Why don’t sharks need dentists? Believe it or not this isn’t the set up of a rather surrealist joke but instead it’s an interesting biological question. Sharks continuously loose their teeth throughout their life time and can have up to 30,000 teeth over that period. That’s over 900 times as many as you or I will ever have. When a tooth becomes damaged or worn a shark can simply shed it and have it replaced in a matter of days. Since the cavity left by a rotting or broken tooth is quickly filled by a pristine new one the scope for infection is limited. Anyone who’s ever had to sit through a root canal can only look on in envy at this arrangement.
So what’s this got to do with palaeopathology you might, reasonably, ask? In 2011 a study was published documenting the earliest known case of a severe tooth infection in a land-living animal. The patient was a smallish reptile from the species Labidosaurus hamatus. To us it would have resembled a large-headed lizard, around 30cm long, with a single row of small, conical teeth. The fossil was 275 million years old and some of its teeth was missing.
In and of itself missing teeth are not unusual in a fossil. Even otherwise well-preserved skulls are often missing many of their original teeth but this was different. CT scans of the jaw revealed severe damage to the underlying bone, a sure sign of a deep, wide-spread infection. The most likely explanation for this was that damage to the teeth had allowed bacteria to get into the pulp cavity where it spread to cause a devastating infection in the jaw.
What is fascinating about this is what it tells us about the lives of these animals. The teeth themselves were deeply embedded in the jaw, which is very different to the set-up in earlier reptiles, which was much more similar to modern-day sharks. Their teeth were only loosely attached to their jaws which would have meant they would fall out far more easily (sharks can loose up to 100 individual teeth a day) but they could also be replaced much faster. In animals like us and Labidosaurus the tooth is much more deeply rooted. This makes it far less likely to come out when you’re eating and allows you to actually chew your food. It does also mean though that tooth replacement is much slower. We humans only get two sets over our entire lives, Labidosaurus was a little more fortunate and could eventually replace a lost tooth but it would have taken a long time.
Chewing your food has significant advantages and allows you to also tackle a wider range of different foods. It releases more energy per mouthful and means you can spend less time digesting. Labidosaurus seems to have been an omnivore, one of the earliest known from the fossil record, with its deeply embedded teeth allowing it to tackle both meat and plants. This would have given it far more options in lean times which would have boosted its chances of survival and reproductive success. The downside is it also left them open to tooth infections, something their ancestors never had to worry about. Given that fossil of Labidosaurus and their relatives are extremely abundant in the geological record, and are found all over the world, this seems to have been an evolutionary trade that ultimately paid off.
So next time you find yourself sitting in the dentist’s chair just reflect on the fact that, painful as it might be in the moment, your root canal is actually the result of an evolutionary novelty that allowed your tiny reptilian cousins to conquer an entire planet.
Robert R. Reisz, Diane M. Scott, Bruce R. Pynn, Sean P. Modesto. Osteomyelitis in a Paleozoic reptile: ancient evidence for bacterial infection and its evolutionary significance. Naturwissenschaften, 2011