Fossilized fish captured mid-swim offer a rare glimpse into extinct animal behavior — and suggest that swimming in schools developed at least 50 million years ago.
A limestone shale slab from the Eocene Epoch reveals that extinct, thimble-sized fish called Erismatopterus levatus may have coordinated their motion similar to how fish in groups move today, researchers report May 29 in Proceedings of the Royal Society B.
The fossil captures a mass of 259 fish apparently swimming in the same direction. It’s unclear what killed the fish. But a suddenly collapsing sand dune, for example, could have buried them in place in a flash, knocking just a few askew in the process, the researchers suggest.
Analysis of the fish’s positions and orientations suggests they followed the same rules of “attraction” and “repulsion” that govern fish shoals today: The fish are repelled from their nearest neighbors to avoid collisions, but stick with the group by tracking with farther away fishes.
Because collective behavior is seen in so many animals, including the flocking of birds or swarming of insects, scientists believed such behavior evolved long ago. But there has been scant evidence in extinct species, says Nobuaki Mizumoto, a behavioral ecologist at Arizona State University in Tempe.
Mizumoto, whose research usually focuses on how termites build together, stumbled across the fish shoal fossil in a museum in Katsuyama, Japan in 2016. The fossil originally came from sediments in the Green River Formation, a geologic formation spanning what is now Colorado, Wyoming and Utah.