All the better to chew you with, my dear
Hadrosaurs were the ‘duck billed’ cows of the Cretaceous. They got this reputation firstly for their unusual ‘beaked’ mouths, and secondly for their apparent ability to chew food with amazing efficiency, often depicted grazing like cattle.
Hadrosaurs had teeth arranged into what are called dental batteries – rows of stacked teeth designed for crushing and grinding tough, fibrous plant matter to increase its digestibility. These tooth batteries could contain up to 300 individual teeth in each part of the jaw!
Despite not being as ‘media sexy’ as giant sauropods or ferocious theropods, these dental batteries are the most complex tooth forms ever developed in the whole history of vertebrate evolution. And that’s pretty damn awesome.
What is a little odd though is that despite wide interest in these amazing teeth, very little is known about their evolutionary origins. Which is even odder because dinosaur researchers are usually really good at obsessing over little things like this.
Research by Aaron LeBlanc and colleagues looked at the development of the different tissues making up hadrosaur teeth through individual growth series to see just how these complex tooth batteries formed.
They did this by slicing up the teeth of hadrosaurs into really thin slices called thin sections. These are thin enough that you can look in great detail at the three-dimensional microstructure of the bone under a microscope, and see the different and exquisite tissue types and textures.
What the team discovered is that the dental batteries formed through a combination of being able to stop the process of normal tooth replacement and then also allowing the root of the teeth to form part of the grinding surface – also known as the occlusal surface (because it’s the surface with which the tooth sets ‘occlude’).
This is unusual as usually in other amniotes (egg-laying animals), the enamel forms a complete covering crown over the core of the tooth (you can feel this right now with your tongue). But in hadrosaurs, the enamel shifts to one side, and the tissue that makes up the core, called cementum, forms the other side of the tooth.
This tooth form exists not just in the adult hadrosaurs, but also in teeny embryonic specimens and those preserved as hatchlings. What this means is that even at the earliest stages in development, both the enamel and the cementum start forming at the same time as opposed to one after the other.
Because hadrosaurs also did not shed their teeth, like most other vertebrates, this means that they retained multiple ‘generations’ of teeth in stacked positions on top of each other. What seems to happen here is that instead of teeth being pushed to the side as new ones emerge, older teeth are resorped quite a bit, which leads to a tight and solid interlocking structure.
The result of this is a highly complicated and integrated ‘matrix’ of different bits of tooth, including living replacement and dead grinding teeth, all connected by a flexible network of ligaments.
Rather than a single, solid block then, it is better to view hadrosaur dental batteries as more similar to scales on medieval armour or shark skin – here, each single element is strong and rigid, but the material connecting them is flexible and makes interactions between groups of teeth more efficient. This complexity is undoubtedly one of the reasons for the evolutionary success of hadrosaurs, and while they might not look as spectacular as other groups of dinosaurs, they were certainly no less impressive!
Reference
LeBlanc et al. (2016) Ontogeny reveals function and evolution of the hadrosaurid dinosaur dental battery, BMC Evolutionary Biology. 16:152, DOI 10.1186/s12862-016-0721-1 (OA link)