Trilobites, fossil hunters’ favorite arthropods, dominated Earth’s ancient oceans for hundreds of millions of years. Now, a new study suggests that trilobites migrated across the ocean floor in long, orderly queues—just like many modern arthropods. Some lobsters, for instance, line up each spring in a single file to march into shallow waters to breed, then march resolutely back to deeper waters in the fall. The researchers examined 365-million-year-old fossils from Poland’s Holy Cross Mountains and found 78 examples of trilobite queues, each containing up to 19 dime-sized trilobites of the eyeless species Trimerocephalus chopini. The creatures were sometimes found touching or even on top of each other, suggesting that the arthropods used a combination of physical contact and chemical signals to stay in formation. (Today, the spiny lobster uses contact as well as the scent of urine to stay close in poor-visibility, turbid ocean waters.) The T. chopini queues thus represent both the oldest known mass migration and the first fossil record of chemosensory cues, or chemotaxis, say the researchers, who report their findings this month in Palaeontology. The reason for their migration isn’t known, but the trilobites, like lobsters, may have been following a seasonal path to mating and breeding grounds.