A smattering of white spots found among the ink in medieval books aren’t just printing errors — they’re actually an amazingly detailed “fossil” record of European beetles, new research finds. The dots represent spots, or wormholes, where hatching beetles chewed their way out of the woodblocks used to print art and illustrations between the 1400s and 1800s.

This literary record reveals that two species that now overlap in Western Europe once kept their distance from each other along the entire continent. Without evidence of the wormholes, this history would have been impossible to discern, said study researcher Blair Hedges, a biologist at Pennsylvania State University.