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Sea Monster Battle Preserved for the Ages
5 May 2011 5:30 pm
One hundred and twenty million years ago, when a Cretaceous sea covered what is now South Australia, an ichthyosaur had a really bad day. Exactly what happened is a mystery, but telltale tooth marks found on a specimen of the 7-meter-long fishlike marine reptile indicate that this unfortunate individual was bitten on the snout during a prehistoric tussle. Fossil forensics has now identified the most likely culprit: another ichthyosaur.
When paleontologist Maria Zammit of the University of Adelaide in Australia and her colleague Benjamin Kear of Uppsala University in Sweden examined the remains of the ichthyosaur (Platypterygius australis), which was discovered in Marree, South Australia, they found a pair of tooth marks on the animal's lower jaw. Definitive evidence of interactions between prehistoric marine reptiles is rare. Tooth marks on bones have been found before, but often as damage left by attacking predators or scavengers that tore at the carcass after the animal died.
That doesn't seem to be the case here. The researchers found that the wounds on the Australian ichthyosaur had partially healed; remodeled bone around the bite marks confirmed that the animal lived for a long time after the trauma. Determining the identity of the attacker was a matter of narrowing down the line of potential suspects. Platypterygius swam in the same waters as large sharks and the monstrous, 10-meter marine reptile Kronosaurus—a short-necked, big-headed creature called a pliosaur—but these open-water hunters would have left very different bite patterns. Sharks would have left thin slashes in the bone, whereas a devastating Kronosaurus bite would be evidenced by deep punctures around a huge, missing chunk in the victim's skeleton. Likewise, these predators probably would have targeted the fleshy underbelly of the ichthyosaur rather than going for the jaw. Instead, the relatively shallow wounds on the ichthyosaur's lower jaw correspond to the tooth arrangement of another ichthyosaur, and the placement of the bite seems more suggestive of an aggressive interaction than an attack by a predator, the researchers will report next month in Acta Palaeontologica Polonica.
"We can't completely rule out an accidental encounter with another marine reptile," Zammit says, "but we think the spacing and shape of the bite marks, their position on a non-vital part of the body, and the range of aggressive interactions between reptile species favor our hypothesis."
What led one ichthyosaur to bite another on the snout is unknown. The orientation of the wounds hints that the attack most likely came from below; in the paper, Zammit and Kear hypothesize that the wound might represent a "restraining bite" left when the other ichthyosaur "attempted to neutralize the threat of a counter attack by clamping onto and forcing aside [the victim's] elongated jaws." Perhaps the two animals were competing over mates or territory, says Zammit, but unfortunately there is no sure sign of what might have sparked the conflict.
Paleontologist Anthony Martin of Emory University in Atlanta agrees that the injured Platypterygius was probably wounded in a fight with one of its own kind. "Whatever bit this ichthyosaur wasn't trying to kill it, just hurt it," he says. "And that's more likely to happen from competition or mating." Beyond that, however, what the fossil might mean for ichthyosaur behavior is unclear. Martin notes that the future discovery of similar fossil wounds may support the idea that ichthyosaurs had complex social behaviors. "Too bad we don't have enough information to say for sure it was a love bite," he laments, "because that would make for some really sexy paleontology."