Crushing Evidence
Round holes preserved on some ammonite shells have long been assumed to be bite marks left by mosasaurs—Late Cretaceous marine reptiles. However, Royal Tyrrell Museum invertebrate palaeontologist Dr. Paul Johnston felt the thinking flawed. He noted that the holes had smooth edges, rather than rough, broken edges that would result from a violent puncture. He developed an alternate hypothesis—attributing the round marks to limpets. Limpets are small snails that graze on algae and bacteria formed on hard surfaces. They have tiny teeth on their tongues that grind as they feed—resulting in impressions known as "resting marks" or "home scars." To test his hypothesis, elaborate, steel robotic jaws were fabricated to emulate those of a mosasaur. The jaws were then transported to the Philippines to be tested on Nautilus shells (the closest analogue to extinct ammonites). The resulting marks had rough broken edges precisely as Dr. Johnston predicted. This creative testing supported the new hypothesis. Subsequently, a fossilized limpet was discovered attached to an ammonite shell, enhancing the credibility of this position. However, despite robots, exotic locales, and unconventional analogues, the scientific community remains divided, and further investigation is required.
Originally published January 6, 2011.