Re-thinking the Fearsome Tyrannosaurs

Bob Whitby: Hello and welcome to Short Talks From the Hill, a science and research podcast from the University of Arkansas. I’m Bob Whitby, a science writer at the University of Arkansas, and today we’re speaking with Celina Suarez, associate professor of geosciences, regarding her work with a group of scientists who recently published a study concluding that tyrannosaurs were in fact social hunters more akin to wolves than the fearsome solitary predators of popular imagination. Interesting findings, welcome Celina.

Celina Suarez: Thanks Bob, great to be here.

BW: Just give us a really quick background on what you do.

CS: So here at the university I mostly focus on teaching geochemistry and teaching about Earth history and Earth system history, how the Earth acts as a system over time, and so with that I’ve kind of amassed a number of different skills, everything from paleontology and sedimentology, which is like looking at the rocks, to geochemistry and understanding the chemistry of rocks, rocks, and fossils and how that can kind of tell us a little bit about the past, not only the environment, but also the biology of organisms from the past.

BW: This particular finding had its basis in a quarry in Utah with the great name of Rainbows and Unicorns.

CS: Yeah.

BW: Which I’d? I’d like to ask you how that got named. But first let’s talk about the actual discovery. What did they find when they when they discovered this site?

CS: So all of this material comes from the Grand Staircase Escalante National Monument, which is a monument basically on the Kaipairowits plateau in south central Utah, and it has wonderful paleontological resources, as well as archaeological resources, actually, as well. So paleontologists have been working with the Bureau of Land Management for the last almost 20 years trying to get an assessment of the paleontological resources out there, and they usually have field seasons out there every summer in collaboration with the University of Utah and the Denver Museum of Natural Science. And you know they were out prospecting, looking for fossil sites, and one of the people, who is now the lead preparator at the University of Utah, stumbled upon some toe bones that were fairly large, theropod toe bones, and most of the large theropod toe bones out in this formation are tyrannosaur-type dinosaurs, so they found that site first. Of course they found it near the end of the field season. That always happens, and they returned the following season and found just more and more of these teratophoneus organisms. And one of the other really interesting things is they started to find a large variety in sizes. Everything from like a little bitty baby hatchling, not hatchling but baby probably that it was around four years old, to full-size adults which definitely got the imagination going. So that’s kind of how the sites got found and it was such a spectacular site when they first found it, one of the workers was paleontologist Alan Titus, who’s actually in Arkansas alum, you know, was super excited about it and one of his workers were like, “Oh yeah Alan, with you everything is rainbows and unicorns,” and the name kind of stuck, because it just continued to be a really productive quarry site, so they just started calling it the Rainbows and Unicorns site.

BW: It’s a great name, very memorable. So one of the things that really struck me about the finding was that, you know, all these bones were jumbled, they were not, I guess, laid out as you would expect them to be laid out if the dinosaurs had all died in a group. And your work helped recreate a very specific narrative of how they got to where they are, and that led you to the conclusion that they had all died at the same time. So it’s a bit of a detective story, it sounds like.

CS: The paleontologists had suspected this for a while, that they were probably all buried at the same time, but Alan kind of came to me, I had been working with Alan and my former Ph.D student, Daigo Yamamura. We’ve been working with Alan on various projects within the Kaipairowits formation, the last, I don’t know about five years, and he came to me asking, “Well, is there anything that we can do to tell that these all were fossilized at the same time?,” and he gave me his rundown of what he thought happened. And initially we started focusing on isotope work. So stabilized isotopes, which are just atoms of the same type of element but different amounts of neutrons within the nucleus, so it gives them different masses. And I said sure we could take a look at the isotope work and take a look at, say, some of the soil nodules that were found within the site as well as embedded within the cavities of the bones, to see if they have very similar isotopic composition, which they did. And so I was like, well that’s probably a clue that they probably were buried within a similar soil. That soil was probably reworked and redeposited in this channel sandstone. There wasn’t a dead ringer yet. So I was like, you know, the other thing we can look at is trace element geochemistry, which is work that I did early on in my career as like a master’s student and Ph.D. student. And those elements are what are known as the lanthanides. So those are the elements, if you think about the periodic table of elements and everybody focus on the main part and then at the bottom there’s those two rows that most people are like, oh, don’t worry about those, they’re just weird, but it includes things like uranium and thorium and things like that. So the lanthanides, that top row that includes the elements lanthanum through lutetium. They’re heavy elements, but one of the interesting things about them is they really love apatite. All of our bones are made up of apatite, just a particular type of mineral, and so they usually suck up trace elements really quickly when bone that used to be part of an organism is buried and comes into contact with soil fluids. And you can look at the patterns of them to tell kind of the environment that they were fossilized in, as well as if they were coming from a number of different places. So if a bone is fossilized in a lake somewhere far away and another bone is fossilized into soil far away, and then a stream reworks them into a channel, they should have different trace element concentrations. So, we did. I got it processed and Eric Pollock at the stable isotope lab started running on the ICPMS that we have and yeah, they came back all pretty much the same trace element had relative proportion of the different elements. Everything from the turtles, the fish, the different types of tyrannosaur bones that we analyzed. And so that basically tells us they were all buried and fossilized in the same environment. And since we have different ages of the same type of animal, most likely they were all living together. Whether or not they were hunting down a hadrosaur together, hard to say, but they certainly were all living together.

BW: And it’s even more specific than that, as you thought that perhaps they had died in a flooding event? Is there evidence of that, or is that just kind of a way to explain why they’re all in the same place at the same time?

CS: Yeah, so the reason why we think they probably died in some sort of flooding event and were deposited across the landscape initially within a soil is because we see little bits of soil carbonates. So especially like if you live in Texas, if you go out and you dig down into the dirt, you’ll find little white blobs. Those are soil carbonates and they typically form where you have kind of seasonal events. They probably all died at one point somewhere; whether or not that was in a flood or not, it’s hard to say, but they certainly were reworked back into a channel setting where we have like a channel sandstone that you would typically see, like a little a small river or something like that preserved, and we also look at the other associated animals that have been found with them. So there are several turtles that were found with them as well as gars, like large fish. Those typically are found in water environments, so we know that there was a water environment nearby. They had very similar trace element patterns as well, so they were probably swept into this body of our small body of water during a flooding event, and we’re all fossilized about the same time.

BW: Are you able to say what this means, what are the implications of this finding in terms of, I guess in terms of further research and or what we know about tyrannosaurs?

CS: Yeah, sure. I mean it certainly gets the mind going as far as social behavior, like social structure. So we know they were probably living together and they probably all died together and were buried and fossilized together because they have this similar trace element concentration, whether or not they were behaving as like what you would imagine as a pack, like a wolf pack for example, it’s hard to say. It’s really hard to definitively prove behavior in the fossil record. But one way we might do that is looking at stable isotopes, and that’s something that Alan and I are hoping to do in the near future. You are what you eat, and so one of the elements that we can use is carbon. Carbon isotopes are a particular isotope. You can use to look at dietary affinity. And so we can try to look at carbon isotopes within the different ages of the tyrannosaurus to see if they are all very similar, or if they’re all very different. If they’re very similar, then most likely they’re all eating the same thing. So say a large tyrannosaur brings down a hadrosaur, eats it and all of the babies and younger organisms come and eat the same thing.

BW: So we can’t rule out the idea that they may have just been hanging out together, further work remains on that.

CS: Yes, work remains.

BW: OK, well thanks again for joining us. Great to talk to you again.

CS: Thanks Bob. Good to be here.

Matt McGowan: Music for Short Talks From the Hill was written and performed by local musician Ben Harris. For more information and additional podcasts, visit Arkansas Research. That’s arkansasresearch.uark.edu, the home of science and research news at the University of Arkansas.