in France who was not involved in the work, said Hagadorn's conclusions make sense.
"What was the function of the powerful frontal appendages of Anomalocaris? They seem to be directed towards the mouth but there [is] no clear evidence that they might be used for crushing food," Vannier wrote in an e-mail. "To me their main function might have been to stir up the bottom sediment."
Hagadorn said the mouth parts are often illustrated as capturing trilobites and other Cambrian critters, but emphasized that little if any research backs up the popular depiction. Instead of eating solid food, Hagadorn suspects Anomalocaris stuck to softer items on the menu 500 million years ago, much the same way modern arthropods such as shrimp, crabs and lobsters do.
"They mostly eat soft things, worms in the mud or soft microorganisms floating in water," Hagadorn said. "We have no positive evidence Anomalocaris did eat this way, but that's not surprising. How are you going to tell the difference between mushed-up worms, mushed-up phytoplankton or mushed-up snails in the fossil record? They’re all going to look like mush."
To reconstruct Anomalocaris' ancient mouth parts, Hagadorn and his team examined 400 fossils of the structures, picked the best-preserved ones, then plugged the data into a 3-D computer model. They also did the same for 12 groups of trilobites, "including spiny ones, flat ones, round ones, and so on," Hagadorn said, noting the shell strength was modeled after crab and lobster shells.
"Basically, we tried to capture the full range of prey sizes and shapes as well as predator mouth sizes and shapes," he said.
The computerized model's stress tests showed Anomalocaris' two feelers were very inflexible and the armored mouth, at least for non-juvenile trilobites, would break before the trilobites did.
"There’s that, plus no positive evidence in fossilized gut contents, feces or otherwise suggesting that Anomalocaris could eat trilobites or anything else with shells or cuticles," Hagadorn said.
Some trilobite fossils have bite marks and scars resembling Anomalocaris' nibble. Hagadorn suggested that perhaps the creature "ingested things and then spit them out," including hard-shelled trilobites, but never ate them.
Jean-Bernard Caron, a paleontologist at the Royal Ontario Museum in Toronto who was not involved in the work, said that doesn't mean Anomalocaris never ate trilobites, however.
"The animal was likely well-adapted for preying on soft preys," Caron wrote, noting that trilobites did shed their carapace during molting. "It is possible that they could have preyed upon them during that time ... and before the carapace mineralized again, thus explaining numerous healed injuries on trilobites."
Paleobiologist Danita Brandt of Michigan State University in East Lansing, who was independent of the research team, joked that "no court of law" could convict a predator of trilobites. But she said Hagadorn's work is an important step in getting there.
"His model takes a lot of the guesswork out of the equation of what Anomalocaris was capable of," Brandt said. "I think what he's done here is fantastic."
Images: 1) Illustration of the bottom of Anomalocaris canadensis, with two feelers and an armor-plated mouth./Wikimedia Commons/Nobu Tamura. 2) Anomalocaris' distinctive pineapple-ring-like mouth, in the open position, with the four cardinal plates visible./Hagadorn et al. 3) Fossils of trilobites, creatures Anomalocaris may not have been able to eat while hard-shelled./Flickr/kevinzim.
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