Fossil Overturns Thinking On Tyrannosaurus

A newly described dinosaur species, Raptorex kriegsteini, is one-hundredth the size of T. rex, but has all hallmark anatomical features of the larger dinosaur. Paleontologist Paul Sereno describes the find, and why it changes what scientists thought about Tyrannosaurid evolution.

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IRA FLATOW, host:

You're listening SCIENCE FRIDAY from NPR News. I'm Ira Flatow. Up next, a tiny T. rex. Well, not that tiny, and not exactly a T. rex, but it's very close.

Scientists say they have found a new species of dinosaur, unearthed in northern China. The creature is only about nine feet tall, but it has all the hallmark anatomy of the giant Tyrannosaurs rex: those puny little arms, the massive head at the jaws, the long hind legs. And that has thrown paleontologists for a loop because the prevailing wisdom was that those features slowly evolved as T. rex got bigger, but now finding these characteristics in the earlier, smaller versions is unexpected. The information was published in Science Express. That's the online edition of the journal Science.

And joining us here is, Paul Sereno, explorer-in-residence at the National Geographic Society. Dr. Sereno is also a paleontologist at the University of Chicago. Welcome back to SCIENCE FRIDAY, Dr. Sereno.

Dr. PAUL SERENO (Paleontologist, University of Chicago): Oh, thank you very much.

FLATOW: So it's nine feet tall.

Dr. SERENO: We say long.

FLATOW: Nine feet long, well of course, that's the way it runs out, right?

Dr. SERENO: Yeah, they're very bird-like, and we've made that mistake a couple of times, reconstructing them something more in our own likeness. But yeah, they have a horizontal back, and we measure tail-tip. Half of that's tail. So you're really only dealing with an animal that weighs about as much as a human, the average human, about 150 pounds, 60 kilograms, something like that.

FLATOW: So this really is a new species.

Dr. SERENO: Oh yeah.

FLATOW: We should not call it a tiny T. rex.

Dr. SERENO: Well, you know, it's been called a mini-me T. rex, a tiny T. rex. It does look like a T. rex, or Tyrannosaurus, technically, because of the - probably there's a couple things that we associate with these animals. Of course, the roaring, huge head, powerful jaws, but then the puny arms. That's sort of the signature of this design type, and the ability to run. And all that is present in this dinosaur that we are calling Raptor rex, but that's 50 million years earlier than T. rex.

FLATOW: And do you believe that this one evolved into T. rex?

Dr. SERENO: Yeah, you know, I cringe sometimes when I hear the word missing link because we really don't find, on average, missing links that would be, as it were, right on the stem leading to something. Because to be right on the stem, to be, as it were, a missing link in a chain, you have to have almost nothing unusual to your own immediate line of evolution. But that's really the way Raptor rex is.

There's so few features that we can call its own. And instead, it's grabbed on to all these features that were known only from the Late Cretaceous, 50 million years later, characterizing Tyrannosaurus and the closest of kin, some of which are about half the size but still multi-ton predators. And so it's real exciting to find this ability for evolution to scale up what obviously was a good plan.

FLATOW: Yeah, so it didn't - as we said before, it did not evolve gradually over time. It just was right there 50 million years before.

Dr. SERENO: That's right. You know, we see sort of three stages at this point in Tyrannosaur evolution, the branch that would include all the big ones and everything else that we've found, and a lot of everything else that we've found has been found in the last decade. And all of it from northern continents: Europe, North America, Asia.

FLATOW: Right.

Dr. SERENO: And these earlier ones had maybe one or two features that an expert might spot, but no one else would: a tooth here, a shape, the length of the iliac, the hipbone or something like that. Then comes Raptor rex at 125 million and really puts the body plan together for Tyrannosaurus.

FLATOW: 1-800-989-8255 is our number. Well, Dr. Sereno, could we not then have an Ankylosaurus mini - you know, all the other giant dinosaurs, the Brontosaurus minor, junior version of that?

Dr. SERENO: Exactly.

FLATOW: They all came fully formed 50 million years before.

Dr. SERENO: That was the question that I asked at the end of the science paper because unlike these beds where this dinosaur was found, and part of the story, of course, is that it was spirited out of China, collected illegally under wraps, sold in the United States, a private collector - Henry Craigstein(ph) an eye surgeon and avid amateur paleontologist - contacted me. I said you've got to give it up, give it up to science. Give it back, and I will name it and describe it. And he did.

And so that's the background of the specimen. But it was found in an area of northern China, which we can tell from the stuff that was stuck to the bones. And it came out in a big block of clams and fish bones that preserved these small dinosaurs.

On average, they're rare. Those beds tend to preserve them better than anywhere else in the world, and so my question was exactly yours. Are we just looking at an unusual mini in the case of T. rex that will eventually be found to exist for Allosaurs, for Carchar dinosaurs and maybe for Ankylosaurs or the proverbial Brontosaurs? I personally don't think so. I think that many of these animals have precursors, small precursors, that don't have the full suite of features that the very large animals have because there's so many things - body size is one of the best ways to adapt to a new environment and encounter new things, new food types and all sorts of things. A couple of genetic changes, you can change your growth pattern. There you are, but other changes occur to allow you to adapt to that kind of - those kinds of pressures.

This is a body plan which was successful across two orders of magnitude, 100 times the body weight, a T. rex which has exactly the same proportions of skull to body within the forelimb, hind limb. The bones to be sure are bulkier, the vertebrae stronger to handle the weight. But it weighs six tons, and it basically has exactly the same body design.

FLATOW: Well, Dr. Sereno, if someone came up to me and say hey, you know, I've got this dinosaur here. It's a fossil of a T. rex, but it's only 50 million years old, and it's a baby, looks like a baby dinosaur, wouldn't you be a little suspicious? How do you, you know, make sure it's not a fake or a phony or something you're, you know, buying off the back of a truck someplace?

(Soundbite of laughter)

Dr. SERENO: Absolutely, and of course I was incredulous as well as suspicious. Fortunate for the fossil and the fossil-collector and for science, the specimen came out largely in a block, and so you could see the bones connected. It was very complete.

But secondly, we didn't take anything for granted. We saved virtually everything that was stuck on that block, the sediment, fish bones, the clams as big as your palms stuck right to the dinosaur. That tells you the dinosaur was buried near a lake margin, and that's a real key to where it came from in that part of Asia.

But was it still a juvenile? Was it a juvenile of one of the big ones? For that, we had to prove the age of the specimen, and there's a couple ways to do that. The first is to examine the skeleton grossly because this dinosaur, like you and I, have bones that fuse with growth and maturity, and many of those bones were fused. Enough were fused, as in the pelvic girdle and the vertebral column, that clearly this dinosaur was not going to get much more than about 10 or 15 percent bigger.

But then we took one of the thigh bones and sectioned it, thin-sectioned it, because this has become now a cottage industry to figure out, almost like reading tree rings, pretty much how old the animal was, five to six years, and it was still growing, although we think toward the very end of its growth range.

Now, we've done that with T. rex, and we've done it with all sorts of Tyrannosaurs, and so we have a really great comparative sample, and it fits right in where it should: a small animal, maturing at about six to seven, old by 10. T. rex, by comparison, matures at about 15 years, is getting old by 20.

Sue, the one I pictured this dinosaur with in skull and forelimb, is 28 and by far the geriatric among the Tyrannosaurs.

FLATOW: 1-800-989-8255. Let's go to Larry(ph) in Oregon. Hi, Larry.

LARRY (Caller): Yes, sir, thanks for taking my call. I've always wondered, whenever we talk about whether they're ancestors of man or any new species, as they say, is it possible that you've just found one freak individual as opposed to a whole population? What are the numbers you're talking about?

Dr. SERENO: Well, there are fossils that - that's a great question. Individual variation does play a role, and when people say T. rex is the biggest, and this other dinosaur is the biggest and so on, that's a feature which we know varies quite a bit with a population sample, body size, within 10 or 15 percent. Now, if you're trying to say that this dinosaur, at 150 pounds and nine to 10 feet in length is going to somehow grow to 40 feet, I think that's not plausible. It depends on what kind of question you're trying to ask.

The first thing is the quality of the specimen. We have 95 percent of the skeleton. So I think we can say with quite a bit of a surety what the anatomy of this animal is like. And if you're willing to accept, as we do, and even give a higher bound than might be actually true, 10 feet, when we actually measure eight feet, a foot or two here or there, I think we're on the mark.

But you're absolutely right. We need more specimens, but sometimes we only get one. We cannot tell the sex of the animal. It's in good health. There was a little injury here or there. It does not look at all like we would find - you know, huge argument over the hobbit as to whether it's, you know, a sick individual or a healthy individual. And here we have a complete skeleton, every bone of the skeleton. This is not a sick individual. It's a very healthy predator.

FLATOW: Well, 150 pounds and nine feet long is one skinny animal.

(Soundbite of laughter)

Dr. SERENO: Now, when - yeah. When you look at it, Tyrannosaurus have the longest hind legs for their body size. They're built like horses - the thigh bone is very short and the leg gets longer proportionally as you go distally. They're really leggy, especially when they're at this body size, which would be relatively small. So the main mass is in the trunk.

And then you've cantilevered out this heavy skull, which has, again, for it size, bigger jaw muscles. They say, well, okay Dr. Sereno, but you didn't see the jaw muscles on the fossil. That's true. But we have really good marks, sagittal crests and fossae, that show us the size of the jaw muscles. And off the scale for predators who have really, really strong jaws. So what the Tyrannosaur mold is, is to put this big head and muscled jaws on the end of running legs, really some of the fastest legs we have for predators.

And what got shuffled to the side - and I think what the importance of the specimen is, is what got shuffled to the side is the fore limb. It's still function. It would have scratched you pretty hard, but it's not the frontline of defense or offense.

FLATOW: Mm-hmm.

Dr. SERENO: It really is the jaws, and the skull. You can't have everything. And especially if you cantilever out on a vertebral column, a big skull like that, you want to retain your speed and your agility.

FLATOW: Let's go to Miles(ph) in Boston. Hi, welcome to SCIENCE FRIDAY.

MILES (Caller): Hi. I was just curious what made you to believe that it's not just a case of parallel evolution, that they got the same form because it works as opposed to being related?

Dr. SERENO: Right. It's a great question. Why isn't this fossil just a Tyrannosaur parallel? Well, what we do is we look at all the features of the animal, and we see and score it in all Theropod dinosaurs. And that's the meat-eating dinosaurs, because we know it would fit somewhere in there. And, you know, this - ultimately be a certainly boring part of the research because it's so obvious. It fits without any contradiction into the Tyrannosaur radiation.

There's no other rational place we can put it. In other words, there's no other features in the animal if you - that would ally it with another group of dinosaurs that might have a short arm. And so there are a few other predators that do have short arms - they look quite different. We plotted the arm out and it smack plots right with the Tyrannosaurus. All the features there suggest Tyrannosaur relationships. I think we have, you know, a classic Tyrannosaur.

FLATOW: If this specimen was smuggled out of China, is it going to go back to China?

Dr. SERENO: It is going to go back. And we're thrilled (unintelligible) are from China. I've done a lot of work in Inner Mongolia. I've visited the tunnels that private collectors sometimes die in, in China - trying to extract these specimens. There have been hundreds of feet in the bowels of the mountains there in Liaoning and Inner Mongolia. And this specimen is going to go back to a museum, really a big museum they're building in Hohhot, the capital of Inner Mongolia.

FLATOW: And so this area where it was found now must be crawling, so to speak? But…

Dr. SERENO: It's been calling since we first discovered a bird there. We call it the China bird Sinornis, about 10 years ago. That was revelatory, but even more revelatory was when a local found the first feathered dinosaur. There are feathered dinosaur beds where you get the skeleton of these small forms, including another primitive Tyrannosaur Dilon(ph)that actually preserved the carbonized remains of rudimentary branching feathers. That's why we reconstructed those on Raptorex the way we did in the flesh model. We have solid evidence they were feathered, not scaled, over much of their much of their body.

FLATOW: Calling - talking about Raptorex this hour on SCIENCE FRIDAY from NPR News. Is this area any close to the digging spots of Michael Novacek and his folks, American Museum and Natural History?

Dr. SERENO: Yeah. I mean, they've described, with colleagues too, specimens from this area. Their dig sites, however, are in Outer Mongolia.

FLATOW: Right.

Dr. SERENO: And it was a thrill to be traveling in that area of the woods 20, 30 years ago. It's the Gobi Desert, split between Outer Mongolia, a separate independent country, and Inner Mongolia, a province of - an autonomous region of China. This area is Inner Mongolia - it belongs - but it is on route as you travel northwest from Beijing, going towards the great fossil beds of Outer Mongolia. These are earlier - they are lake beds. And when you find fish bones and clams, you're not finding the kinds of sediment and animals you'd expect to find in Outer Mongolia. It really clues you in that it did come from Inner Mongolia.

FLATOW: I have a tweet here from Gentfred(ph) who says, does this mean in 50 million years, humans might evolve to be several stories tall with short, stubbly arms?

Dr. SERENO: Well, see, this is actually the reasoning that led us, to some extent, astray. It's perfectly reasonable to think that what we see in development could just be tweaked and extended and we would look different because different parts of our body are evolving at different rates. Humans, for example, is born with a large head, our bodies catch up. Our legs are small - they catch up when we start walking in about two or three, they start growing faster. What if we just extend it then, we'd be really long legged pinheads. Well, that's - if we found something like that in the fossil record, that'd be a reasonable statement.

FLATOW: Mm-hmm.

Dr. SERENO: That's what we thought of transfer evolution before Raptorex was discovered. We thought that the arms got small as the animal got large. And what we have to admit now, is that this design emerged as small body size, and that's not what happened. That's what's revolutionary about the specimen in terms of mechanics, evolution and so on. It was all put together at pint size. And then I think, rarely, can this actually occur in evolution, where you can take something that's 150 pounds and basically, bone for bone, ratio for ratio - allowing for some robustness to handle the weight, scale it up to five or six tons. But it happened in the case of Tyrannosaurs. The evidence is at hand.

FLATOW: And as you say, it might have happened with the other big dinosaurs too?

Dr. SERENO: It might have, but I think we probably would have found more tell-tale signs, but I - we're always missing some small ones and so we're waiting to see what happens. I have something in store, in a couple of months.

FLATOW: Oh.

(Soundbite of laughter)

FLATOW: Give me a hint.

Dr. SERENO: Another lineage.

FLATOW: Give me a hint.

Dr. SERENO: Another lineage that reduced its arms - believe it or not, it's small body size. And it will be another reversal of what we thought actually happened in evolution.

FLATOW: So, this was another fully formed one of this - a young…

Dr. SERENO: Another predatory lineage. It got a small arm when it was small and then scaled up to be actually a quite a different kind of predator, but still with the smalls. When you try to - evolution is incredible. What we've learned about dinosaurs of the past is just remarkable in terms of the detailed piecing together of the evolutionary tree of dinosaur life. But if you go trying to predict things, you'll often find yourself out on a limb, as it were.

FLATOW: Well, all right. So, we're going to have you back, right?

Dr. SERENO: Absolutely.

FLATOW: You give us a call first?

Dr. SERENO: Sure.

FLATOW: All right. We're going to wait to see that new specimen that you came up with. Thanks for taking time to be with us today, Paul.

Dr. SERENO: You're welcome.

FLATOW: Paul Sereno is an explorer in residence at the National Geographic Society and also a paleontologist at the University of Chicago, talking about this little T-Rex, tiny little T-Rex, Raptorex. And we'll wait and see what else he has - sitting on the edge of our seats.

We're going to take a short break. We're going to switch gears and we're going to come back and talk about the old famous Irish potato famine. It's back, and this time it's attacking tomatoes. So, stay with us. We'll be right back after this break.

I'm Ira Flatow. This is SCIENCE FRIDAY from NPR News.

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