Scientists have discovered macabre evidence suggesting 300 million-year-old Orthacanthus sharks ate their own young, as fossil poop of these fearsome marine predators contained the tiny teeth of juveniles.
These prehistoric sharks used protected coastal lagoons to rear their babies, but it seems they also resorted to cannibalising them when other food sources became scarce.
Three hundred million years ago, Europe and North America lay on the equator and were covered by jungles (the remains of which are now compacted into coal seams).
The top predators of these so-called "Coal Forests" were not land animals, but huge sharks that hunted in the oily waters of coastal swamps.
The fossil evidence for shark cannibalism comes from distinctive spiral-shaped coprolites (fossil poop) found in the Minto Coalfield of New Brunswick in Canada.
The poop is known to have been excreted by Orthacanthus because this shark had a special corkscrew rectum that makes identification easy.
The poop is packed full of the teeth of juvenile Orthacanthus, confirming that these sharks fed on their own babies. This is called "fillial cannibalism".
"Orthacanthus was a three-metre-long xenacanth shark with a dorsal spine, an eel-like body, and tricusped teeth," said Aodhan O Gogain, PhD candidate at the Trinity College Dublin in Ireland.
"There is already evidence from fossilised stomach contents that ancient sharks like Orthacanthus preyed on amphibians and other fish, but this is the first evidence that these sharks also ate the young of their own species," said Gogain.
"As palaeontologists cannot observe predator-prey relationships directly in the way that a zoologist can, they have to use other methods to interpret ancient food webs," said Mike Benton, professor at University of Bristol in the UK.
"One method is by probing the contents of coprolites (fossil poop) as we have done here," Benton said.
"We don't know why Orthacanthus resorted to eating its own young. However, the Carboniferous Period was a time when marine fishes were starting to colonise freshwater swamps in large numbers," said Howard Falcon-Lang, from Royal Holloway University of London.
"It's possible that Orthacanthus used inland waterways as protected nurseries to rear its babies, but then consumed them as food when other resources became scarce," Falcon-Lang said.
"Orthacanthus was probably a bit like the modern day bull shark, in that it was able to migrate backwards and forwards between coastal swamps and shallow seas," said Gogain.
"This unusual ecological adaptation may have played an important role in the colonisation of inland freshwater environments," he said.
The findings were published in the journal Palaeontology.
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