Juvenile fish sometimes cannibalised their younger fellows during the dinosaur era. They may have done so at times when other foods were scarce.
The behaviour was revealed by an exquisitely preserved set of fossils, which had been largely forgotten since the 1800s.
“This kind of stemmed from an accidental discovery,” says at the State Museum of Natural History Stuttgart in Germany.
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Cooper studies an extinct group of ray-finned fish called the Pachycormiformes, which lived during the Jurassic period. The group included Leedsichthys, one of the largest fish to ever exist. They were close relatives of the teleosts, which make up 96 per cent of modern fish species. “They became extinct right at the end of the Cretaceous, along with the dinosaurs,” he says.
During his PhD, Cooper found held in the British Museum in London. It included a long list of specimens of a species now called Pachycormus macropterus. Two supposedly had “undigested, young Pachycormus” in their guts, but no detail was given.
Cooper visited the collection, now at London’s Natural History Museum. He found there were actually three fossils of P. macropterus that had smaller members of the same species in their guts.
Other than cannibalism, the only possible interpretation is that P. macropterus gave birth to live young, in which case the internalised fish are fetuses rather than prey. However, Cooper says this probably isn’t the explanation. One of the specimens also had the remains of another, unrelated fish in its guts, and in any case all three specimens are juveniles.
“The great thing about gut contents is a lot of the ambiguity is gone,” says at the University of Tennessee in Knoxville. “It’s definitely cannibalism.”
The P. macropterus specimens all came from a quarry near Curcy in north-west France. The site no longer exists so it isn’t possible to re-excavate it.
Cooper says it is significant that Curcy is the only site where P. macropterus has been observed engaging in cannibalism. “We’ve got lots of specimens – this is a very abundant fossil fish from the Jurassic,” he says. That implies cannibalism wasn’t their normal behaviour.
Instead, Cooper suggests conditions at Curcy were unusually difficult. “Usually, when we see cannibalism at such a high level in a population in fishes, it’s because the environment is stressed,” he says.
The fossils from the site are mostly juveniles, with almost no adults. That suggests it was a nursery area: many fish leave their young in relatively sheltered, shallow regions where they are safer from big predators. Cooper suggests that other prey were scarce, so the older juveniles resorted to eating the younger ones. “It’s kind of like teenagers eating babies,” he says.
That makes sense, says Drumheller. Previous studies suggested that adult P. macropterus ate squid, but the juveniles ate fish – so they could potentially have been cannibals.
“Cannibalism is one of those odd behaviours where we know it happens a lot in modern ecosystems with modern carnivorous groups, but it’s something that doesn’t end up in the fossil record very often,” says Drumheller.
Journal of Vertebrate Paleontology
