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Ask the Expert

Dr. Mary Schweitzer

Dr. Mary Schweitzer is a paleontologist and Associate Professor in the Department of Marine, Earth, and Atmospheric Sciences at North Carolina State University. She is also Associate Curator in Vertebrate Paleontology at the university's Museum of Natural Sciences, and research associate with the Montana State Museum of the Rockies. Schweitzer and her team made headlines in 2005, when they reported, in the journal Science, on their discovery of soft tissues still surviving within the 68 million-year-old bones of a Tyrannosaurus rex. Schweitzer's research interests include determining how and why molecular remnants, including soft tissues, may have survived; what biogeochemical conditions may have led to that survival; and how such molecular remnants have been chemically modified from the living state. She is also interested in the evolution of physiological and reproductive strategies in dinosaurs and their bird descendants, including the question of whether dinosaurs were warm-blooded, cold-blooded, or something in between. Schweitzer earned her Ph.D. in Biology from Montana State University in 1995.

On July 31, 2007, Mary Schweitzer answered selected viewer questions about her discovery of what may be T. rex blood vessels and red blood cells, the implications of that and similar discoveries, and other matters dinosaurian and paleontological. Please note we are no longer accepting questions, but see Inside the Bones and our Links & Books section for more information.

Q: It looks as if the T. rex may have nucleated red cells. Is this so?
Judith Chester, Santa Fe, New Mexico

A: Well, there are small, red structures within the vessels that look like nucleated red cells. So on the surface, this is a case of "if it looks like a duck…." But after 70 million years, just because something looks familiar doesn't mean that that is what it is. The fossil record can mimic many things, so without doing the chemistry to show that there are similarities to blood cells at the molecular level, I do not make any claims that they are cells.

However, we do know that, except for mammals, all living vertebrates (fish, frogs, birds, and reptiles) have nucleated red blood cells in circulation. Mammals are unique in having their blood cells "spit out" the nucleus before they go into circulation (unless there is a disease). So, because dinosaurs' closest relatives are crocodiles and birds, it makes sense that their blood cells would have been nucleated.

Q: What do you intend to do with the possible T. rex blood vessels if you find out that they contain T. rex DNA?
Recie, Alorton, Illinois

A: I don't work with DNA, and my lab is not equipped to work with ancient DNA—it is not as stringent as it needs to be. So I won't be doing anything with them. However, there are a lot of other questions those blood vessels raise, and that is what I am pursuing now. And again, we have not ruled out that the vessels are some kind of artifact. They look and act like vessels, and have all the characteristics, but until the chemistry shows what they are made of, I can't state definitively that they were vessels actually produced by the dinosaur.

Q: I have a stereomicroscope and I love looking at bugs and other stuff with it. What did it feel like when you first realized what you were looking at, I mean the dinosaur tissues? Did it blow your mind? Thanks so much for your answer!
Olivia Tyson, Ottoson Middle School, 7th Grade, Arlington, Massachusetts

A: Yeah, it did sort of "blow my mind." Still does. I spent about three weeks saying that I couldn't be seeing what it looked like I was seeing. I kept looking at them over and over, and I would get goosebumps. I kept thinking that there had to be some kind of mistake, and I had my technician repeat the studies over and over and over with new chunks of bone to be sure we could get the same results.

Q: The NOVA scienceNOW show and its Web site both state that these are possible blood vessels and possible red blood cells. When will you know if they definitely are, and if they are, what questions about T. rex (and perhaps other extinct animals) might that confirmation help answer?

A: I can't make any claims for those structures that appear to be like their modern counterparts until the chemistry reveals whether they are molecular remnants of the original structures, even if altered greatly, or if they are some kind of microbial pseudomorph or even some kind of as yet unknown biogeological process unrelated to structures or molecules produced by the dinosaur itself. If, for example, I were able to isolate those round red structures in the vessel and analyze them separately, and if I were to see any signals that are consistent with heme or hemoglobin, I would be much more likely to believe they are related to the dinosaur cells and proteins. For right now, I am assuming they are not.

They are pretty intriguing tho, aren't they?

Q: I found your work as shown on the NOVA scienceNOW program fascinating. Has your discovery of surviving soft tissues in fossilized bones sent other paleontologists scurrying into the field or into museum drawers to test more bones? Did it shake up your field?
Lisa Ennis, West Chester, Pennsylvania

A: Well, thanks! I find it really interesting too! But, so far as I know, no other dinosaur paleontologists are keen to do this. This is for a lot of reasons, of course, not the least of which is that when they have spent all that time and money exploring for, collecting, transporting, and preparing their dinosaurs, the thought of taking large chunks of the bone and dissolving them just doesn't seem all that—well, productive, I guess. I am not sure whether it "shook up the field" or not, that would be better put to one of my colleagues. I think they are all very appropriately just waiting to see how much information we really can get out of this material first.

Q: What does your discovery of soft tissues still surviving after many millions of years say about our understanding of how fossils become fossils? Will that understanding now have to be greatly revised? Also, did other paleontologists initially doubt your discovery, seeing how radical it was? Keep up the good work!
Alexander McCurdy, East Blue Hill, Maine

A: I think that our fundamental understanding of the fossilization process may be somewhat in need of revamping—but only if it can be shown that this material is original to the dinosaurs and not some unknown artifact. One thing I am focusing on in our lab is to try to tease out the pathways that would go from what we know of modern tissues and cells to whatever these structures have become. But in reality, although the mechanisms of fossilization have been presented to the public in relatively simplified form, we in the profession have always known that it is more complicated, and that microenvironments within a single bone can vary greatly, resulting in a highly varied preservation. This is just an extension of that, I guess.

As for an acceptance of this work, what needs to be realized is that it is the job of my colleagues to be very skeptical. That is how we keep one another in line! Peer review is crucial to the scientific process, and the skeptical opinion of my colleagues is something I rely on very much. They are probably not as skeptical as me though. I think many of them have a "let's just wait and see" attitude that is very, very appropriate. We have a saying that goes, "Extraordinary claims require extraordinary proof." That is what we are trying to come up with, and the community will wait until then to weigh in fully, I think. They will also wait until the work is repeated by other colleagues. Those are things that are necessary to validate our work.

Thanks for the encouragement. We are still plugging away.

Q: As I recall, you soaked fossils in a mild acid to dissolve the mineral deposits on the inside of the bones. Why does the acid not harm or dissolve the vessels as well, but instead leaves them intact and pliable?
Paul Moffett, Indianapolis, Indiana

A: That is a good question. The type of acid we use is very commonly applied to remove the mineral from modern bone to reveal the structural proteins that are so intimately linked to the mineral. It is a very mild acid and is more accurately a metal chelator than a true "acid." So it removes the mineral while leaving the protein intact, and it does not harm cell membranes or vessels in modern bone, so we hoped that it would not in our ancient material either.

Q: Why has it taken so long to extract or attempt to extract DNA from this tissue? Has it been tried and failed? I would think intact DNA, even in fragments, would be the most important finding from this soft tissue.
Terence Milligan, Edgewater, New Jersey

A: Well, it has taken so long because I don't work with ancient DNA and so have not tried to extract DNA. My lab does not meet the requirement of an ancient DNA lab. I am not particularly well-trained in DNA studies, and I personally think that it is much more informative and less risky to begin by demonstrating first ultrastructural preservation and then protein preservation.

Many in the scientific community believe that DNA is too labile to have possibly survived, even in tiny fragments, for 70 million years. It is more generally accepted that some proteins, like collagen, have a higher preservation potential than DNA. And my lab is more suited to do protein studies than to work with ancient DNA. Others are certainly welcome to try to obtain DNA from this or other dinosaurs. It probably won't be me directly, though if we get to the point where funding and equipment allow, I will certainly do the supportive work.

Q: From the first fossils found, what previously made scientists think dinosaurs were or are cold-blooded without having tissue and blood samples?
Lydia Lloyd, Modesto, California

A: Many of the first dinosaur scientists in fact proposed that these were active beasts, particularly the theropods. Some of the very early work by some artists show theropods in active poses (see for example by Charles Knight). But we also knew from the bones that they were (and are) closely related to reptiles, and modern reptiles are cold-blooded. So it was largely an assumption that dinosaurs shared this trait, and that assumption dictated many of the questions asked about dinosaurs for many decades. This is true in many scientific fields actually.

And, in my own case, I was not looking for vessels and cells in the first dinosaur we found them in, because like every other paleontologist, I did not think there could possibly be any remnant of those structures surviving for that long. The vessels and soft tissues we discovered were actually the happy accident of looking for and trying to describe something else entirely.

Q: I have two pieces of amber with insects in them, and I've always been amazed that it's essentially the actual creatures still there after 20 million years. Did you feel something similar when you found the T. rex soft tissue? Does it bring the dinosaur and its world that much more alive to you than fossil bones can? I think I would get a shiver down my spine.

A: Yes, I spent about three weeks feeling like every day I was living in a science-fiction movie. It was one of the most amazing things I have ever experienced in my professional career.

Q: Will science be able to use the discovered tissues to recreate living dinosaurs, a la Jurassic Park
Glenn Martin, Grand Junction, Colorado

A: In my professional opinion, no. There are way too many obstacles beyond getting real dino DNA and proving that it is dinosaurian. That, believe it or not, would probably be the "easy" part, and we have not yet accomplished that. I am not exactly sure why anyone would want to do such a thing! Like the Jeff Goldbloom character in the movie says, "Just because you can do something doesn't mean you should."

If you want more detail, check out a paper I wrote on this subject, available as a pdf at:

Q: Did the internal structures you found previously show up in X-ray and X-ray tomographs of dinosaur bones? If they did not, why not?

Also, in the Colgate University museum in about 1965, I saw fossilized clamshells that had had their limestone "shells" dissolved away to reveal an internal structure somewhat like a rib cage made of iron pyrites. Why was this apparently known method of dissolving fossilized material to reveal potential inner structures not used on other fossils before now?
Hal Lane, Durham, North Carolina

A: In answer to your first question, these structures we see are very, very tiny (the cells are about 20 microns), and again, if you don't think that they exist, why would you look? I think that part of the reason they were preserved is because they were covered with a mineral "shell," and therefore there would not be sufficient resolution or density difference to image with a standard X-ray or CT.

Regarding your second question, I sure don't have an answer for that. All we did was apply a very standard methodology that is commonly used to study modern bones. It wasn't really anything special. I think, if you look at the answer to the previous question, it has more to do with expectations. Many of the questions we ask as scientists are built upon accumulated wisdom and assumptions, and so fit within that framework. Sometimes we need to reevaluate those assumptions to make sure they are still valid, and this will happen more often as technology becomes more sensitive.

In addition, I would like to point out that a Polish researcher by the name of Roman Pawlicki was reporting cells similar to what we saw as early as the 1960s. But no one followed up on his work, and some of these findings were not published in widely read journals so may have been overlooked.

Q: Many creationists claim that the Earth is much younger than the evolutionists claim. Is there any possibility that your discoveries should make experts on both sides of the argument reevaluate the methods of established dating used in the field?
Carl Baker, Billings, Montana

A: Actually, my work doesn't say anything at all about the age of the Earth. As a scientist I can only speak to the data that exist. Having reviewed a great deal of data from many different disciplines, I see no reason at all to doubt the general scientific consensus that the Earth is about five or six billion years old. We deal with testable hypotheses in science, and many of the arguments made for a young Earth are not testable, nor is there any valid data to support a young Earth that stands up to peer review or scientific scrutiny. However, the fields of geology, nuclear physics, astronomy, paleontology, genetics, and evolutionary biology all speak to an ancient Earth. Our discoveries may make people reevaluate the longevity of molecules and the presumed pathways of molecular degradation, but they do not really deal at all with the age of the Earth.

Q: Do you feel dinosaurs were warm-blooded or cold-blooded, and what evidence supports each case?

Also, there seems to be a great deal of evidence that most dinosaurs could not live in today's gravity without total engineering failure of their body structures. What is your take on this?
Robert Therriault, Sunnyvale, California

A: In regards to your first question, I think that dinosaurs probably spanned a range of metabolic strategies. Most of the very early and very primitive dinosaurs already possess some characteristics we associate with an elevated metabolic rate. For example, only warm-blooded animals today are obligate bipeds, with upright posture (legs directly under the body, not splayed as in crocodiles, lizards, and turtles). Only warm-blooded animals have sustained and rapid growth or possess insulatory body coverings. All of these traits have been observed in some dinosaurs, although not all are features of all dinosaurs. I think that the evidence supports the idea that dinosaurs had a higher metabolic rate than modern lizards and snakes. Whether any or all dinosaurs attained the metabolic rates possessed by living birds is still open to debate.

We wrote a paper reviewing this evidence, if you want more information. To see an abstract, go to:

Re your second question, I do not know of any evidence in the peer-reviewed literature to support such a claim. The biomechanical evidence that I am familiar with shows that dinosaurs were remarkably efficient and well-adapted to their habitats, lifestyles, and environments. I can think of absolutely no arguments in any field that something so fundamental as the law of gravity has varied even slightly over the course of geological time. I would recommend papers by John Hutchinson and others that address biomechanical issues in dinosaurs.

Having seen many dinosaur fossils in my time, I think it is quite apparent that they were very, very successful, and I have seen no signs at all, even on the microscopic level, for engineering failure. And I think all research in physics supports the "gravitational constant." The principle of uniformitarianism guides historical sciences and states that "the present is the key to the past." Without that understanding, or invoking ever-changing conditions in which certain fundamental rules like gravity do not apply, we cannot ever understand or make statements about events that we can only observe indirectly. The evidence is very strong that by applying principles like the constancy of gravity or the speed of light, past events are understandable and future events predictable, and science moves forward. And these have not yet been disproven, so in my mind there is absolutely no reason to question that dinosaurs would have done just fine in today's gravity.

Q: I have two questions, if that's okay. What work are you up to now? Are you testing the bones of yet more types of animals than were reported on in the show and the NOVA scienceNOW Web article Inside the Bones?

Also, what does it take to do the kind of work you do? What kind of training would someone need to do it? Thanks for your time!

A: In answer to your first question, yes. We have been seeking to verify the extent of the phenomenon we observed in our "B. rex" by looking at a range of bones from other organisms, from dinosaurs in the Mesozoic to mammals and birds in modern times. See our 2007 Proceedings of the Royal Society paper, "Soft Tissue and Cellular Preservation in Vertebrate Skeletal Elements From the Cretaceous to the Present," an abstract of which is available at:

My research goals for the immediate future are to determine if any original remnant molecules comprise the structures that look like blood vessels and cells, and to try to characterize the chemical changes that took place to get from original, functional dinosaur material to what remains of this today. We have so much to learn about preservation, and about these wonderful beasts, I just can't wait.

Regarding your second question, I think it helps if you are slightly crazy. Other than that, I think a firm grounding in chemistry, biology, molecular biology, and evolutionary biology as well as a lot of chemistry. I also think you need to be willing to take chances and to think outside the box. And a deep passion for and love of the subject matter certainly helps in those dark times when nothing goes the way you think it should.

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