NSF's Dig This!
It's National Fossil Day! Biologist Mary Schweitzer discusses her discovery of medullary bone in a T. rex fossil and how researchers can now tell the sex of a dinosaur
National Science Foundation
Biologist Mary Schweitzer discusses her discovery of medullary bone in a T. rex fossil and how, for the first time, researchers can tell the sex of a dinosaur
Interviewer: Jordan D’Eri
Interviewee: Dr. Mary Schweitzer
JORDAN: Dinosaurs. They captivate people of all ages… all over the world… and it’s easy to see why. We have this romanticized idea of them and the world they lived in: gigantic beasts with sharp teeth fighting to survive on a planet infested with deadly tar pits… massive earthquakes… and volcanoes. Skimming over the fact that this isn’t most accurate depiction of our planet… dinosaurs are so much more than that. And when we look from the fossil record to their modern descendants… there’s a lot of amazing things we can still uncover. Amazing things that could have huge implications for our world.
MARY: We live on a changing planet and dinosaurs saw probably more change than mammals ever will so why not see how – if we can figure it out – how they responded at the molecular level. How did they do things different that made them so incredibly successful and still are today?
JORDAN: That’s biologist Mary Schweitzer, at North Carolina State University, who made headlines this year when she confirmed medullary bone has been found in a T-Rex. Now for those of you unfamiliar with the medullary bone… it’s a type of bone found in modern pregnant birds, and it only exists within the bone for an extremely short period of time. It’s laid down before the eggs are laid… and once they’re finished laying… the bone disappears. And because birds are distant relatives of dinosaurs…well… put two and two together… that T-Rex was pregnant! So not only was Mary’s discovery a possible tiny needle in the fossil-discovery hay-stack… but for the first time… researchers are able to distinctly identify the sex of a dinosaur. Now… before we begin I’d like to include a little disclaimer. We’re about to try and cover a ton of stuff in a relatively short period of time… so if you’re at all confused about anything we talk about… just shoot me an email at jderi at N-S-F dot gov…. that’s j-d-e-r-I at N-S-F dot gov… and I’ll do my best to answer your question. This is the first of my two-part interview with Mary… covering everything from how the bones got to her in the first place… to how this discovery impacts modern birds and even our consumption of birds. I may even jump in from time to time to clear some things up and give a little background. But we’ll get to all that and more. For now… let’s start from the beginning.
MARY: So, we first reported this tissue in 2005 but we relied only on morphology and histology. There was no chemistry done at the time and in the intervening ten years, not only have we learned a lot about what things are preserved in the rock record and what we can learn from them, we’ve also learned that there are a few other things that might superficially resemble medullary bone and might be mistaken for it if you’re not really careful. So, we started to see the need to do the chemistry and to capitalize on what we know of medullary bone in modern birds and it’s pretty exciting, actually, when you look at the processes of evolution and why this tissue exists that we can say now that medullary bone in dinosaurs shares common chemistry with medullary bone in birds. It is different than all other bone types and that probably has to do with its function which is to mobilize calcium really, really quickly so that these eggs can be shelled rapidly. And so we can say that this novel bone type, using keratin sulfate instead of chondroitin 4 or 6 sulfate in the bone matrix originated at least before dinosaurs and birds or before T-Rex and birds diverged from one another. So it wasn’t a common ancestor and that’s pretty exciting. That pushes the timing of this evolutionary novelty way back.
JORDAN: See? Here I am already jumping in. So I’d like to add a little footnote about medullary bone here. Childbirth tends to do some rotten things to our bones… and that’s a constant across many species. When a baby is in the womb… in the case of humans and other mammals… it needs calcium for things like teeth and… well… bones. So where does that adorable little leech get it from besides what mom eats? The mother’s bones. Of course… this can erode and create little holes in her bones. But birds have super thin bones to help them fly… so taking calcium from structural bones for the shelling process would make them too weak to support the bird’s weight. And when they’re shelling their eggs… birds need about as much calcium per day as a woman would if she’s pregnant and nursing for eighteen months. That’s why they have medullary bone… which mobilizes easily to send calcium to the shell gland. That way… the bird… or dinosaur… wouldn’t have to draw calcium out of structural bone. Their bones remain stronger. So, how long does medullary bone last within the bone either before or after either a bird or a dinosaur lays eggs?
MARY: So, what we know about birds is that there is a spike of estrogen that birds produce when they first ovulate and that estrogen also goes through the bloodstream to the medullary cavities of the bones and causes the osteocytes, osteoblasts there to produce this medullary bone. So, basically on the membrane surface of the osteoblasts that line the medullary cavity in birds there are estrogen receptors so when it spikes estrogen to say ovulate, it also triggers these bone cells in birds to lay medullary bone down. That bone lasts as the calcium draw until the last egg is shelled. So in modern birds depending on how many eggs they produce, this can be maybe two weeks.
JORDAN: Wow. So it’s a really tight window then?
MARY: Yeah. It’s a very tight window which probably explains part of why we haven’t seen it before in dinosaurs.
JORDAN: So these estrogen sites are tiny molecules on the surface of cell membranes. They’re like a lock that the estrogen key fits into to “turn on” the cell. They don’t leave a trace… but maybe the estrogen will. It’s something Doctor Schweitzer’s looking into.
MARY: Well, it’s actually something I want to test but haven’t yet. But it may leave a longer – and this is something that we have to do in a complete survey of modern birds. But it might leave a small narrow lining of medullary bone that persists longer there is some evidence for that but we have not tested it or published on it yet so we don’t know that for sure. That would be my guess, though, is that there is a little residue of medullary bone holding base cells that last a little bit longer.
JORDAN: OK. Well, that would be exciting to find.
MARY: Yeah, it would. And, I mean, I think, you know, when you think about all of the variation in the human species, if you took all the skulls and lined them up that you could, you would probably not be able to put these as a single species.
JORDAN: Right. And with genders too.
MARY: And if we didn’t have any soft tissue. So the question is then in dinosaurs, how much variability is allowed before they become a different species and we don’t have a comparison but if we used just humans as a model, it may be that a lot of dinosaurs have as much variation and we’re calling things different species that are simply different. And so one of the cool things about being able to tell male and female is, of course, that’s a huge source of variation in all animal taxa or most animal taxa is that males and females are pretty different. So now if we have a marker for femaleness in theropod dinosaurs or at least in T-Rex, we can go back and look at things that might be a little bit more durable, lasting longer than a couple weeks.
JORDAN: So… for example… maybe the dinosaurs that have medullary bone also have a proportionally longer lower jaw… or heavier bony bosses over their eyes or other features not so dependent on reproduction. Then… researchers may be able to discern what features might go along with “femaleness” and they can ask questions of the population structure… like growth rates and juvenile versus adult changes… to name a few. So we’ve heard a lot about changes for carnivorous dinosaurs… are herbivores any different?
MARY: Well, herbivores, at least among dinosaur taxa, they don’t have the hollow bones of theropods so that hollow bone is kind of right there at the base of where theropods diverged from sauropods. So, there is some evidence that some ornithischians may have a type of medullary bone but that has not been chemically tested. I think that before we can say anything we need to do the chemical testing. And that is in progress, actually, but it may be that medullary bone is different in ornithischian dinosaurs because they have a different physiology or because they don’t have quite the demand for this tissue. On the other hand, if we find it for sure and positive and it’s chemically confirmed in ornithischians, that means that this tissue arose sometime after divergence of crocodiles from the theropod bird lineage so it would push that origin of medullary bone back even further.
MARY: And I think it’s kind of cool because I think, personally, and I have no evidence at all for this but I think that it might be tied to an elevated metabolic rate where these animals might be shelling their eggs and the calcium demand might be greater because they have a higher physiology and so maybe the need for medullary bone, especially if it arose in the origin of dinosaurs or basal to all dinosaurs might coincide more with this elevated physiology than with something like thin bones or hollow bones.
JORDAN: OK. So where would that come from? The idea of there being the higher metabolic rate. Because there are only two types of warm-blooded animals… right? Mammals and birds.
MARY: Well, we have a lot of evidence and back, like, a zillion years ago now, it was – we first – when dinosaurs were first being discovered, they had some features that linked them to reptiles and, of course, all modern reptiles are sluggish and cold-blooded. They’ve only been around for, oh, 300 million years or more so they’re not very successful. (laughter) But I guess, and that date is – I don’t have my calendar in front of me. (laughter) But anyway, they’re not, you know – it was thought from a mammalian perspective that they were kind of small and stupid but there was a lot of evidence starting with some of the stuff published by Bocker and others in the mid-80s that dinosaurs probably – well, actually it goes back to the 60s with Holstrom – dinosaurs might have been as active as mammals and so rethinking the whole dinosaur physiology thing, there is a lot of evidence, it’s mostly circumstantial, we can’t really go back and stick a thermometer up their butt but, you know, it is mostly circumstantial and based upon bony correlates that dinosaurs had a higher metabolic rate – resting metabolic rate than crocodiles. And these things include things like only warm-blooded animals today walk with their legs directly under their body rather than with a splayed posture. Except for crocodiles, only warm-blooded animals have a four-chambered heart with greatly increased circulatory systems and we have evidence that that was true in dinosaurs because how else are you going to get blood up all of that length of neck in a saurapod. They had to have efficient hearts and reptilian three-chambered hearts can’t do it. and so we have a lot of circumstantial evidence that dinosaurs probably had a higher metabolic rate, whether it was warm blooded or not, we don’t know but they probably had a higher metabolic rate than, say, crocodiles or lizards or snakes. And so when you acquire a higher metabolic rate, it requires massive alterations to pretty much every system in your body. So, for example, cold-blooded animals, their digestive enzymes work across a broad spectrum of temperatures but in warm-blooded animals, you have a very narrow window in which they operate otherwise they just don’t work.
MARY: And same with reproductive strategies. For example, crocodiles, which are the closest cold-blooded animal related to birds, they have temperature dependent sex selection. So, whether you are male or female depends more on where you happen to end up in the nest than your genes. So, if you elevate your metabolic rate, what does that do if you can’t compensate for it? so, there is some evidence that they did have higher metabolic rates and it’s circumstantial, like I said, but if that’s true perhaps medullary bone is one of the responses to that elevated rate, either because the dinosaur needed more calcium – I mean, you can’t – your muscles don’t contract without calcium so the need is pretty great – or the developing embryos needed more calcium or something in between.
JORDAN: So, this discovery really is kind of a solid indicator not just for theropods the potential is that this could reach beyond just to dinosaurs as a whole.
MARY: And I think that, you know, we can start asking some really cool questions that were thought to be unanswerable in the fossil record which is kind of why I do what I do. It’s very controversial. It’s very hard. And it’s fraught with – I mean, it’s expensive and it’s fraught with a lot of controversy but I don’t want to sell the fossil record short and I think as – you know, we live on a changing planet and dinosaurs saw probably more change in that single image and continued to than mammals ever will so why not see how – if we can figure e it out – how they responded at the molecular level. How did they do things different that made them so incredibly successful and still are today?
JORDAN: Right. So these more complex questions that you’re talking and, you know, answering with research like this would be kind of in the realm of how did these dinosaurs respond to their environment, to the changes like you were talking about and just what that could mean for us in the future.
MARY: That’s kind of one of my goals. And there’s all kinds of side projects because, you know, we study molecules. I mean, we’ve been studying molecules for a very, very long time at various levels but we really only study them in living organisms and what happens to a molecule when it sits completely encompassed in a very safe environment of bone for millions of years, you know, and does that have an implications for how we might maybe engineer molecular structures to be more durable? And so, you know, I think it goes beyond paleo. I think it’s – and beyond evolutionary biology. I think we need to think outside the box. What can molecules do, whether, you know – besides tell us an awful lot about the animals, it may be looking at them independently and figuring out what else we can learn about them. And that’s something I’m not going to do but I’m hoping somebody smarter than me is. (Laughter)
JORDAN: And this could be the key to help unlock those things.
MARY: Yeah. It could be. We work really hard in my lab to develop these new approaches and we try very hard not to overstate any data that we have and so I think that, you know, if other people want to apply these methods, and I’ve had many people in my lab to learn them, I think it’s very exciting. It’s an amazing time to be in paleo.
JORDAN: That was Mary Schweitzer, a biologist at North Carolina State University, and curator at the North Carolina Museum of Natural Sciences. In part two of our interview, we’ll cover her scientific process and what her discovery could mean for the future use of chemistry in paleontology research. If you have any super cool N-S-F-funded archaeology or paleontology-themed research you think I would dig… send it along to Jordan D’Eri… that’s me… at jderi at N-S-F dot gov. That’s j d-e-r-i at N-S-F dot gov. And to finally bring it all home… here’s a fun fact you can dig: Did you know the largest tooth of any carnivorous dinosaur ever found is from a T-Rex? It may have been almost a foot long, including the root. That’s all for Dig This.