Stone slab reveals oldest evidence of schooling behavior by fish

May 29 (UPI) -- Fish were swimming in unison at least 50 million years ago. Scientists have discovered 259 fossilized fish embedded in an ancient stone, bunched together and all facing the same direction -- the oldest known evidence of schooling behavior.

Researchers at Arizona State University found the fossilized fish while examining collections at the Oishi Fossils Gallery of Mizuta Memorial Museum in Japan.

Scientists identified the small fish as Erismatopterus levatus, an extinct species that lived in alpine lakes. Though researchers were unable to confirm the cause of death, the team of scientists suspect a sand dune suddenly collapsed, trapping and killing the entire school of fish.

Because all the fish are facing the same direction, researchers suspected the fossils represented a school of fish. To investigate their hunch, scientists took detailed measurements of the fish and their positioning. Using the measurements, researchers built a model to simulate the behavior of the fish.

The model ran dozens of scenarios, each with varying water flow and spatial distribution. The simulations suggest the ancient fish formed schools for the same reason fish do today -- to protect themselves from predators. The model showed the density of fish was greatest at the center of the school, where it is safest.

The simulation results -- detailed this week in the journal Proceedings of the Royal Society B: Biological Sciences -- also suggest fish schools are governed by the same two rules that dictate modern schooling behavior: "repulsion from close individuals and attraction towards neighbors at a distance."

Aversion to the closest fish helps prevent collisions, while attraction to neighbors that are a bit farther away helps the fish school keep its shape.

"Our study highlights the possibility of exploring the social communication of extinct animals, which has been thought to leave no fossil record," researchers wrote.