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How did chemistry help reveal pregnancy in a T. rex fossil? Biologist Mary Schweitzer describes one of the most important fossil discoveries ever made
National Science Foundation
How did chemistry help reveal pregnancy in a T. rex fossil? Biologist Mary Schweitzer describes that and more in part two of an interview about one of the most important fossil discoveries ever made
Interviewer: Jordan D’Eri
Interviewee: Dr. Mary Schweitzer
JORDAN: Hello… I’m Jordan D’Eri… and welcome to Dig This… an archaeology and paleontology-themed radio show from the National Science Foundation. For those of you who aren’t aware… this is the second part of our two-part interview with Doctor Mary Schweitzer of North Carolina State University on her super cool pregnant dinosaur discovery. If you haven’t listened to part one yet… go back and check it out. Otherwise you’ll be LOST in another WORLD… ha! See what I did there? No but really… you’ll get confused so do yourself a favor and check it out.
Anyway… we left off last time just as Doctor Schweitzer was about to talk about the process of acquiring… and finding out… if her T-Rex was pregnant. Unfortunately… our interview cut out part-way through her telling the story… so I’ll do what I can to catch you up with what you missed. Side note: this is very much an abridged version of what she sent me… so if you want to know the full story… email me at jderi at n-s-f dot gov. That’s j-d-e-r-i at n-s-f dot gov.
And I mentioned this in the beginning of part one… but again… we’re covering a lot of stuff in a short period of time. If anything sounds confusing throughout the rest of the interview or if you need clarification on something… just shoot me an email at that address I mentioned just a minute ago and I’ll help you out. So in nineteen-ninety five… Doctor Schweitzer was doing her PhD work on a different T-Rex and found some variations in the fossilized bone. She published a few papers on her findings… but her research was… in her words… ‘pretty robustly ignored’… so she wanted to repeat her studies on another T-Rex.
And nine years later… she got her wish. Another T-rex skeleton was found and after the long and arduous dig was completed… parts of the skeleton were sent to her lab. Now… I don’t want to completely gloss over that delivery because… to me… it sounds like a nail-biting ordeal which had the potential to ruin the whole thing. The seven-ton bag of bones was delivered via helicopter… a helicopter which… by the way… was not equipped to carry that much weight. So in the mix of all the jostling and adjusting to accommodate the heavy load… several bony fragments from the femur fell out that could not be put back in proper orientation. They were the very innermost parts of the long bones… close to the medullary cavity.
But that ended up being Doctor Schweitzer’s lucky break… so to speak. Those fragments were collected without preservatives in the field and sent to her at N-C State where she decided her first project at her new job would be to repeat her studies from before. So when she opened the box for the first time… the first piece she picked up out of the box was the one that had medullary bone. She recognized it immediately and turned to her lab tech and said… quote- ‘oh my gosh… it’s a girl… and it’s pregnant.’ I’ll let her take the story from there.
MARY: So, I went into the literature to figure out how people study medullary bone in birds and realized that one of the things they do is remove the mineral from the bone and compare how the collagen fibers in medullary bone are oriented relative to normal bone. And so I told my tech, you know, let’s try to etch it because we might be able to see – I wasn’t expecting to see collagen on anything but just how the fibers were oriented and it went faster than we thought and so when we went to pull out this little fragment of what we thought was medullary bone, it was very stretchy which was not what I expected at all.
JORDAN: So this is as good a time as any to mention that bone… even in humans… is a combination of mineral… to make it hard… and collagen… which is a protein… to make it flexible. The collagen is ‘spit out’ by the bone cells… and in bone that grows slowly… these fibers are regular and oriented. In bone that is deposited rapidly… like medullary bone… the fibers are random… so that bone will be structurally different than the normal bone that’s always present. If you just etch some of the mineral away from bone… using a weak acid… the way the fibers are oriented is revealed even if the collagen is gone.
MARY: And so that led to a whole different focus of study from pregnant dinosaurs to what the heck are these things and is it present only in this medullary bone, which would make the tissue really weird, or is it present in cortical bone as well. And we found that it is so we got stretchy transparent tapering blood vessels with contents, in some cases, and bone cells, the osteocytes that make up the bone, and the collagen – well, it would be collagen if it was modern bird than a matrix. So, you know, then it was like, OK, these are really cool things and so we published pictures of them and then we went and said, OK, how widely distributed are these things because if it’s just a one off deal, I mean, we happen to have the only dinosaur on the whole planet with all this stuff on it, pregnant and it’s got all these so we did a time point survey and we showed just using microscopy, nothing else, no analytics, that in about half of the specimens that we looked at from modern chicken bone from the refrigerator to Triassic material, at least three out of these four components were present in about half of them. And so then we decided we would try to chemically characterize, try to figure out what the heck it is starting with the assumption that if it truly is, say, collagen in the matrix, then it should have features in common with modern collagen.
JORDAN: So doctor Schweitzer and her team went after those features and tested them… the fibrous matrix… the vessels and the cells. They showed that these materials reacted to our chemical tests the way modern bone did. That really supported the idea that they were made by the dinosaur. They were even able to get protein sequences from them! Then… they decided to start fresh.
MARY: And then we went out specifically into the field to look for another dinosaur buried under similar conditions to what this T-Rex was to see if we could make a prediction, these kind of environments are really good for recovering soft tissues. And so then we had a duckbill dinosaur on the other lineage, the ornithischian lineage and we were able to show not only presence of soft tissues but better preservation in a 20-million-year-old older dinosaur. And it’s kind of been crazy since then. (laughter).
JORDAN: Yeah. I can imagine. So how did they know prior to this whether or not there was, you know, a dinosaur was a male or a female?
MARY: We’ve never been able to tell.
MARY: So, there’s all kinds of features that have been suggested. For example, in the crested dinosaurs, maybe those with bigger crests were male or whatever so you start with, OK, what's – birds, are the males the big ones or are the females the big ones? Are the males showier or the females showier? And then you say, OK, well, can we extrapolate that back to dinosaurs and maybe those with big head crests were male and others female. Or, you know, one proposal was that if you look at the first vertebrae beyond the tail, they have these little bones called chevrons and in some cases, they’re missing. So, maybe the female doesn’t have that first chevron because she needs to lay eggs than the male does. But it could also been taxonomic and maybe they just don’t preserve. So, there was no real hard core way to tell male from female and I think that’s part of why we did this study.
JORDAN: So one important note here. This tissue is an evolutionary novelty. So it’s important to know when it arose.
MARY: It’s been proposed that medullary bone exists in pterosaurs in both juveniles and adults only in the jaw. Now, it’s not found in the jaws of modern birds and it’s not found in anything naturally but mature females so is this tissue medullary bone, thus we have redefine it or is some other weird tissue and my feeling is it’s based upon the evidence, it’s the latter. Pterosaurs have – their beak area grows really weird so I don’t think you can say anything about the gender of or anything about medullary bone in pterosaurs. We just don’t have the evidence yet. But chemically, we could show that it’s different and test that rigorously.
JORDAN: The release also suggested that this could shed light on the evolution of egg laying in modern birds. That’s a quote from there. So one, what would the connection be there and two, what's important about that? Why would we need to know?
MARY SCHWEITZER: Well, because we eat a lot of them. I mean, it’s a huge industry worldwide so anything we can know about birds can make us more efficient at feeding the world, but I think – crocodiles, again, we use crocodiles and birds to bracket dinosaurs as the extent groups so in crocodiles, they lay their eggs en masse and they shell them en masse so they may have 25 to 30 eggs in their body and they shell them all at once. They don’t have medullary bone so you can actually take the bone of a female crocodile, adult crocodile and hold it up to the light (inaudible) and tell if it’s female or male just by looking because it’s got all these holes in it and it’s pretty impressive. So, birds, on the other hand, shell their eggs one at a time usually separated by 24 hours. So if a bird lays, say, 15 eggs, that’s 15 days so it’s a massive calcium draw that lasts longer and because crocodiles don’t have these hollow bones and birds do, it’s an enormous physiological stress on birds. And so understanding some of these traits – you know, when did birds switch and we have some evidence from the fossil record that at least in theropods, they had gone from an en masse type of shelling to at least two eggs maybe ever 24 hours. And so understanding when these traits arose then lets us ask why and then we can maybe take what we learn and apply it to different aspects of modern life.
JORDAN: All right. what was your “a-ha” and then second of all, where do you see this research going in the future?
MARY: So, I guess, you know, my “a-ha” moment, the first one was when I picked that first piece of bone of the rock – out of the box Jack sent and realized we had a female. That was – I mean, basically I had a period of about a month there where every day was like Christmas. It was just – it was extremely cool and I feel very blessed to be able to have that experience because a lot scientists can work their whole career and never feel like that. So it was very cool. And I guess with this study, realizing – I mean, I’ve been saying for 25 years, “Don’t sell the fossil records short. There’s stuff there.” But here, you know, I mean, in the holy grail for anything molecular is sequence data but I think here we can show that even without sequence data, even if the molecules are fragmented or – there’s a lot of problems with sequencing fossils and we’re in the process of writing that up but we show that retention of original chemical signaling can still let us address these questions even if we can’t ever get sequence. So, I think understanding modern material and understanding how things are preserved and understanding bone biochemistry are really, really crucial to asking these questions even when sequence data is unattainable, and I don’t think it’s unattainable but I do think we need to modify our methods considerably. I feel like paleontology is opening up to these new methods and I feel like it can be held to really rigorous standards. If we’re really careful, if we do all our controls and if we don’t overstate the data, I think that we can quit sort of apologizing for the things we’ll never know about dinosaurs and start saying, yeah, we can figure this stuff out. So, I think that’s really cool but it takes a lot of money. It takes a lot of time. It takes a thick skin, and hopefully we’ll get a lot of people more interested in doing that kind of work.
JORDAN: So yeah okay it’s really cool that science has confirmed a pregnant T-Rex. And there are a lot of amazing implications just from that discovery alone. But another really big takeaway? We can use chemistryas a tool to test ideas in paleontology. The methods doctor Schweitzer used in this study could be applied to fossils to possibly answer a wide range of questions. Who knows what’ll be uncovered? That was Mary Schweitzer, a professor in the department of biological sciences at North Carolina State University, and curator at the North Carolina Museum of Natural Sciences. I’m Jordan D’Eri… co-editor of the National Science Foundation’s Science three-sixty news service… and writer… editor and co-host of the N-S-F Science three-sixty super science show… here at the National Science Foundation.
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: current estimations of the bite-force of a T-rex is almost thirteen thousand pounds per square inch. To compare… saltwater crocodiles have a bite force of a little less than four-thousand pounds… and the average adult human has a bite force of less than three-hundred pounds. That’s all for Dig This!