The 50-million-year-old slab of limestone suggests that fish have been swimming in unison for far longer than previously realised
Fish can band together, sometimes in the millions, to form a school or shoal. They will move as one, while many animals, from birds and fish to insects, are known to travel in formations, just when this behaviour arose in the evolutionary timeline isn’t yet certain.
“It looks like an actual fish shoal,” said Dr. Mizumoto, who studies animal behaviour and presented his research in Proceedings of the Royal Society B on Wednesday.
The fossil captures a mass of 259 fish apparently swimming in the same direction. All identified as the tiny extinct species, Erismatopterus levatus. It’s unclear how exactly they died, but the researchers say the evidence suggests they were all in the same place at the time of the event. And, their demise took place ‘near instantaneously.’ 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.
‘Considering that dead bodies in an assemblage of carcasses would be positioned all over the slab, the observed localised aggregation is likely to be the result of behaviour rather than an artefact of fossilisation,’ the researchers note in the paper.
‘Also, the sediment is fine-grained mud, which is one criterion for an in-situ rather than transported assemblage.’
Researchers analyzed the position and orientations of hundreds of tiny fish, from 10 to 23 millimeters long, to see if individuals followed rules of schooling similar to what fish follow today.
In 2016, Arizona State University biologist Nobuaki Mizumoto chanced upon an unusual fossil while exploring the collections of Japan’s Fukui Prefectural Dinosaur Museum. Dated to around 50 million years ago, the limestone slab—originally discovered in the United States’ Green River Formation, an area now occupied by Colorado, Wyoming and Utah—appears to depict a school of fish frozen in the midst of migration.
It’s difficult, for instance, to find evidence of schooling fish in the fossil record. You need just the right circumstances to fossilise something like a school of fish in place within a rock. Then, that rock has to survive intact long enough for a palaeontologist to discover it and study it.
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 neighbours to avoid collisions, but stick with the group by tracking with farther away fishes.
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.
To better understand what might have happened, the researchers simulated 1,000 possible scenarios incorporating the measurements of the fish as well as different types of water flow and spatial distribution.
And, these simulations suggest they were swimming in unison much like fish are known to do today.
The prehistoric school seems to adhere to the laws of attraction and repulsion, with members maintaining enough distance between neighbours without straying too far from the group (Mizumoto et al.)
“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.” Dr. Mizumoto explained.
Because collective behaviour is seen in so many animals, including the flocking of birds or swarming of insects, scientists believed such behaviour evolved long ago. But there has been scant evidence in extinct species, says Nobuaki Mizumoto, a behavioural ecologist at Arizona State University in Tempe.
Other researchers were impressed by the specimen, although they question whether it proves the existence of prehistoric schooling behaviour.
‘Because we found evidence of approach from a distance in our fossilised group of E. levatus we can reasonably infer predator avoidance as a selective pressure leading to shoaling behaviour,’ the researchers write.