A slab of grayish limestone shale from the Eocene Green River Formation, the United States, caught a school of Erismatopterus levatus, an extinct species of freshwater fish.
Collective behavior in a school of Erismatopterus levatus. Image credit: Mizumoto et al, doi: 10.1098/rspb.2019.0891.
The limestone shale slab, about 22 inches long by 15 inches wide (57 by 37.5 cm), was collected from the Green River Formation, a fossil site located in western Colorado, eastern Utah and southwestern Wyoming in the United States.
It formed approximately 50 million years ago (Eocene epoch) and contains fossils of 259 fish, most of them heading in similar directions.
“We identified the species as Erismatopterus levatus based on their dorsal fin rays having two spines with six to seven soft fin rays, the base of their pelvic fin having a subthoracic location, and their anal fin rays having two spines with seven soft fin rays,” said Dr. Nobuaki Mizumoto of Arizona State University and his colleagues.
“Moreover, Erismatopterus levatus is known only from Lake Gosiute and Lake Uinta deposits of the Green River Formation.”
Lengths of the individuals ranged from about 0.4 to 0.9 inches (1.1-2.4 cm), which is much smaller than the descriptive specimen of this species (2.6 inches, or 6.5 cm, long), indicating that they were juveniles or larvae.
“The structure of the group shared characteristics with extant fish shoals,” the paleontologists said.
“The group showed an oblong shape longer in the direction of movement. This is the usual shape of extant shoals and is thought to protect against ambush predators by reducing the frontal area, where these predators tend to attack.”
The fossilized fish group also showed a high level of individual alignment to one another.
“Fossils with animals preserved while doing something are referred to as ‘frozen behaviors’,” Dr. Mizumoto and co-authors said.
“Examples include fighting dinosaurs, queuing trilobites and insects in copulation. These fossils are assumed to result from rapid burial, which preserves individual positions during interactions.”
“Our fish shoal was fixed near instantaneously so that individual positions and heading directions were preserved in the fossil.”
“Rapid fixation of the fish shoal might be possible by sand dune collapse on shallow water, which can produce a bed in only seconds or minutes,” they added.
“Sudden freezing caused by supercooling could also explain rapid fixation, although this seems unlikely, given the warm climate estimated for the Eocene Green River Formation.”
“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 researchers concluded.
A report on the discovery was published in the Proceedings of the Royal Society B.
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Nobuaki Mizumoto et al. 2019. Inferring collective behaviour from a fossilized fish shoal. Proc. R. Soc. B 286 (1903); doi: 10.1098/rspb.2019.0891
