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Why did some dinosaurs grow so large? Researchers have new insights

EYDER PERALTA, HOST:

If you've ever seen a cut-down tree, you've noticed the rings that mark each year of growth. But those rings don't just show the tree's age. They show how fast the tree grew. A new study took this idea and applied it to dinosaur bones to learn just how some dinos got so big. Michael D'Emic is a paleontologist at Adelphi University. He joins us now. Welcome, Michael.

MICHAEL D'EMIC: Hi. Thanks for having me.

PERALTA: Yeah. So your research focused on a particular group of dinosaurs, right? How did you analyze their bones?

D'EMIC: So we looked at a group called theropod dinosaurs that includes some of the most famous examples, like T. rex and Velociraptor, Archaeopteryx. And, importantly, that is the group that includes birds living today. So we had to take samples or slices from each of the leg bones and affix them to a glass slide - a large microscope slide - and sand that down to about the thickness of a human hair, so thin that light could shine through. And then we could count and measure the growth rings.

PERALTA: So you studied theropods, these big dinosaurs. And what did the study find about the way that they grow?

D'EMIC: Our study found that there is no one way to grow a dinosaur, that the largest dinosaurs sometimes took as little as 10 years or so to get to their truly immense sizes, and some others would have taken decades. So there were vastly different growth rates and durations in the largest dinosaurs. And we actually found that that was true for even medium- and small-size theropod dinosaurs, as well.

PERALTA: And does that challenge conventional wisdom, what we thought about how dinosaurs evolved to get so big?

D'EMIC: It does. It was thought that in the group Dinosauria, which includes the group that we studied, it was thought that the predominant mechanism for evolving a larger body size was through developmental acceleration, so having a faster growth spurt. And what our paper shows is that it's just as equally likely that they actually slowed their growth but grew for longer.

PERALTA: Are there evolutionary advantages to, like, these two types of growth?

D'EMIC: Yeah, there's advantages and disadvantages. There's tradeoffs. So evolving to grow faster than your ancestors means that you can possibly outcompete other species in your environment for resources. So maybe you can reach taller trees or get to environments that smaller species can't get to. And you can also then outpace the growth of predators in your environment. So you're not small for as long, and so maybe you're not potential dinner for as long in your life. The disadvantage is that it takes a lot of energy. And so if lean times come, like a drought, there's not that much food around, then you would be more prone to extinction.

PERALTA: Can we talk about living animals? Do we see that kind of variety in how fast or slow large species grow in animals that are living now?

D'EMIC: Absolutely. Yeah, we see a whole range of growth rates and patterns in animals today. The tricky thing is that the sample of animals that we have today, those are just the animals that happen to be with us, right? I call them animals that happen to not be extinct. So it's not a true evolutionary sample. To get a sense of how evolution proceeds, you need to sample fossils in the past. There's a famous quote - nothing in biology makes sense except in light of evolution. We can get led astray by just looking at animals today.

PERALTA: That's Michael D'Emic, a paleontologist at Adelphi University. Michael, thank you so much.

D'EMIC: Thanks for having me. Transcript provided by NPR, Copyright NPR.

Eyder Peralta is NPR's East Africa correspondent based in Nairobi, Kenya.