500 million years ago, trilobites were sly hunters

Fossil tracks suggest trilobites attacked from above, moving alongside to use their many legs for more effective grappling of their prey. (Credit: Shinichi Higashi/Flickr)

Most fossils from the Cambrian Period, which occurred between 541 million and 485 million years ago, preserve the physical remains of organisms. But others tell a surprising story of how a savvy predator hunted prey, a new study shows.

Trilobites are a fossil group of extinct marine invertebrate animals with external skeletons. As predators and scavengers, they flourished in the Cambrian period and were very prominent in the oceans that once were located in what is now the state of Missouri.

“The Saint Francois Mountains in southeastern Missouri have been the focus of geological research for decades and were once islands in the Cambrian ocean,” says Kevin Shelton, professor of geological sciences at the University of Missouri.

“I’ve worked as a geologist studying ore deposits in the area for more than 30 years. In that time, I’ve run across thousands of fossilized trilobite burrows. It is rare that we get to study the activities of 500-million-year-old organisms, yet the fossils in this locality are helping us determine how these organisms behaved.”

The field area near the mountains is home to an abundance of trilobite trace and body fossils. James Schiffbauer and John Huntley, both assistant professors of geological sciences worked with Shelton and Tara Selly, a graduate student in Schiffbauer’s research group, to collect slabs of rocks from the site.

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Using sophisticated 3D laser scanning and digital photograph analyses, sections of the rocks revealed burrows or trails left behind by trilobites and their prey—often worm-like creatures—in ocean sediments.

illustration of a trilobite hunting a worm
A trilobite detects a lumpy worm burrow by sight and perhaps smell, then burrows down and grasps its prey with its many legs. (Credit: Stacy Turpin Cheavens of the Department of Orthopaedic Surgery/University of Missouri)

To the scientists, these intersecting trails show how the predators caught their prey. Previous studies have shown that trilobites had very large eyes, so the researchers were looking for clues as to how their anatomy played into their feeding habits.

Tracks from the site showed the predators attacked from above, moving alongside to use their many legs for more effective grappling of their prey. The predators preferentially selected smaller prey, indicating that they attacked their food rather than randomly bumping into it.

“Predation, or the action of attacking one’s prey, is a significant factor in evolution; this discovery is extremely important in the study of how organisms evolved in the Cambrian Period,” Schiffbauer says.

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“In this study, we provide evidence that these trilobites were likely visual predators, displaying selectivity in seeking and hunting their food.”

“Because we had an abundance of samples from the site, we were able to conduct more rigorous statistical analyses” Huntley says. “Our findings are important not only because of the large sample size, but because these early arthropods displayed such sophisticated predatory behavior.”

The team published a paper on their work in the journal Palaeogeography, Palaeoclimatology, Palaeoecology.

Source: University of Missouri