A slab of rock and the methods used to study it could offer clues to when a behavior common in fishes first evolved.
Fish can band together, sometimes in the millions, to form a school or shoal. They will move as one, like a flock of birds, so long as each fish stays in line with the fish that surround it.
Modern fish, as well as other kinds of animals, already know how to move as one. But unraveling when in Earth’s deep past this behavior evolved has been a tough fish to fry for scientists.
It’s difficult, for instance, to find evidence of schooling fish in the fossil record. You need just the right circumstances to fossilize something like a school of fish in place within a rock. Then, that rock has to survive intact long enough for a paleontologist to discover it and study it.
But that is just what may have happened.
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In 2016, Nobuaki Mizumoto, a biologist at Arizona State University, was on vacation with his wife when he came across a slab of grayish limestone rock at the Fukui Prefectural Dinosaur Museum in Katsuyama, a small city in Japan.
The slab, about 22 inches wide and 15 inches tall and estimated to be about 50 million years old, preserves what looks like a school of fish belonging to an extinct species called Erismatopterus levatus. There are 259 fish in the slab, all of which are under an inch long, and they are all facing the same way.
“It looks like an actual fish shoal,” said Dr. Mizumoto, who studies animal behavior and presented his research in Proceedings of the Royal Society B on Wednesday.
“Gee god, that’s an interesting specimen,” said Roy Plotnick, a paleontologist at the University of Illinois at Chicago, who was not a part of the study. “I’ve never seen this kind of preservation.”
If you look closely at a modern fish school, Dr. Mizumoto explained, there are a few rules that each fish follows so that the school stays together. These rules include attraction, whereby the fish move closer to their neighbors, and repulsion, whereby the fish increase their distance from their neighbors.
Dr. Mizumoto and his team think that the fossil fish show these behaviors in action, which would reveal that fish had the know-how to form schools much earlier than previously known.
Given the direction each fish seemed to be heading as the school died, the team created a projection of what the next “snapshot” of the school would have been had it not been frozen in stone.
The anticipated trajectories of the fish, Mr. Mizumoto said, could suggest a school that’s sticking together.
But does the fossil truly represent what the school looked like in life? Other scientists say more proof is required to show that the fish became buried within the space of a few seconds.
“The slab surely does represent a shoal of young fishes,” said Michael Benton, a vertebrate paleontologist at the University of Bristol in England. “Whether the spacing represents the original is tricky, to be sure.”
Dr. Mizumoto and his colleagues “argue for an instantaneous burial model but don’t have evidence for that,” Dr. Benton added.
If burial took any longer, then it would not be possible to draw conclusions about what the fossils reveal about schooling behavior.
“I can’t picture a three-dimensional school of fish sinking to the bottom and maintaining all their relative positions,” Dr. Plotnick said. “That makes no sense to me.”
In the paper, Dr. Mizumoto and his colleagues acknowledged the possibility that the fish were already dead when they gathered, and not engaged in schooling behavior at the time they fossilized.
Further study is also difficult because of mysteries about the slab’s precise origin. This species of extinct fish comes from rocks found in Wyoming. Aside from that, little is known about where or how it formed, even though such knowledge could help shed light on how the fish died.
While the exact behavior of these fish may remain unknown, Dr. Benton knows of no other fossil that comes as close to showcasing how long-extinct fish may have schooled. Dr. Mizumoto and his team’s methods could have other applications, and they did the best they could to bring the school to life, he added.
“I think they did everything plausible.”
Earlier reporting on fascinating fossil finds