UMN research sheds light on evolution of carnivorous dinosaurs

[MUSIC - JOHNNY CASH, "DINOSAUR SONG"] (SINGING) Dinosaurs lived a long time ago. They were terrible lizards, don't you know? Some ate plants, and some ate meat. Some ate fish, and some ate meats.

CATHY WURZER: It's true, country legend Johnny Cash did a children's album in 1976, and this is The Dinosaur Song. If you have kids in your life, you may have fielded thousands of questions about dinosaurs, like why do some dinosaurs have such big heads and tiny arms?

Well, we may have an answer for you. University of Minnesota professor Peter Makovicky wondered the same thing, and he's one of the authors of a newly-published study shedding light on the evolution and biology of these dinosaurs. And he's with me right now. Welcome to the program.

PETER MAKOVICKY: Thank you, and good afternoon, Cathy.

CATHY WURZER: Good afternoon, professor. OK, so it sounds like the tiny arms and big heads, that those dinosaurs have been kind of a scientific mystery for some time. Say more about that.

PETER MAKOVICKY: Yeah. So obviously, this mystery that you refer to started with the discovery of T. Rex over 120 years ago now. Why would you have this incredibly large animal with a giant head full of big teeth and then incredibly short arms?

So just for people to visualize it, T. Rex is a 9 ton animal with a 5 foot head but has an arm about the same length as an adult human. What were they used for? What good were they, basically?

And so that's a question we've pondered as scientists ever since that discovery. And it turns out it's not just T. Rex. There are actually at least three different families of meat-eating dinosaurs that have that same body plan. Large size, big heads, and short arms.

And so with the discovery of a new species from Patagonia, we've actually been able to look at this question statistically in this recent study you referred to.

CATHY WURZER: And they weren't even geographically close to one another, is that right?

PETER MAKOVICKY: Yes. So the interesting thing is these three lineages-- the tyrannosaurs that we're all familiar with, the abelisaurids and the carcharodontosaurids, which are better known from the southern continents, achieved the similar body plan independently of one another and at slightly different times in the geological record.

So they're sort of replacing each other, as it were, and doing this ecological thing, rather than sort of coexisting. So there is some structure to that, which is also interesting.

CATHY WURZER: So how did you in the other researchers determined that this could help shed some light on other already discovered meat-eating dinosaurs? How did that happen?

PETER MAKOVICKY: Yeah. So in 2012, I was working with my collaborators in Argentina on an international collaboration, and we discovered a new species of meat-eating dinosaur, which we named Meraxes gigas in this recent paper.

And the neat thing about Meraxes is it's a carcharodontosaurids, so it's in the same family as Giganotosaurus, a T. Rex-sized predator that folks may be familiar with from the recent Jurassic World, Dominion movie.

And as luck would have it, Meraxes has the most complete form of any of these carcharodontosaurids from South America. And one thing we noticed immediately is that it's proportionately tiny, the same way and comparable to what we see in T. Rex and its relatives.

And we wondered, was that a coincidence, or does that actually indicate some kind of active selection over evolutionary time for these short arms? And we undertook a number of advanced statistics, taking into account that we have three different lineages doing this.

And we showed that the similarity, the degree of forelimb reduction in tyrannosaurs, carcharodontosaurids, and the abelisaurids is very unlikely to have happened by chance, that these animals ended up looking like one another for a very good reason.

So that was sort of the first clue we had that maybe there's more to these arms than we think. And then we sort of did some broader comparisons, looking at the body proportions of meat-eating dinosaurs more broadly.

And what we basically showed is that this isn't just a function of size. There are other big meat-eating dinosaurs whose forelimbs are not as reduced. So it's something special about these three species.

And what we were able to zero in on statistically is that they have really big heads. So what we think is that over evolutionary time, these three lineages are, independent of one another, they're getting bigger, and they are getting really massive skulls with huge bite forces. And as a consequence, they're reducing their forelimbs.

So they're sort of actively evolving towards this body plan that obviously is famous and well-known to everybody.

CATHY WURZER: So OK, if someone's just tuning in, they're listening, they're thinking, OK, this sounds kind of cool. So what's the takeaway for folks on this?

PETER MAKOVICKY: Yeah. So I think the takeaway is that we, as humans, are incredibly focused on our dexterous forelimbs and our ability to manipulate the environment around us using our hands and our arms.

So for us, it's incredibly counterintuitive to think of another group of bipedal animals basically reducing their forelimbs and making them less functional. But in fact, we have these lineages of dinosaurs that are transferring, as it were, the functions of the forelimb to the functions-- to the skull. They're sort of re-optimizing their skeleton.

And so that's an interesting point in an evolutionary sense, because the three lineages of dinosaurs we're talking about are the largest bipedal animals to have lived and to have acted as top predators in their ecosystems. So I think it's one of these things that sort of, again, challenges our understanding of nature and challenges our understanding of how ecosystems worked in the past versus how they work today.

CATHY WURZER: So I understand that this particular dinosaur was discovered some 10 years ago. So this research is just coming out now, which is kind of interesting. That's quite a lag time.

PETER MAKOVICKY: Yeah. So I think folks are maybe a little misled by the sort of impressions you get watching Jurassic Park. I remember the early ones, they just sort of brushed some sand off, and there's the whole skeleton.

CATHY WURZER: Yes.

PETER MAKOVICKY: And then they use some imaging, and hey, presto, it appears. And then in one of the subsequent ones, they just sort of 3D print the whole thing in sort of movie time, in a couple of minutes. And of course, it takes much longer palaeontologically.

So as I mentioned, we discovered the specimen in 2012. It took four digging seasons to actually get the whole skeleton.

CATHY WURZER: Ah. OK. It took you a while.

PETER MAKOVICKY: Yeah. It took a while to actually get it out of the ground. And then when you collect fossils, you collect them with some of the surrounding rock. You encase it in plaster jackets to protect it.

So then there's a very long process of what we call preparation, where you have skilled technicians basically removing that surrounding rock, cleaning up the bones, and stabilizing them with a variety of adhesives to actually allow us to move them and study them.

And that's when the analytical part starts happening. And obviously, there's some delay because this is an international collaboration. My co-authors, my partners are in Argentina. There's some delay, obviously, inevitably caused by the pandemic and our inability for me to travel down there and work with them.

So you know, it actually takes time. So 10 years is maybe on the longer end of things, but it's not unusual for a dinosaur. Remember, it's been in the ground for 100 million years.

CATHY WURZER: Exactly.

PETER MAKOVICKY: So 10 years is just-- we have a margin of error.

CATHY WURZER: Well, professor, thank you for your time. It was very interesting. I appreciate it.

PETER MAKOVICKY: Yeah. Thank you, Cathy. You have a great day.

CATHY WURZER: You, too. U of M Professor Peter Makovicky has been with us.