In 2016, Nobuaki Mizumoto was visiting the dinosaur museum in his hometown of Katsuyama, Japan, when he came across an unexpected display—not of a dinosaur, but of a school of fish. It was embedded in limestone shale and exhibited in a corner with no particular fanfare. Yet the 50-million-year-old fossil was clearly extraordinary: 259 tiny fish bodies with eyes and spines and even fins. All but a few faced the same direction, as if frozen mid-swim.
Mizumoto does not specialize in fish or fossils, but he does study the collective behavior of termites at Arizona State University. He became intrigued by the idea that this fish fossil showed collective behavior, too. When fish form schools, they have to coordinate their swimming, staying together without crashing into one another.
Of course, behavior doesn’t usually fossilize well. Scientists have discovered other fossils featuring large groups of fish, but the fish tend to be jumbled, facing every which direction—as you would expect if a school of fish had died, sunk to the bottom of a lake, and been slowly buried in sediment.
Mizumoto wondered whether the fossil he saw had actually preserved the fish in position the very instant they died. He teamed up with a fish-fossil specialist in Japan, and they went back to the museum to photograph the fossil. Then they charted the position and heading direction of every single fish. Mizumoto found that if he looked at where the fish were and where they were going, they seemed to obey two rules that live, modern fish schools follow: repulsion from close neighbors (as if to avoid bumping into one another) and attraction to more distant fish (as if to stay together as a school). Their positions didn’t appear to be random.