Bite Marks Tell of Tussling Ichthyosaurs
The prehistoric world was intensely violent. So I believed when I was a kid, anyway. Almost every book I read or movie I saw about now-fossilized creatures showed them as ferocious monsters that were constantly biting and clawing at each other. I spent hours with plastic toys and mud puddles reenacting these scenes myself, never thinking about whether there were any fossil traces of such epic battles.
Finding fossil evidence of ancient conflicts is very difficult. A predator might leave behind traces of feeding—such as toothmarks on bone or undigested muscle tissue in their fossilized dung—but the signs of prehistoric fights are very rare. Sometimes, as in the case of the holes in the frill of the horned dinosaur Nedoceratops, what were thought to be injuries inflicted by fighting with animal turn out to be a different kind of pathology or a strange bone growth pattern. Nevertheless, a few signs of prehistoric conflicts have been found.
A little more than 100 million years ago, the large ichthyosaur Platypterygius australis swam the seas of Cretaceous Australia. It was not a dinosaur—not even close—but a marine reptile belonging to a lineage that had returned to the sea many millions of years before. Thanks to new fossil evidence reported by Maria Zammit and Benjamin Kear in an in-press Acta Palaeontologica Polonica paper, we now have evidence that one of these ichthyosaurs may have had a painful run-in with one of its own kind. SAM P14508, a Platypterygius found in South Australia, has a distinctive set of healed wounds on its lower jaw that were most likely made by another ichthyosaur.
The key to the Platyptergius puzzle was the fact that the animal survived its injuries. Had a predator been feeding on the carcass, Zammit and Kear would have found distinctive toothmarks without any signs of healing. Since the injured bone had grown and remodeled after being damaged, though, the ichthyosaur clearly lived for quite some time after being attacked. The bones were scored rather than deeply punctured or broken; while painful, the injuries would not have debilitated the ichthyosaur.
Naturally, predators aren't always successful and might injure an animal without killing it, but the pattern of the wounds lead Zammit and Kear to propose the marks were made by another Platyptergius. The only other large predators in the area at the time were the enormous pliosaur Kronosaurus and large sharks, both of which would have left very different bite marks and probably would have attacked part of the body containing vital organs rather than the snout. Zammit and Kear are tentative about their conclusions—without a time machine and some scuba gear, we can't know for sure what happened—but the wounds on the specimen are consistent with the damage another Platyptergius could have inflicted. "t is tempting to reconstruct the positioning of the marks on the ventral side of the mandible as the result of a restraining bite," they write, "delivered when another ichthyosaur approached SAM P14508 from below and attempted to neutralize the threat of a counter attack by clamping onto and forcing aside its elongate jaws."
Zammit, M. and Kear, B.J. (2011). Healed bite marks on a Cretaceous ichthyosaur Acta Palaeontologica Polonica, 5 : 10.4202/app.2010.0117