Millions of years ago, a large shoal of fish were minding their own business in the rocky rivers of North America when suddenly, somehow – Bam! – their lives came to an abrupt end. By a stroke of luck (for us, not them), their final moments managed to become fossilized under a blanket of mud, suspending a snapshot of their life for perpetuity.
Not only is this fossil an incredible sight to behold, but it’s also teaching scientists about social interaction and collective behavior in ancient fish. Perhaps unsurprisingly, they appear to follow the same codes of conduct as present-day species; stay clear of your neighbor, but don’t stray from the flock.
The fossil was discovered in the Green River Formation, an Eocene geologic formation of fossil beds that spans 5 million years, recording the sedimentation of a group of intermountain lakes in the Rocky Mountains of present-day Colorado, Utah, and Wyoming, from around 50 million years ago. The specimen measures around 57 centimeters by 37 centimeters (22 x 14.5 inches) and contains the fossilized remains of at least 259 fish belonging to the extinct species Erismatopterus levatus. As you can clearly see, the fish are in very close proximity to each other and all appear to be heading in the same direction, strongly suggesting this was a traveling shoal of fish.
Reporting the journal Proceedings of the Royal Society B, life scientists from Arizona State University have now taken a deeper look at the position of the 250-odd fish and created a simulation model to estimate how the shoal and individual fish moved.
The collective behavior is a fine balance of repulsion and attraction, with the fish simultaneously being repulsed from close neighbors, so they don’t crash, but also show alignment attraction towards distant neighbors, so they don’t get lost from their pals.
Just like modern fish, the shape of the shoal suggests it served to reduce the risk of predators by diluting the risk of being eaten and confusing predators. It appears that the density of fish was higher in the safer central area, while lower at the edge of the group, where predators can easily attack. It’s also likely that the larvae and juvenile fish were kept hidden in the core of the shoal, while the outside was left to the healthier grown-ups.
It’s unclear how the fish shoal's structure was preserved in the fossil, however, palaeontologists have unearthed a number of similar fossils containing “frozen behaviors,” such as the mind-blowing fighting dinosaur fossil of a Velociraptor and a Protoceratops seemingly locked in battle.
"Although it remains unclear how the fish shoal's structure was preserved in the fossil, these findings suggest that fishes have been forming shoals by combining sets of simple behavioral rules since at least the Eocene," the study concludes. "Our study highlights the possibility of exploring the social communication of extinct animals, which has been thought to leave no fossil record."