In an astounding new discovery, researchers have come across a totally preserved large school of fish that swam around 50 million years ago.

Researchers from Arizona State University and the Oishi Fossils Gallery of Mizuta Memorial Museum in Japan discovered a slab of limestone shale containing fossils of nearly 300 fish and is thought to be a school that swam in unison some 50 million years ago.

Close examination of the slab and the fossil discovery showed that an amazing amount of 259 fossilized fish were all the tiny extinct species, Erismatopterus levatus, as per Daily Mail.

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Though it’s unclear how the school died, but the team believes that their demise took place ‘near instantaneously’ and were all in the same place when the event happened. However, the researchers hypothesize that a sand dune in shallow water might have collapsed right on top of the fish, wrote Science Alert.

“Considering that dead bodies in an assemblage of carcasses would be positioned all over the slab, the observed localized aggregation is likely to be the result of behavior rather than an artifact of fossilization,” the researchers said. “Also, the sediment is fine-grained mud, which is one criterion for an in situ rather than transported assemblage.”

In order to understand what might have happened, the team stimulated 1,000 possible scenarios consisting of the measurements of the fish and also of various types of water flow and spatial distribution. All the simulations suggested that the fish were swimming in unison (shoals), similar to how fish do today, which helped them stay safe from predators.

“Because we found evidence of approach from a distance in our fossilized group of E. levatus we can reasonably infer predator avoidance as a selective pressure leading to shoaling behavior,” the researchers wrote in the study.

“Consistent with this, the density within the group was higher in the safer central area, while it was lower at the edge of the group, where predators often attack,” they continued in their study published in Proceedings of the Royal Society B.