Voices Shell
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John Hunter: Teeth are just a fantastic and fascinating subject on their own on, but there is a practical reason. Because of their great durability, because of their thick resistant enamel covering, which helps to maintain their shapes over the lifetime of the animal that has them, they make excellent fossils. 

Jen Farmer: From the heart of the Ohio State University on the Oval, this is Voices of Excellence from the College of Arts and Sciences, with your host David Staley. Voices focuses on the innovative work of Arts and Sciences faculty and staff. With departments as wide ranging as art, astronomy, chemistry and biochemistry, physics, emergent materials and mathematics, and languages, among many others, the college always has something exciting happening. Join us to find out what's new, now. 

David Staley: I am joined today by John Hunter, Associate Professor in the Department of Evolution Ecology, and Organismal Biology at the Ohio State Newark Campus, where he teaches courses [00:01:00] in human anatomy, biology, and evolution.

His research focuses on the evolution, paleoecology and biogeography of Cretaceous and Paleogene mammals. Dr. Hunter, welcome to Voices. 

John Hunter: Thank you for having me. 

David Staley: And I wanna talk about your most recently published research that is examining tooth wear, like the wearing down of teeth in both juvenile and adult duck bill dinosaurs,

and I'm gonna use that as the setup and you can tell us about this research. 

John Hunter: Sure. This is work that actually originally did quite a long time ago, but I'm just publishing now. But it focuses in on the worn teeth of some baby dinosaurs in comparison to a closely related adult dinosaurs.

They belong to the duck-billed dinosaur group which were these herbivorous dinosaurs common in North America during the Cretaceous period. 

David Staley: When was that roughly? 

John Hunter: Roughly the late Cretaceous, [00:02:00] so a hundred million years ago to about 66 million years ago. And these particular fossils come from about 75 to 80 million years ago from what's known as the Two Medicine Formation of Montana.

And the youngsters are part of a set of fossils that were discovered in the late 1970s, early eighties. They're found in nests and the ones that I particularly studied are not the smallest ones, not the embryos or potentially hatchlings, but they're animals that probably had been growing but kept in the nest for the first maybe 45, 80 days,

it's been estimated, retained in the nest while parents were feeding them. And they're in terms of like full body length, imagine about a meter. The very smallest ones that I've had a quick look at and mentioned in the paper were much smaller, more like about 45 [00:03:00] centimeters, and if you curl that up, that's about what you would fit into the size of a typical duck bill dinosaur egg.

The genus is Maiasaura, and what was interesting about these particular nestlings that I studied is they have, the tooth wear looks very different from the kind of tooth wear you see in adults. Now, let's back up a little bit and I need to tell you about how, what the tooth wear can tell, you can tell you about diet and how duck bill dinosaurs teeth differ from the way our teeth are arranged. Now, we're used to thinking about a small number of very durable teeth that have some complex shape to them. We have multiple cusps, we have just like in each quadrant of our jaw, we have three molars and a couple of premolars. These animals were different. They didn't have the evolutionary history of reducing the number of teeth over time the way mammals have. Instead, these dinosaurs had the legacy [00:04:00] of having waves of replacement of multiple teeth through their lifetime, and that's the primitive condition for vertebrates and seen in fishes and in reptiles. But what these dinosaurs did of the duck bill dinosaur group, the Hadrosauridae and the Ceratopsian dinosaurs, the horned dinosaurs also did this convergently, a convergent evolution is they took their teeth and compressed them together in what's known as a dental battery. They did this basically with two steps; you end up instead of like immediately replacing a tooth when a new replacement tooth comes in, you would get the teeth to stick around for a little while by cementing them together with their neighbors. And if you do that and then wear the teeth down, you would end up with a washboard like surface at the top with some surfaces, some edges where the enamel was, that's the outermost hardest coating which is the most durable and would stand proud of these [00:05:00] lakes of dentine, which are a little softer.

And we're talking about, dentine is about as dense as bones, so it's not like water or clay or something, but it's certainly less durable than enamel. So, the enamel would stand up and the dentine would be at a slightly lower level. And if you compress all these teeth together, you have lots and lots of cutting surfaces and basically like a big washboard.

Now over that, the adults tended to have a single washboard shape with a small crushing edge on the extreme end of their lower but dental batteries, but with the youngsters, the nestlings, that area where there's crushing wear was significantly bigger, more like two thirds shearing surface and one third crushing surface.

Now if you then compare that with the kind of tooth wear that you see among living mammals, particularly living mammal herbivores and omnivores, you tend to see that that dominant shearing wear in [00:06:00] animals that eat high fiber diets, grass leaves, that sort of thing, whereas animals that have more the kind of proportion of crushing wear to shearing wear, let's say one third crushing two third shearing, you tend to find that in animals that eat low fiber diets. So basically the kind of differences in diet that we saw between nestling Maiasauras and adults is enough of a difference that in mammals would characterize different diets entirely. And with evidence that the adults were keeping the youngsters in their nest for significant amount of time,

say over weeks while they grew very quickly means that the adults were not only provisioning their young, which has been well documented for a while, but they were provisioning their young with a higher quality, low fiber food, more so than they themselves were eating. 

David Staley: So I have many questions.

John Hunter: Certainly. 

David Staley: The first is you study wear, wear patterns.

How do you distinguish where as coming [00:07:00] from diet versus the wear and erosion that comes from in this case a fossil, millions and millions of years old. How do you draw that distinction? 

John Hunter: For one thing, repeatability. So, if you tend to find the same kinds of wear

repeatedly. 

David Staley: So not just on one tooth or one sample, many, many samples. 

John Hunter: Many, many samples and found from different preservational environments. So for example, if we just look at the adults which are known from many different localities through time and in many parts of the world. Sometimes they're found in geologic deposits that look like they were deposited during flooding events where they might represent lakes, which would be low energy environments or streams, which are high energy environments, and if you end up seeing the same kinds of surfaces, then those wear surfaces are not being produced by other natural kinds of processes.

[00:08:00] They're being produced by eating 

David Staley: Well, and you said that these dinosaurs kept their young in nests, in fact, you even gave a timeframe, I think 40, 45 days, something like that. 

John Hunter: Yeah. That does not... 

David Staley: How are you able to determine that? 

John Hunter: It's interesting. It's not my work, but some work by a group of paleontologists led by someone named Woodward, did some studies of growth rates in Maiasauras looking at the tibia, which are a bone in the leg, and this is now done for a large number of dinosaurs. It's a good way of calculating growth rates. So, if you look at a cross section of the bone, you can see little lines, circular lines that are concentric, like concentric circles, almost like tree rings, that look like they represent seasonal slowdowns in growth.

They're called lines of arrested growth or lags for short, and you can use those to calibrate the amount of time that it takes to deposit a certain [00:09:00] thickness of bone, and you can use multiple say, where in an adult where you have multiple lags or lines of arrested growth, you can calibrate the thickness of bone, how long it takes to deposit that bone, and then you can apply that to the youngsters, determine how fast that the youngsters were growing. And the estimates for the growth from embryo, hatchling stage to a nestling stage probably took 45 to 70 days,

which is really incredible work. But that's, but it's also incredibly rapid rates of growth. In general, when people have been applying these methods to a large variety of dinosaurs, I've been teaching the kids in my Dynamics of Dinosaurs class about this, that in general rates of growth among many groups of dinosaurs

tend to be similar to what we see among birds and mammals today, and rather different from what we see today among crocodilians and turtles and other slowly [00:10:00] growing reptiles. So, it looks like dinosaurs had a more rapid growth to adult size, rapid growth to maturity more like birds and mammals today.

David Staley: Mm-hmm. You said you were comparing the teeth of duck bill dinosaurs to mammals today. Why the comparison with mammals? Why not reptiles, or...? 

John Hunter: Well reptiles do not today just simply do not show the kind of dietary range and range of tooth complexity that's really needed to come up with a comparative sample for judging the diets of duck-billed dinosaurs and other dinosaur groups too.

In general, living reptiles today have much more simple teeth. If they wear their teeth, it happens to be in a more kind of haphazard, irregular way. What is interesting about reptiles though, is that certain reptiles change their diet as they grow, but more often the case it's that the youngsters will eat insects and then the adults will switch when they get to a certain size [00:11:00] threshold and start going after larger animals, vertebrate prey, for example. And then birds are not a good example 'cause they don't have teeth , nowadays, which basically leaves us mammals. 

David Staley: This work on duck-billed dinosaurs suggests that parents may have fed their young differently than how they fed themselves. Does this tell us anything about,

I don't know, dinosaur family life or how dinosaurs were as parents? Is that the right question to ask? 

John Hunter: It certainly is the right question to ask, and at least for certain dinosaurs, and I think it's pretty well documented in Maiasaura, that the level of parental care was really quite sophisticated.

That doesn't seem to be the case for all dinosaurs. There's a closely related, another duck-billed dinosaur, Hypacrosaurus, which actually replaces Maiasaura over time in the same sediments, but other people have looked at the Hypacrosaurus growth series, which, and it's another [00:12:00] group of dinosaurs that's known from embryos and older individuals all the way through adults,

and there is a change in tooth wear and a change in diet, but it looks to me, and I reviewed this in, in our recent paper, that at an equivalent age to our nestlings, that the Hypacrosaurus were eating more fibrous food. And there's some other evidence that Hypacrosaurus started off from larger eggs,

didn't grow as quickly as Maiasaura, so there's a lot of lines of evidence suggesting that Hypacrosaurus was more like the kind of bird today that hatches out of its egg like a chicken, and is ready to go out and forage on its own, whereas Maiasaura is like a bird that hatches out today that needs a little bit more care, but then that grows more quickly early on.

So, even among closely related dinosaurs, and these belong to different subfamilies, but they're still duck bill [00:13:00] dinosaurs, there probably would've been a range of different strategies taken depending on what worked for those particular animals. 

David Staley: You had hinted at the beginning of our conversation that this research began a while ago, and it's only recently been published. Care to explain why, or...? 

John Hunter: Okay. Originally, this is work that I did in my undergraduate thesis at Brown University a very, very long time ago, 1988. 

David Staley: Oh my goodness. 

John Hunter: And it was, after presenting it at a meeting, I wrote it up, it was gonna be a book chapter, and the book never materialized.

I had an entire career focused on fossil mammals and didn't really look back to it until recently when I saw that there was going to be a special issue of a journal, paleo geography, paleoclimatology, paleoecology focused in on micro wear and ecology in mammals, and dinosaurs. And I was thinking, perfect.

This is the [00:14:00] perfect venue to submit this old work, and I really wanted to publish it before my undergraduate advisor, Christine Janis, while she was able to contribute to it and work with me to bring it out into the world. I'm glad it really worked out. 

David Staley: And so you were studying mammals and then shifted to dinosaurs?

John Hunter: I think originally I started with dinosaurs with this very project, and then the mammals and spent a whole career on mammals. That doesn't mean I've been ignoring dinosaurs. In the nineties through early two thousands, I did a lot of work in North Dakota collecting fossils and working with

a group of amateur fossil collectors in southwestern North Dakota and we ended up building up a data set on vertebrates occurrences, including dinosaurs close in the last million years of the Cretaceous in the Hill Creek formation. And I believe it, it was at the time and still remains I think the one of the best detailed records for [00:15:00] dinosaur occurrences as you approach the time of their dinosaur extinction.

So that's my other contribution to dinosaur science. 

David Staley: So, you've a 2024 study that has traced. 30 million years of primate tooth evolution. Can you walk us through what was actually happening to this teeth over that stretch of time and tell us the significance?

John Hunter: Okay. That is a project that came together from the coalescence of a number of different things coming together.

I had a graduate student a few years ago who was investigating rates of dental evolution in early primates, and while she was visiting museums and making observations, I asked her to make notes of the shapes and morphology of the various fossil primates she was looking at in a particular way,

and the particular way actually has to do with an old method that some colleagues and I devised [00:16:00] back in the nineties called crown types. We used crown types, which reduced teeth of complex teeth down to a few simple variables: the number of cusps close to the cheek, the number of cusps close to the tongue, the blades that unite cusps together. And a general variable for cusp shape. And with those few variables, we were able to describe pretty much the diversity of mammalian upper teeth that was available to us at the time. And we did some work. 

David Staley: All mammals? 

John Hunter: Well ,there's one study of all living mammals that my colleague, Jukka Jernvall at the University of Helsinki did back in 1995,

that's in his dissertation monograph, and then together Jukka and I and our colleague Mikael Fortelius applied crown types, upper molar crown types to groups of herbivorous mammals through time belonging to the artiodactyls, the even toed ungulates, the [00:17:00] perissodactyls, the odd-toed ungulates, and the archaic ungulates, the archaic hoofed mammals that really are the group that I specialize in, that's my primary group. We used crown types to investigate morphological diversity as an alternative way of measuring biological diversity overall, as an alternative to say simply counting numbers of species,

and actually some of our work was among the earliest applications of the concept of morphological diversity to fossil vertebrates. But for a number of reasons, that work got put on the back burner, people started getting interested in new technical ways of capturing the shapes of mammal teeth.

I contributed that a little bit myself, but mainly focused in on images, CAT scan images, 3D scans, that sort of thing. But all this time I felt that there was something to be said for the simple methods, and the simple methods that anybody could apply by taking a [00:18:00] tooth, looking at it from arm's view and describing what and then pulling some simple variables out of that.

What I did is I asked my student when she was looking around at museums to see if she could apply crown types to the lower teeth, 'cause we had only looked at upper teeth before, and to apply them to pre-molars as well as molars. Pre-molars are teeth that are behind the canines and are replaced, whereas molars are behind the canines, behind the premolars and are not replaced. And I asked her to do one more thing: I asked her to quantify the crown types, describe the crown types separately for the front half of the tooth and the back half of the tooth. Now, the reason why I asked her to do that was because primitively, in mammals, ancestrally, the front half of the lower molariform teeth is much higher, we call it the trigonid, and the talonid, which is the back half of the tooth, is much lower, it's a crushing basin, and the [00:19:00] trigonid, the higher front end of the tooth is where the shearing blades are. And my colleague Jukka, who's primarily a developmental biologist, had long ago developed the hypothesis that the lower later forming parts of the tooth crown, later forming development are more variable, and it would be interesting to check and see if the lower parts of the teeth that are more variable within populations are also more evolvable. Do they evolve more quickly? Do we see more evolutionary diversity?

And so we applied that to fossil primates 'cause it was a convenient data set to look at. And the short answer is yes, at least initially when there's a big difference in the height of the back half of the tooth, which is low, and the front half of the tooth, which is high, that most of the diversity that we see evolving is in the lower, a later developing more variable part of the tooth, and later when by the time we get to the end of our little view through Paleogene primates, what's actually happening is the [00:20:00] lower back end of the tooth is getting higher and higher, much more like our teeth, which are nice and level, and the difference evolutionarily in terms of the numbers of crown types we see through time and the rate at which they form, becomes more even.

What we are hoping is that the next steps is to bridge the two data sets that we have now, or the several data sets that we have now developed, and look at some of the crown type evolution in the upper teeth in primates through time and see whether we have matching patterns in upper and lower teeth.

And really, for me, what I'm interested in, because I'm interested in archaic ungulates, the hoofed mammals who seem to be doing very similar things to what the primates are doing, but the primates are up in the trees and my hoofed mammals are on the ground, to see whether they're similar parallel patterns in the diversification of [00:21:00] primates in the trees

and the archaic hoofed mammals on the ground. 

David Staley: What's the mechanism, what's causing this evolutionary change? 

John Hunter: A lot of it is ultimately climate driven. As we go from through time over the past, 65, 66 million years since the extinction of the dinosaurs, there has been pretty much a constant cooling of temperatures that we're doing our best right now to reverse, as we all know.

But in general, the trend through time has been cooling, and with that there's been more opportunities for herbivores to make a living, essentially, as essentially habitats have opened up. During the early part of the Paleogene, which is the first half of the Cenozoic era, there were tropical like rainforests all the way up into Northern Canadian latitudes,

but those restricted over time [00:22:00] and habitats opened up, and so, it became more of a stable evolutionary strategy to be an herbivore. Initially in making the transition from the ancestral diet for most mammals, for placental mammals in particular, insectivores, there was probably a transitional state where the animals included more fruit and then eventually more foliage, and primates in the trees and archaic hoofed mammals on the ground were in the middle of making that transition.

David Staley: Your study of primate tooth evolution; does that include us, homo sapiens, humans? 

John Hunter: Oh, no, no. These are really archaic primates. A lot of them, at least early on, belong to a group called Plesiadapiformes. They are probably outside of what we think of as the crown group of primates. In general, the crown group of any group is the group that is kinda within the range of the living forms. So, if [00:23:00] we imagine all the living forms of primates, from lemurs to aye-ayes, to humans, to colobus monkeys and go back to their last common ancestor, and then consider all the descendants of that common ancestor,

that's crown primates, and a lot of the Plesiadapiformes are outside of that, so they're more closely related to primates than anything else, but probably outside of the crown group. And some of the earliest crown group primates made their way into our sample, though. They belong to a couple of families of North American primates, Omomyids, which if you imagine are probably very, we're probably very tarsier like, and Adapids, which might have been more lemur like. 

David Staley: Earlier you mentioned a class you teach, Dynamics of Dinosaurs. I love the title. What's the course about? Dinosaurs.

John Hunter: Well, I have to give a brief shout out to a couple of people on that. It was or iginally a course devised by Tom Heatherington, who's a retired faculty member in EEOB [00:24:00] who originally designed the course for Zoology and EEOB majors, basically to teach them a little bit about dinosaur paleobiology and the kinds of entrances we can make from fossils. Currently it's taught on the Columbus campus by Erin Lindstedt, and I teach it on the Newark campus.

We've worked it so that it's part of the general education curriculum and it's part of the Origins and Evolution theme. 

David Staley: I was very interested to see that your Bachelor's is in both Biology and Classics. So, you have tell us the story of how a classics major, at least a double major, ends up studying dinosaur teeth. 

John Hunter: Well, I, in high school I had studied Latin and Greek, five years of Latin, four years of Greek, and I think it was an easy transition when I went to college to become a Classics major. I had gotten, to be perfectly honest, a bit turned off of sciences in high school and only rediscovered sciences [00:25:00] as being a fascinating set of subjects that I wanted to do myself only in college, and it was really sort of wandering around and exploring around while I was doing my undergraduate work at an arts and sciences college, I was at Brown University, t hat I, through the back door, found my way back into what ended up occupying the rest of my career.

So, I started taking courses in geology and in anthropology and linguistics, and eventually made my way into biology and developed some collaborative ties with professor there, Christine Janis, that I still collaborate with from time to time. 

David Staley: If I'm remembering correctly, at Brown, students can design their own path.

Am I remembering this correctly, and is that what you did at Brown? 

John Hunter: There I think you may be referring to the new curriculum, Ira Magaziner from the seventies had proposed this, and there is a certain amount of academic freedom that's allowed to students [00:26:00] because of majors might involve a smaller number of courses, which allowed me to get a double major, and some people can frame their own majors, but it does have to be guided by faculty input. And the one thing that helps Brown make this situation work is they have a really well-developed system for advising students, and that was in place way back in the eighties, and I'm sure it's even better now.

Students can dev, as I recall, students can devise their curriculum, but they really do benefit from faculty input and guidance, like any student. 

David Staley: Tell us what's next for your research, what are you working on now? 

John Hunter: Oh my, let's see. The next things that I'm gonna be working on, I've, with a colleague in the Royal Tyrell Museum of Paleontology,

he's at a similar state in his career as I am, where we're both seeing, maybe a decade down the road, we're gonna end up probably be looking at retiring, and so we're trying to help each other [00:27:00] get out all the work that we started. So he's been working on Paleocene aged or early Paleogene mammals of Alberta in Canada,

I have some unfinished projects involving Paleogene mammals from North Dakota and Montana, and we're sort of 

working together to bring those publications to light.

David Staley: How did you end up with teeth? As a paleontologist, of all the things that you could study, why teeth? 

John Hunter: Okay, well, teeth are just a fantastic and fascinating subject on their own on, but there is a practical reason, and I'll tell you that first. Practical reason is that teeth because of their great durability, because of their thick resistant enamel covering, which helps to maintain their shapes over the lifetime of the animal that has them, at least among mammals,

that also feeds back into their durability over time and they make excellent fossils. And so, even if the rest of the animal might be broke into bits and be shards of bone, the teeth will [00:28:00] stand proud and preserve at least a view of what the animal was eating over time, over its lifetime. But even if they weren't so readily preservable, which they are, no system in the body is so informative of not only how they were formed in development, but also how they were used to chew food and gives you an insight into the dietary ecology of the animal.

In terms of doing paleoecology with mammals, I can think of no better system. 

David Staley: John Hunter. Thank you. 

Jen Farmer: Voices of Excellence is produced and recorded at The Ohio State University College of Arts and Sciences Marketing and Communications Studio. More information about the podcast and our guests can be found at go.osu.edu/voices. Voices of Excellence is produced by Doug Dangler. I'm Jen Farmer. [00:29:00]