The skull of the nearly 20-foot-long (6-meter-long) predator bears deep gouges and scratches on its slender snout.
"It was a really aggressive encounter" and featured a marine battle tactic not unfamiliar today, said paleontologist Benjamin Kear. "Modern marine animals tend to concentrate bites on the face, where all the dangerous hardware is at," said Kear, of Sweden's Uppsala University, who studied the ichthyosaur fossil with student Maria Zammit.
The attack, though, wasn't entirely successful for the aggressor—the wounds show signs of healing.
Swarming With Sea Monsters
Ichthyosaurs were dolphin-like reptiles that lived some 120 million years ago, during the Cretaceous period . Their toothy, three-foot-long (meter-long) snouts were equipped with roughly a hundred teeth to nab fish and squid in chilly southern oceans.
"Most of what you'd recognize in the oceans today were there, including sharks, shellfish, squid, starfish, and so on," Kear said. "Just take out whales and seals and replace them with ichthyosaurs, giant crocodile things with flippers, weird Loch Ness creatures, and other monsters."
(Explore an interactive sea-monster time line.)
One of those "giant crocodile things" was Kronosaurus, which grew to more than 40 feet (12 meters) long, had a head the size of a small car, and boasted teeth as big as bananas.
But while Kronosaurus may have eaten ichthyosaurs, the narrow bite pattern on the study fossil suggests the assailant was actually another ichthyosaur—a telling detail.
"Ichthyosaurs have no living relatives, so we have almost nothing to go on how they behaved or lived," Kear said. "These bite marks are priceless droplets of information, and they suggest ichthyosaurs have fought over carcasses, territory, or even each other."
Such insights were far from reach until recently. Though Australian miners digging a pipeline found the first fossil piece in the 1970s, the skull wasn't completed until after an Aborigine had stumbled upon the other bits in 2001.
Now that the skull and the bite study are complete, Kear and Zammit plan to probe the fossils for insights into how ichthyosaurs swam through the shallow inland oceans of the southern supercontintent Gondwana, which would eventually split into Africa, South America, Antarctica, and Australia.
(Also read "When Monsters Ruled the Deep" from National Geographic magazine.)
The research about fossilized ichthyosaur wounds will appear in an upcoming issue of the journal Acta Palaeontologica Polonica.