The Experiential Education Office at UTM, in partnership with the City of Mississauga Central Library, hosts a monthly lecture series featuring UTM faculty and their research. Last Tuesday, I found myself at the Noel Ryan Auditorium at the Central Library, equipped with little-to-no knowledge of dinosaurs and dentistry. I was ready to cover professor Robert Reisz’s talk, a paleontologist from the biology department, titled “Dinosaur teeth and what they can tell us about their life and times.” The lecture surprised me when I walked out with a new-found interest in dinosaur teeth.
Did you know that you, as a human being, have something in common with alligators who are evolutionarily the closest thing to dinosaurs?
The alligator’s tooth structure from day one to day 40 is pleurodonty, which Reisz explained means, “They are actually solidly fused to the jaw [and] come in from the side instead of the top like mammals” but after day 40 the alligator “has become like the mammal, in that the new tooth is forming right underneath and will grow upwards.”
Reisz emphasized that “crocodiles are like us in that they retain the gomphosis in which the teeth are held in the jaw by these ligaments.” Meanwhile, other reptiles’ teeth are fused to the jaw, like the Komodo dragon and the lizard.
Following this revelation, he talked more generally about dinosaurs. “Dinosaurs in their primitive condition,” are relatively close to crocodiles in the evolutionary scheme of things. As Reisz explained, they have the same tooth structure as mammals and crocodiles.
The next question the paleontologist posed was, “What is primitive for amniodes?” I blacked out for a second, although I am certain I blacked out every time he introduced a new term. There were two six-to-eight year olds sitting at the very front, their jaws wide open in amazement. Inspired by their interest, I decided to make an honest effort to understand the lecture and follow the case for dinosaur phylogeny.
According to Reisz, there are two major categories of dinosaurs: saurischian and ornithischian dinosaurs. Ornithischians are all plant-eaters. Most saurischian dinosaurs were theropods, and thus, meat-eaters or carnivores while the rest were herbivores. Hence, the clear majority of all dinosaurs are plant-eaters.
Reisz mentioned that interestingly, “there are 1,200 teeth in a skull of an adult dactyl dinosaur.” Noticing a blanketing disbelief in the crowd, he added, “Now, how does that happen?” He described that these dinosaurs are unique in how there are up to five generations of teeth in one tooth position. In comparison, humans only have two.
Why are there five generations? “Initially, the tooth looks normal—it has an apex top, then a layer of enamel, followed by dentine and pulp cavity,” said Reisz. However, the tooth is bent over, and does not have a pulp cavity. In dentistry terms, Reisz explained, no pulp cavity means a dead tooth.
“These teeth are being used to grind plant material, and the new teeth are growing and then being pushed up,” His laser pointer hovered over an image on the screen. “The back teeth,” he circled, “will eventually be ground down.”
Reisz then moved onto the theropod dinosaurs. Theropod dinosaurs, literally meaning beast-footed dinosaurs, were described by the University of California Museum of Paleontology to “include the largest terrestrial carnivores ever to have made the earth tremble.”
“Their teeth are humongous,” said Reisz, pointing to a picture on the screen of one of his Ph.D. students standing next to the skull of a theropod. The skull is approximately half his size.
The teeth are like “sophisticated steak knives” because the edge of each tooth has serrations. Reisz added, “There’s something very peculiar about these teeth, when you cut them [where the serrations are], there’s a crack inside the tooth that leads to a hole,”
Reisz and his students didn’t agree with the argument that the crack was formed because of pressure and the creation of the space would serve to stop the crack from further extending. Reisz then wanted to determine whether the “ziphodont tooth was development or ontogenetic,” he said, “We love our technical terms,” he remarked to a laughing audience.
Reisz’s research group decided to compare the erupted tooth against the tooth that hasn’t still formed inside the gum. If the channel and space right beneath, or “globular dentine,” was formed due to pressure, they shouldn’t be able to see it in the still-forming tooth. But “Lo and behold! The globular dentine is seen,” he exclaimed. All theropod dinosaurs have them—the ones that don’t are those that became plant eaters.
“The reason has nothing to do with how hard the animal bites,” he continued, “The argument can be made that these interdental folds strengthen the tooth. The enamel is the hardest tissue, and it’s going very deep into the tooth which means it’s stronger than if it’s just on the surface,” Reisz concluded. This is not seen in other animals with similarly serrated teeth.
In the end, he shared his argument on the existence of soft tissue protecting dinosaur teeth, also commonly referred to as lips. He argued that had dinosaur teeth been exposed, there would be signs of wear on the teeth. “Elephant tusk, which are entirely exposed, have no enamel,” he emphasized. “This isn’t the case for dinosaur teeth.”
Soft tissue or not, understanding the evolution of dinosaur teeth has given me some theories of myself to chew on. Although it slipped my mind to ask the aforementioned children for their thoughts, I hope to catch up with them at the next “Lecture me!” event.