480-million-year-old trilobite ‘traffic jams’ may reveal ancient migratory behavior

For decades, scientists have been digging up groups of fossilized trilobites—tough, armored arthropods that lived in ancient oceans—only to find the creatures lined up like well-behaved kindergartners on their way to lunch. Now, researchers studying ancient fossils in Morocco think they know why.

Previous theories argued that the lined-up fossils had something to do with how the trilobites died—maybe the arthropods were swept into lines by strong currents, or maybe several of them made their way into burrows one after another and then perished. But to a team of researchers in Europe and Morocco, those theories didn’t quite add up.

When the team examined several clusters of 480-million-year-old trilobite fossils from Morocco (above), they noticed that the queued trilobites were mostly adults or semiadults—no babies allowed—and that the majority were facing the same direction. If the lines were the result of random currents, there would probably be a wider variety of ages, and some of the trilobites would likely be facing backward. The researchers also didn’t find typical signs of burrows in the sediment surrounding the creatures.

Taken together, these factors led the researchers to speculate that the trilobites had died in the midst of their ill-fated conga line. The trilobites may have lined up to trek to distant mating grounds—much like modern spiny lobsters—or to escape environmental upheavals such as storms , the researchers write this week in Scientific Reports. In either case, the creatures may have assumed their arrangement to reduce drag, similar to geese flying in V formations.

The trilobites were blind, so they couldn’t have queued up by sight. That means they likely found each other and organized using touch, chemical signals, or a mix of both—a sophisticated behavior for such primitive creatures.