The corpses of the victims are roughly 750 million years old.

An autopsy of their fossilized remains attests to gruesome deaths. The single-celled organisms, discovered in the cliffs of the Grand Canyon, are covered in tiny holes—telltale signs of microscopic vampiric attacks by predators that punctured their outer skin and then ate their innards, piece by piece.

“They’re clever little organisms,” says Susannah Porter, a paleobiologist at the University of California, Santa Barbara, who recently published a postmortem study of the microbial feeding frenzy in the journal Proceedings of the Royal Society B: Biological Sciences. She’s identified four distinctive types of wounds, suggesting that multiple species of mini-vamps hunted in Earth’s young oceans.

For Porter, it’s the timing of the