Researchers studying 500-million-year-old fossils have discovered that trilobites – extinct marine invertebrates with exoskeletons and large eyes – actively attacked their wormy prey by breaking into their burrows and then trapping them in the grasp of their legs. The findings, published in Palaeogeography, Palaeoclimatology, Palaeoecology this week, suggest that trilobites were far more sophisticated killers than we have given them credit for. 

Behavior is hard to fossilize. Based on the physical remains of extinct animals, we know that species were rapidly diversifying during what’s known as the Cambrian Explosion half a billion years ago. Finding evidence of their activities and interactions with other species is rare, though predatory activity can be found in the fossil record in the form of drill holes, repair scars, bite marks, and skeletal fragments in feces and gut tracts. It's far less common to find fossil snapshots of predators caught in the act of eating their prey.

A University of Missouri team led by James Schiffbauer stumbled upon exactly that when they looked at an exposure of the Cambrian Davis Formation on the north side of Highway 8 in western St. Francois County, Missouri. Mountains in the south-eastern part of the state were once islands in the Cambrian ocean. Using 3D laser scanning and digital photography, the team found traces in the sediments left behind by Rusophycus trilobites and their worm-like prey, called vermiforms. 

The trilobite traces intersect the vermiform burrows more often than the researchers would expect from just random chance alone, and the diameter of the intersected burrows is significantly smaller than that of the non-intersected burrows. That means trilobites deliberately sought out the worms, and they preferentially selected smaller prey. 

Based on the low angle of the tracks, the team also thinks that the predator attacked from above and moved parallel alongside the worm to get a better handle on it. To the right is an illustration of a trilobite detecting a worm (using sight or maybe smell), and then burrowing down to grapple the worm with its many legs.

"Predation, or the action of attacking one’s prey, is a significant factor in evolution," Schiffbauer says in a statement. "We provide evidence that these trilobites were likely visual predators, displaying selectivity in seeking and hunting their food."

^{Image in the text: Stacy Turpin Cheavens/Department of Orthopaedic Surgery, University of Missouri}