Dinosaur Dentistry, Part 3: steak knives and dental batteries

In the third and final entry about the Dinosaur Dentistry event held at the University of Alberta, I wanted to talk about what makes studying dinosaur teeth so interesting. I’ve pointed out previously how at a fundamental level dinosaur teeth and human teeth are built from the same building blocks. That’s because the teeth in dinosaurs and mammals are made of the same dental tissues: enamel, dentine, and the periodontal tissues. Sure, there are differences in the thicknesses and arrangements of these tissues around a given tooth, but the tissues themselves are the same. But what this entry focuses on are some of the creative ways that dinosaurs used these dental tissues to make some very different and complex structures. This is a story about steak knives and dental batteries!

 

Hidden inside the steak knife teeth of theropods

Ah yes, the theropods. The media hype-machine surrounding these meat-eating dinosaurs is enough to make a dino enthusiast giddy with excitement by the mere mention of the word. It’s also enough to drive a non-dinosaur palaeontologist mad with jealousy. Say what you will about theropods or theropod specialists (when will you NOT be in the news??!!), but these dinosaurs have impressive teeth.

The blueprint of a theropod tooth is simple: slender or slightly more rounded, recurved, and often serrated like a steak knife. The appeal is obvious. They look built to do some damage to a hapless victim and help the bearer of this bad news tear the flesh from the bones of its prey. The most exciting part of a theropod tooth in thin section is the cutting edge, which, as I said, is often serrated.

Tooth serrations are little bumps on the cutting edge of a tooth that give it extra cutting power. In theropods, the serrations are often quite large and you can still feel them when you run your finger along the cutting edge of a tooth. Dr. Kirstin Brink and I (along with several other colleagues) examined these serrations more closely in different kinds of theropod dinosaurs. Each serration is made from an enamel cap and an underlying dentine core, but between each pair of serrations (also called denticles when they have a dentine core) is a curious little structure. Where the enamel and dentine of two neighbouring denticles meet is a thin channel that extends deep into the tooth, terminating in a flask-shaped structure.

Brink et al Fig1

Here’s a figure from the original paper (Brink et al., 2015) where you can see the serrations on the leading and trailing edges of a theropod tooth in thin section. The enamel is just the outer white band in the thin sections. The rest is dentine. The skull illustration was made by palaeoartist Danielle Dufault.

Charcharodontosaurus ampulla

A closeup of the “ampulla” structure along the cutting edge of a theropod tooth in thin section (image courtesy of Kirstin Brink). The pretty colours come from using a special microscope effect called a lambda filter.

This structure was called an ampulla by its original discoverer, Abler, who described it as a crack-stopping structure in the teeth of tyrannosaurid theropods. The idea was that these structures prevented cracks from spreading into the deeper layers of dentine when these hard-biting theropods chomped into their food. Our work, however, showed that this weird-looking structure is actually common to many meat-eating dinosaurs, regardless of the build or presumed bite force of the animal. Moreover, Dr. Brink had the bright idea to section the developing teeth in theropods as well (the ones that had not yet erupted into the mouth and were still hidden deep within the jaw) to test if the “ampulla” forms from use of the tooth, or if it is a native feature between each denticle. Amazingly, this structure forms in teeth before the teeth ever erupt into the mouth, and they form from deep folds of the sheets of cells that form each tooth, prior to the formation of enamel or dentine.

This means that the “cracks” between denticles aren’t cracks at all, but features that form with the developing tooth. The “cracks” are actually channels through the enamel that form because of the tight space between each denticle. Basically, the enamel-producing cells just can’t make any enamel in that cramped of a space. Same with the “ampulla”. The ampulla is a dentine feature that is a byproduct of geometry: if you fold a piece of paper to make two big “denticle” shapes, the spot in the middle will make a flask-shaped structure.

Ampulla test

Another figure from that same paper (Brink et al., 2015) showing what the serrations of erupted (top row) and unerupted (bottom row) theropod teeth look like in thin section. Because we found the channel and fold structure (formerly the “ampulla”) even in unerupted teeth, it must form with each tooth WITHIN the jaw and could not possibly be a structure that forms during feeding.

For this reason, we renamed these structures “folds” and “channels” to highlight what they really are: deep folds of the outer surface of the tooth that give each denticle a deeper, better supported “root” into the body of the tooth. It may not be as sexy of an interpretation as the crack-stopping hypothesis, but it fits the data much better. On top of that, these structures aren’t found in the serrated teeth of most other top predators, including sharks, lizards, and our distant relative, Dimetrodon, so chalk one up for the theropods!

 

Looking inside a hadrosaur dental battery

Alright. So the meat-eating dinosaurs had some interesting teeth. But for me, the hadrosaurs (i.e. the “duckbill” dinosaurs) took things to another level. Hadrosaurs have stacked and coalesced their teeth together into four large dental batteries (one in each quadrant of the jaw: upper left, lower left, upper right, and lower right). The net effect is that each dental battery acts as a single large grinding platform with which hadrosaurs could pulverize tough plants. But how did they start with a “typical” dinosaur tooth and evolve such complex structures?

Hadrosaur teeth overview

A collection of images of a hadrosaur skull and – my favourite part – the dental batteries!

This was the subject of a part of my PhD work, published back in 2016. It involved a lot of slicing up of dental batteries to peer inside these structures and get at the individual teeth. So. Here’s what a single hadrosaur tooth looks like:

Hadro tooth in section

In thin section, hadrosaur teeth look more like the incisor teeth in modern rodents, like beavers. The enamel is found on one side of the tooth and cementum is on the other. The black arrows point to the boundaries between the two tissues in the two planes of section.

As I mentioned before, all dinosaur teeth are made of the same dental tissues as our teeth. But the difference here is in how the tissues are arranged around the tooth and how they develop. For instance, instead of forming an enamel cap and a root on the bottom, hadrosaur teeth have enamel on one face of the tooth, and the cementum root on the other. The other major difference is in how FAST the tooth tissues develop in a hadrosaur. When you cut open a dental battery, a vertical stack of teeth actually shows you different stages in the development of a hadrosaur tooth.

Figure S5 Hadro model labeled

A BEAUTIFUL medical-style illustration done by Danielle Dufault showing the internal anatomy of a hadrosaur dental battery (found in the paper: LeBlanc et al., 2016). This is an upper jaw, so the grinding surface is at the bottom. New teeth (at the top) have big open pulp chambers, but as they near the grinding surface, the dentine plugs the inner core. This allows hadrosaur teeth to be ground down completely. This illustration is inspired by an actual fossil where the one wall was prepped away and the surface in the foreground was cut by a saw. The detail comes from actual thin sections of that specimen that helped fill in the gaps.

Each tooth starts off as a small cone of dentine, enamel, and cementum, but very quickly as it erupts towards the grinding surface, the dentine starts to close off the vital internal pulp. This happens so rapidly and so extensively that each tooth “zips up” like a jacket zipper, closing off and destroying the vital pulp inside as the dentine grows inwards. In the end a hadrosaur tooth erupts into the mouth as a solid block of dentine, with hard enamel on one side and softer cementum on the other. As each tooth is ground down, it wears down at different rates, with the harder enamel forming crests, and the dentine and cementum forming valleys. And to make things more efficient, hadrosaurs used every last inch of each tooth and ground them down completely, which we can see in the thin sections through the grinding surfaces of the batteries. This ingenious solution to the problem of heavy tooth wear means that hadrosaurs had a constantly replenished grinding surface that stayed rough, allowing it to efficiently grind up its food!

Hadro occlusal surface

A closeup of the grinding surfaces of an upper and lower dental battery in thin-section. In each case, there are three teeth making up the grinding surface. The oldest teeth are just the very tips of the roots of nearly completely ground teeth!

But there’s more! While a dental battery looks like a solid mass of teeth, none of the teeth within the battery physically touch each other! Time and time again when we’ve sectioned dental batteries, we noticed that each tooth is floating in its own space within the battery. That space between each tooth would have housed the periodontal ligament, the same ligament that suspends our teeth within our tooth sockets, providing a cushion against the force of chewing. It’s this ligament that allowed hadrosaur teeth to continually erupt in unison with their neighbours within the battery.

hadrosaur-tooth-xs1.jpg

I love this section. This is a closeup of a single tooth position in a cross-section through a dental battery. Even though a dental battery LOOKS like a mass of fused teeth, none of the teeth within the battery contact each other. Instead, there’s a bunch of sediment (black and speckled stuff) packed between each tooth. That marks the former position of the periodontal ligament, which has long since decayed and been replaced by this sediment.

 

The next steps for dinosaur dentistry

So there you go. Dinosaurs did some pretty impressive things with their teeth, despite having the same dental “toolkit” as you and I. I had great fun taking part in the Dinosaur Dentistry event put on at the University of Alberta, but what I’m most looking forward to now is the future role of dental histology in dinosaur research. Despite Owen’s pioneering work in the 1840’s, our understanding of what dinosaur teeth are made of and how they evolved such complex and diverse teeth has only recently been revisited. That’s an exciting prospect for me, because that means that there are still a lot of questions waiting to be answered, and even more waiting to be asked. As our database of dino teeth slowly starts to increase (and see the references I’ve included at the bottom to see where things have been going over the last few decades), so too do the piles of questions about how and why certain features evolved. Non-invasive technologies like CT and synchrotron scans are giving palaeontologists glimpses inside the teeth and bones of dinosaurs without having to make thin sections, but to this day, the timeless art of cutting open a fossil and making a microscope slide is still the most important way of gathering the tissue-level detail we need to decipher dinosaur dental evolution. I’m hopeful for the future of this discipline, because it’s still giving us as much new information in the 21st century as it did back in the time of Sir Richard Owen.

 

Keep an eye out for future glimpses into the weird world of dinosaur dentistry!

 

For those wanting to read more about dino dental histology, here are a few studies to get you started, some of which are openly accessible:

Abler (1992): the discovery of the ampulla.

Erickson (1996): figuring out how rapidly dinosaurs replaced their teeth.

Hwang (2011): taking a closer look at dinosaur tooth enamel.

Dumont et al. (2016): dental histology in the last of the toothed birds.

LeBlanc et al. (2017): comparing dental histology in dinosaurs, crocs, and mammals.

He et al. (2018): looking inside the teeth and jaws of early horned dinosaurs.

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