The Early Trilobite Gets the Worm
Researchers who study evidence of predatory behavior in the fossil record generally look for drill holes, repair scars, bite marks, and other signs of predation in fossilized skeletons. But a team of researchers at the University of Missouri has found fossil “snapshots” of predators caught in the act of feeding on their prey. Assistant Professor of Geological Sciences Jim Schiffbauer says predation is a significant factor in evolution, and this discovery represents one of the earliest examples of sophisticated predatory behavior.
The discovery started with a field trip to southeast Missouri. Kevin Shelton, a professor of geology, has been visiting the Davis Formation on the north side of Highway 8 in St. Francois County for the past 30 years. His specialty is metallic ore deposits, and Shelton says this formation is part of a sequence that hosts the major lead and zinc deposits in southeast Missouri. He invited Schiffbauer, Assistant Professor of Geological Sciences John Huntley, and masters’ student Tara Selly to join him on an outing to the formation.
Shelton took the team to his favorite site of the Davis Formation. “I said, ‘Here’s one of the coolest outcrops I’ve seen before,’ and John saw one rock with lots of burrows in it and said, ‘Do you know what you have?’”
What they found was a very large number of fossilized trilobite impressions that intersected with the burrows of worm-like (vermiform) organisms. Trilobites are early marine arthropods that scuttled along the bottom of Cambrian oceans some 500 million years ago. Schiffbauer says scientists generally do not find numerous associations between trilobite and vermiform burrows from a given location—typically up to 10 to 20—but they found hundreds of interactions in the rocks in southeast Missouri. Selly was tasked with documenting all of the trilobite–vermiform burrow interactions in the 82 slabs the team brought back to Columbia.
What she found was that the trilobite traces intersected the worm burrows more often than would be expected by random chance. The fossils also show evidence that suggests the trilobites were selective in hunting their prey, preferring smaller worms, and that trilobites attacked their prey at low angles more frequently than expected, improving their chances of grabbing onto and handling their prey (see illustration).
Huntley says the findings give scientists a unique window into ecological interactions that are usually not preserved.
“We do have trilobites preserved in these rocks, but mainly just little fragments that don’t reveal anything about behavior,” Huntley says. “In this case we have a different view—this is the behavior of the organism preserved as traces in the sediment.”
Huntley says the fact the team found more low-angle attacks than would occur by random chance indicates a certain degree of neural sophistication.
“These are just little trilobites, but it suggests there was a fair amount of sophistication in their approach,” Huntley says.