Fossilised fish flock together

Palaeontologists have uncovered a remarkable fossil that suggests ancient fish followed the same rules of coordinated group behaviour as extant species: don’t swim too close to your neighbour, but also don’t stray too far from the flock.

Measuring just 570 by 375 millimetres, the fossil comprises a stone slab containing 259 fish identified as the extinct species Erismatopterus levatus, which lived in an intermountain lake at least six million years ago. 

“Although it remains unclear how the fish shoal’s structure was preserved in the fossil, these findings suggest that fish have been forming shoals by combining sets of simple behavioural rules since at least the Eocene,” the researchers write in the journal Proceedings of the Royal Society B.

“Our study highlights the possibility to explore the social communication of extinct animals, which has been thought to leave no fossil records.”

The slab was found in the collection of Japan’s Fukui Prefectural Dinosaur Museum, but the researchers – Nobuaki Mizumoto and Stephen Pratt from Arizona State University in the US and Shinya Miyata from Oishi Fossils Gallery of Mizuta Memorial Museum, Japan – estimate that it originally came from the Green River Formation in Colorado, US.

The fish are believed to have been juveniles and larvae, based on their size, and most are heading in the same direction. 

After analysing the positions and directions of individual fish, Mizumoto  and colleagues used estimated behavioural rules to develop a simulation model of collective motion, then compared group-level patterns of the simulated shoal with those of the fossilised fish group.

They found traces of two recognised rules for social interaction similar to those used by extant fish: repulsion from close individuals and attraction towards neighbours at a distance. 

“Moreover, the fossilised fish showed group-level structures in the form of oblong shape and high polarisation, both of which we successfully reproduced in simulations incorporating the inferred behavioural rules,” they write.

It wasn’t an easy task, if for no other reason than that fish swim in three dimensions but fossils exist in two. 

The researchers also had to examine the possible impact of water currents on spatial distribution, and test the alternative hypothesis that the apparent shoaling pattern was just an incidental result of the process of death, decay and fossilisation.

They ran a 1000 simulations in all, and feel confident in suggesting they have found “hard” evidence that modern fish behaviour had ancient origins. 

And the reason is probably the same as it always was – reducing predation risk.  

“To achieve effective avoidance of predation, it has been demonstrated that prey should move towards and align with their neighbours, to prevent being solitary or at the edge of the group,” the authors write.

“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. 

“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.”

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