A restoration of Sinosaurus.
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Art by ДиБгд, image from Wikipedia.
A restoration of Sinosaurus.

The Tale of a Sinosaurus Tooth

A few weeks ago, I had the pleasure of wandering the Natural History Museum of Los Angeles dinosaur exhibit with paleontologist Justin Hall. I didn’t have long to spare, so Hall hit the highlights of the displays. Among them was a set of fused Triceratops vertebrae. No one knows what caused the pathology, Hall explained, but, damn, the mass of gnarly bone sure looked painful.

I have a special fondness for paleopathologies. Dinosaur teeth and bones offer plenty of clues about how those animals lived, but isolated bones and even reconstructed skeletons can often be miscast as static monuments rather than representations of prehistoric vitality. But signs of injury and disease on a skeleton are immediately-recognizable indicators that those fossils represent what was once a living animal, and reflect specific events in the life of an individual. Among the latest of such clues to be described is a damaged tooth socket in the mouth of an Early Jurassic predatory dinosaur.

The injured dinosaur was a Sinosaurus triassicus – a crested, sharp-toothed dinosaur found in the roughly 200 million year old rock of China’s Yunnan Province. Sinosaurus looked something like Dilophosaurus from the Early Jurassic rock of the American southwest, so much so, in fact, that specimens of Sinosaurus were previously and erroneously thought to be a new species of Dilophosaurus from China. But taxonomic identity aside, paleontologist LiDa Xing and colleagues point out, what makes this particular Sinosaurus special is an entirely closed sixth tooth socket on the right side of the upper jaw. This is the first time this kind of pathology has been described in a theropod dinosaur.

By itself, the filled-in tooth socket might not seem especially strange. Carnivorous dinosaurs often damaged their teeth while chomping away on victims and carcasses. Cracked and broken teeth, sometimes found embedded in the bones of other dinosaurs, show that dental damage was a regular part of a predatory lifestyle. But dinosaurs continuously replaced their teeth throughout their lives. When a Sinosaurus lost a tooth, there was already another developing tooth slowly moving into place. So why would a dinosaur with a constantly-replenished tooth supply have a totally closed tooth socket?

Discerning exactly what caused dinosaur injuries is extremely difficult, if not often impossible. But based on the nature of the pathology, Xing and coauthors suspect that the Sinosaurus injury was caused by trauma rather than disease. The closed tooth socket lacks the messy, disorganized bone expected as the result of an infection, and instead hints that this dinosaur damaged the tooth so dramatically that even the replacement teeth in the jaw were wounded. Perhaps Sinosaurus clamped onto a Lufengosaurus just a tad too hard, or otherwise miscalculated during a bite. Whatever the case, that tooth position was so wrecked that the dinosaur’s jaw shut down the dental conveyor belt and closed up the socket.

But the Sinosaurus was tough. The dinosaur didn’t break a tooth and quickly perish. The fact that the tooth socket was fully closed indicates that the Sinosaurus lived for months or years after the injury, taking down prey with one less tooth in its arsenal.