Fifty million years ago, a throng of thimble-sized fish met its untimely end. Now, a team of Japanese scientists may have uncovered the pristinely-preserved mass grave that immortalized these doomed creatures’ final moments. The fossil, described in a study published yesterday in the journal Proceedings of the Royal Society B, appears to showcase a social group, or shoal, of fish in a configuration reminiscent of what’s observed in modern species, suggesting that schooling behavior evolved many eons ago. But because fossils can take a while to form, it might be too early to tell whether this freeze frame accurately reflects how the group moved before its members died.

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“The slab surely does represent a shoal of young fishes,” Michael Benton, a vertebrate paleontologist at the University of Bristol in England who was not involved in the study, told Lucas Joel at The New York Times. “Whether the spacing represents the original is tricky, to be sure.” If these fish were locked into the fossil in their original configuration, this specimen could be evidence of ancient schooling behavior similar to what's seen in modern species. Image Credit: Mizumoto et al., Proceedings of the Royal Society B, 2019 The researchers, however, argue that this could well be the case. Study author Nobuaki Mizumoto, a biologist at Arizona State University, first stumbled across the fossil, a limestone slab believed to date back to the Eocene Epoch, at the Fukui Prefectural Dinosaur Museum in Katsuyama, Japan. Though the rock now resides in Asia, it originally hails from sediments in the Green River Formation, which spans what is now Colorado, Wyoming, and Utah. According to the researchers, the slab enshrines 259 Erismatopterus levatus—an extinct fish measuring less than an inch long—swimming in apparent synchrony.

To test this idea, Mizumoto and his colleagues analyzed the fish’s positions and orientations to see how they stacked up against modern schools. Free-swimming fish in today’s waters follow certain rules of attraction and repulsion to avoid colliding with their neighbors, while still maintaining cohesion. Though the researchers couldn’t track this kind of behavior in the fossil, they used a computer simulation to project how the members of the shoal may have moved, had they lived to do so. Their predictions, they say, match what one might expect of fish undulating along in a coordinated fashion. If that theory pans out, this could be crucial evidence of collective behavior in an ancient, extinct species—something that’s been sorely lacking in the fossil record, Mizumoto told Carolyn Wilke at Science News. But there’s a catch: For now, it’s impossible to tell what exactly killed the fish—or how fast it happened. Something quick, like a collapsing sand dune, could have frozen the fish in a matter of seconds, presenting a portrait of fish caught in the act. Any longer, however, and the burial process could have jostled some swimmers out of place, muddying their original positions in the lineup. By nature, the fossil is also just a two-dimensional representation of a three-dimensional scene. Even with these caveats in mind, the fossil remains noteworthy for the degree of detail etched into its face. “I think they did everything plausible [to bring the school to life],” Benton tells Joel.

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