A CT image of the jaw of a Diplodocus shows five backup teeth available in… (Michael D'Emic /PLOS One )
Dinosaurs almost bankrupted the tooth fairy. New research shows that the lumbering plant-eaters called sauropods produced new teeth as often as twice per month and had up to nine backup teeth in a single tooth socket.
While the fearsome Tyrannosaurus rex is known as the king of the dinosaurs, the sauropods were the real royalty. These creatures, including the childhood favorite Apatosaurus (previously known as Brontosaurus), were the largest animals that ever lived on land.
"A big T. rex is maybe 40 or 45 feet tall, but a big sauropod pushes 100 feet long or more," said Michael D'Emic, a vertebrate paleontologist at Stony Brook University in New York and lead author of the teeth study published Wednesday in the journal PLOS One.
Having never let go of a childhood fascination with dinosaurs, D'Emic wondered how the enormous size — and hence diet — of sauropods may have affected the evolution of their jaws and teeth. Since they were avowed vegetarians, D'Emic assumed their teeth would be well worn from munching copious amounts of tough plant matter.
To investigate, he and his team scoured dozens of museum collections in search of sauropod jaws. Teeth were easy to come by, but persuading a museum curator to let him tear apart an entire jaw was difficult.
"I had to search collections for jaws that had a lot of teeth but that were ratty or fragmentary," D'Emic said.
He lucked out, finding a Camarasaurus jaw dug up in southern Utah and a Diplodocus jaw from Colorado. Both animals lived about 150 million years ago in the Jurassic period, and were types of sauropods that lived side by side.
The next step was to break apart the jaws and remove not only the 30-odd visible teeth individually, but whatever backups were embedded in the sockets below.
In other words, he needed to pull some teeth.
"That had to be done by a professional, somebody with a surgeon's hands and patience," D'Emic said.
Patience indeed. Behind every visible tooth, the Camarasaurus had three backup teeth lined up and ready for use. The Diplodocus had even more — five spares behind each visible tooth.
It took six months for the dino-dentist to extract the teeth, embed them in a special resin, saw them up, mount them on slides, sand and polish them by hand and then photograph them for study.
In addition to the large number of teeth in the pipeline, D'Emic and his team were interested in how quickly those teeth would come in.
Cross-section images revealed lines in each tooth's dentin, the layer below the enamel. Since a new layer grows each day, the researchers could figure out the age of a tooth by counting those lines, much like counting rings on a tree stump. And by looking at the intervals in age between successive teeth in a socket, they could estimate how quickly teeth were replaced.
Camarasaurus, with its larger, broader-crowned teeth, had a new tooth come in about once every two months. Diplodocus, with smaller, narrower teeth, had a replacement about once a month.
When the scientists used a mathematical model to extrapolate their findings to other types of sauropods, they estimated that one genus called Nigersaurus replaced its teeth every two weeks, with nine spares for every tooth.
D'Emic also noticed that more recent sauropods had evolved smaller, narrower teeth that got replaced a lot faster, like Diplodocus. He wondered: Why the shift?
Sauropods didn't actually chew their salad — they clipped it off with their teeth and then swallowed it whole. Perhaps having teeth turn over faster kept them fresh for the hard work of cutting vegetation off trees or bushes, D'Emic said.
Another theory is that given the limited utility of its teeth, Diplodocus invested its bodily resources in other tissues, preferring a quantity-over-quality approach.
"All the tooth had to do was snip," D'Emic said.
Finally, there was the question of how two plant-eating dinosaurs the size of a mega-yacht could coexist in the same environment. D'Emic thinks the answer lies in the teeth.
If Diplodocus grazed vegetation close the ground, it would have accidentally eaten a fair amount of dirt — and the sauropod's narrow teeth could have been an adaptation for this grittier diet, he said. With Diplodocus eating plants down low, Camarasaurus could have foraged up high, providing enough room for everyone.
Paul Sereno, a University of Chicago paleontologist who was not involved in the study, said he found D'Emic's argument convincing.
"Last night I had a salad that I grew in my own yard for the first time, and I really had to wash the lettuce carefully, because there's this grit in it," said Sereno, who also studies sauropods. "It's exactly what we're talking about. You put your muzzle down to the ground and you're going to be incorporating these sand grits that wear your teeth down."