Short Wave - What crocodile bones teach us about dinosaurs

Episode Date: March 9, 2026

Paleontologists have often determined how old a dinosaur was by counting the growth rings in its bones. Just like with trees, it was thought that each ring corresponded to a single year of age. But researchers who studied crocodiles at an outdoor recreation center near Cape Town appear to have poked a hole in that approach. In the crocodiles, which are some of the closest living relatives of dinosaurs, there was more than one growth ring laid down per year. The results contribute to a growing debate over the best way to age animals.Read more of freelance science reporter Ari Daniel’s story here.Interested in more on the future of science? Email us your question at [e-mail] to every episode of Short Wave sponsor-free and support our work at NPR by signing up for Short Wave+ at plus.npr.org/shortwave.See pcm.adswizz.com for information about our collection and use of personal data for sponsorship and to manage your podcast sponsorship preferences.NPR Privacy Policy

You're listening to Shortwave from NPR. Hey, Shortwavers, Regina Barber here. Get excited because today we're talking about dinosaurs with science reporter Ari Daniel. Hey, Ari. Hi, Regina. Okay, I have got a riddle for you. Okay. What do you get when a team of researchers walks onto a crocodile farm?

Hmm. An academic team building exercise. Perhaps one with teeth. No, actually, the answer is. is a different way of thinking about the age of a dinosaur. Yeah, I was not going to get that. Yeah, fair enough. It was kind of a trick question. Okay, since you brought it up, though, Ari, I do want to know how have researchers traditionally

like estimated how old a dinosaur was. It's been a fairly simple process, according to Anusia Chin Sami Taran. She's a paleobiologist at the University of Cape Town in South Africa. She says you just take those fossilized bones and count up the growth rings. We always thought that these rings are formed annually. Meaning like a tree, you might imagine, one ring per year. Exactly. And then you can plot that and you can work out the growth rate of the dinosaur.

So for example, how long did it take T-Rex to grow from a hatchling to a fully grown adult? And that's what all of us were doing, me included. Okay, so how long does it? take for a T-R-X to become an adult, Ari? Well, based on this approach, 20-some years. Whoa. But to really confirm this ring-counting method, you'd need to study live dinosaurs. Which we do not have. They died 65 million years ago about.

But I have done some reporting on dinosaurs, and one of my favorite facts is that, like, birds are dinosaurs. So can they study birds? Yep, that's on the right track. Because the next best thing to look at, as you're pointing out is their living relatives, like birds. So we're going on our lovely crocodile tour now. And Crocs. Oh. We've got some big crocodiles in here.

See, the male's head just moved a little bit to the side. Well, okay, so where is this tape coming from? Where are you at? We are at Le Bonnier Reptiles and Adventures, an outdoor recreation and education center just outside of Cape Town where Quentin Cronier is the head animal handler. So today on the show, how this crocodile park reveals that scientists may have been overestimating dinosaur ages. I'm Regina Barber. And I'm R.A. Daniel.

And you're listening to Shortwave, the science podcast from NPR. Okay, Ari, you're here to explain to us, like, how crocodiles may help us understand dinosaur aging. Let's continue our journey into this reptile center in Cape Town. Perfect. So I want you to picture a flotilla of a hundred and six. 70-some Nile crocodiles lurking in these pools beneath a network of pedestrian bridges. It sounds awful. Well, that's where we're at.

So scary. They're basically the kings of the water bodies in Africa. I mean, they are the custodians of the African rivers. Alzette Muckie manages Labonir. We respect them and we basically let them be. We give them what they need and we just offer varying opportunities. And the scientists you introduced us to earlier, Anusia, does she work with the team at the reptile center?

Yeah, exactly. And I'm accompanying Anusia as she snaps photos of the beasts below. Really gorgeous. Each of them, their skeletons tell a story about how they grew. So we can say so much about the biology of dinosaurs because we have them as a model to understand dinosaur growth. And even though Alzette doesn't study the crocodiles in the same way, she totally feels that resemblance. It's like walking among dinosaurs every day. I'm quite tickled by it, I must say. So did the researchers set out with this dinosaur aging question in mind?

Not exactly, Gina. Anusia says that initially, she was actually interested in understanding how a crocodile's environment impacts its skeletal growth. Oh, okay. So to study that, here's what they did. Anusia and the team at Le Bonnier injected several year-old crocodiles

with an antibiotic over multiple months. Okay. The antibiotic actually gets taken up in the development of the bone. That leaves a signal in the bone. Okay, so they're like making like a tie marker in this bone tissue as like this animal gets older. That's exactly right. Now, even before the analysis, I spoke with this other biologist who was involved with the study. Her name's Maria Eugenia Pereira, and she said it was evident that these four crocodiles

had unique growth trajectories. They hatch together, they grow together, but at the end they have different sizes, different growth trajectory. Individually, they have different stories. The largest individual of the four grew to more than 80 pounds, which University of Cape Town Technical Officer, Andrea Plus, got to see up close when she was coming by regularly to measure, weigh, and wrangle the animals. It almost became too difficult to pick him up on my own. He was definitely a bully, and he tried to bully me, and he won.

So I had to bring in help. Now, Gina, all this happened more than a decade ago, back when this place used to raise and kill the crocodiles to sell their leather and meat. Oh. Okay, so that's not the case anymore? No, the staff says the animals now live out their natural lifespans. But in 2013, when those four crocs were two years old, Labonir got their leather and Anusia got their bones.

Oh, hi, Abby. Welcome. Thank you. Okay, so we're not at the crock center anymore. I'm guessing. You are correct. This is Anusia's lab, which is at the University of Cape Town. Okay.

She rifles through several slides, each one containing a super thin cross-section of a crocodile arm bone or leg bone. I love the sound of science. It's very calming. Yeah. Maria is the one who prepared these. slides, and she polished them until the growth rings were beautifully visible. So the time that the light go through the section and you actually can see all the structures

is the time that you know that the section is the good one. Anusia holds one of these slices up to the light. Look at that. You can see some lines. Look in this area, yeah? Yeah, banding. Yeah. You see that very, very clearly. Under the microscope, they will be more visible. Okay, so what did Anusia and Maria see? once they actually did look at it in the microscope. Something unexpected.

More rings in some of the bones than they were anticipating. Huh. This is a two-year-old crocodile, and in many cases we found up to five growth marks in the bones. So there were extra growth marks formed during their short life. Right. You might have thought they were five years old. Exactly. So what does that mean?

Like, are they not growing at a constant rate? Like, are they growing super fast? Like, does this mean the counts could be off? Perhaps. And this could have some bearing on dinosaur bones. Because if dino bones were formed similarly, then these crocodile findings suggest that at least some dinosaurs may have been younger when they perish than we previously thought. Right. And similar results in other reptiles as well as kiwi birds, back that up as well.

Wow. Okay. It changes how we think about how we can use growth marks to determine tannosal growth patterns. Suggesting, Anusia says, that these marks may be better thought of as cycles of growth. This sounds like pretty huge. Like, what do other researchers think about these conclusions? Well, I spoke with Holly Woodward. She's a paleo-histologist at Oklahoma State University.

Studies like this one are really important in adding to that body of knowledge of how often then growth rings can be reliable. We haven't really done as much ground-truthing as we could with modern animals. But Holly doesn't believe the matter's settled, since some modern animals do show annual growth rings and others don't. It's very weird, but we can't yet say why or what causes it specifically. But does Holly have any ideas? Well, she says it could be due to differences in hormones or maybe day-night cycles,

But until researchers know more, Holly argues that growth rings remain at least a useful starting point for understanding dinosaur growth. Okay, okay. So I also talked with Christy Curry Rogers. She's a dinosaur paleobiologist at McAllister College in St. Paul, Minnesota. It's sort of a cautionary tale not to overinterpret what we can see and know based on bone tissue under the microscope. This confirmed a suspicion that I've often had.

in my own work because we still don't understand everything we need to about living vertebrates and how their bones respond to the environments around them. And what about Anusia, the scientist from Cape Town? Like, what does she think about all this? She agrees that there is more work to be done for sure. We've always estimated the age of a dinosaur. And what this means is that we can still get a rough estimate, but people have to realize that it's an estimation.

And even though researchers may not understand the full picture yet, Anusia believes that the answers may well be waiting for us. It's all in the bones. It's all in the bones. Thanks so much, Ari. My pleasure, Gina. Anytime. Thanks for listening, shortwavers.

And if you like this episode, please follow us on the NPR app or wherever else you may be listening from. And check out some of our other episodes with Ari, like the one about scientists, scavenging dead whale brains. Or the one about sludge. that could potentially save the planet. We'll link to those in our show notes. Reporting for this story was supported by a grant from the Pulitzer Center. I'm Regina Barber. Thank you for listening to Shortwave from NPR.

So, Ari, I do have one more question. Shoot. Do our bones, like human bones, have rings too? I actually asked Holly Woodward about this, and she told me that humans do have growth rings. They're just not reported that often, because we usually stop growing in our 20s or so, and the bone tissue that's usually examined comes from skeletons of older individuals.

And when we get older, our bones remodel and destroy the presence of previously formed rings. Wait, so if you were to dig up somebody who died at like 90, the rings wouldn't tell you anything. Or you might not even see rings? Right. Yep, exactly.