Paleontologists report the oldest known case of mega-predation – one large animal eats another large animal. They discovered the remains of an astonishingly large prey in the belly of an ichthyosaur fossil around 240 million years old. It was about a four-meter-long marine reptile that the five-meter-long ichthyosaur devoured shortly before its death.
The ichthyosaurs were a species-rich group of marine reptiles that formed in the oceans after the mass extinction in the Permian about 250 million years ago and disappeared again about 90 million years ago. Their similarities to today’s marine mammals, to which they are not related, are particularly interesting. Similar to these, the ichthyosaurs evolved from terrestrial vertebrates and maintained lung breathing.
As far as their diet is concerned, some species may have primarily hunted squid, but others could also have assumed the position of top predator: Similar to modern orcas or the great white shark, they may have preyed on larger marine animals in their habitat. The current find now provides a direct indication of this. It comes from a quarry in southwest China and was dated to be around 240 million years old.
Dinosaur in the stomach
As the researchers around Da-Yong Jiang from Peking University report, they initially believed that they were only dealing with the remains of a five-meter-long Guizhouichthyosaurus – a representative of the ichthyosaurs. But then her attention turned to a large bulge in the area of the fossil’s belly. It turned out that there were the bones of the trunk of a Thalattosaurus originally around four meters long. It was a lizard-like water reptile that paddled through the water with its four limbs. The animal’s tail was discovered near the double fossil – apparently it was torn off. “The stomach contents of our ichthyosaur were not eaten up by gastric acid, so it must have died pretty soon after this ingestion,” says co-author Ryosuke Motani of the University of California at Davis.
The finding therefore suggests that about 240 million years ago the Guizhouichthyosaurus tore the marine reptile, which was slimmer but only slightly smaller than itself, to pieces and swallowed its trunk. “Now we can assume that some ichthyosaurs ate large prey, even if they had relatively blunt teeth,” says Motani. Predators that feed on large animals are often thought to have large, sharp teeth that are adapted to cut open their prey. However, Guizhouichthyosaurus had relatively small, cone-like teeth. “It’s clear, however, that you don’t necessarily need front teeth to be a mega-predator,” says Motani.
Big robber with humble teeth
Some of today’s species of predator such as crocodiles also have rather blunt teeth. Similar to these, Guizhouichthyosaurus could have used its teeth to grab and then tear prey apart. “It was previously suggested that a cutting edge might not be critical, and our discovery now supports that,” says Motani.
So far, however, paleontologists have not been able to say with certainty whether the Ichthyosaurus actually killed the Thalattosaurus or whether it only ingested its carcass. “Nobody was there to film it,” says Motani. However, there is evidence that it was not a case of scavenging. According to the researchers, when a dead thalattosaur was decomposed, the limbs would have been separated first rather than the tail. But the thalattosaur’s legs were still attached to the body in the stomach, while the severed tail was discovered a few meters away from the fossil. The researchers therefore suspect that it was torn off by the ichthyosaur when it was caught and left behind.
Whether or not he killed his last meal himself, the fossil now provides the oldest direct evidence that marine reptiles of the Triassic age consumed animals larger than humans. “This is also an indication that mega-predation in marine reptiles may have generally been more common than we previously thought,” says Motani. He and his colleagues will now continue digging at the location of the double fossil. They hope to find further traces there that provide insights into the mysterious world over 240 million years ago.
Source: University of California in Davis, Cell PRess, article: iScience, doi: 10.1016 / j.isci.2020.101347