Did speedy growth spurts ensure dinosaurs' dominance?

For nearly 200 million years of Earth’s history, dinosaurs were the dominant force on the planet. And now we think that their early development might have played a large part as to why. It turns out that dinosaurs, and a few other reptile relatives, hit their growth spurt pretty early on in life. That may well have given them the edge over the competition, by making them rapidly much better developed, and possibly too big to eat. I’ve been speaking to Macalester College’s Kristi Rogers...

Kristi - When we think about the animals we have on Earth today, we can really think about them as reptiles, mammals and birds, as big groups. And we know that birds are actually just modern dinosaurs that have shrunk their body sizes and changed their biology a little bit. Birds actually grow incredibly quickly. When we push back down into the dinosaur fossil record, we can see that dinosaurs also grew quickly relative to other animals living in their landscapes, other reptiles especially. So we know that at some point when we go even farther back in time, we can see that some of the very first animals that were really living on land actually grew more slowly, kind of more like traditional modern reptiles. And so we wanted to see if there was a point at which dinosaurs flipped that switch and made the change to rapid growth. And whether or not anyone else in their ecosystem, that wasn't a dinosaur, did the same thing.

Chris - And you can tell this from the fossils?

Kristi - Exactly. So most of the time people imagine palaeontologists looking at the outsides of bones and measuring bumps and measuring the lengths of bones and characterising the outside anatomy. But there is a branch of palaeontology that cracks open the bones and looks at the insides, especially the microscopic structures that are present. So we use a method that we call bone histology, and we look for signals like the places where blood vessels stream through the bone when that dinosaur, or any other animal, was alive, or the spaces were cells that helped build and maintain that bone when the animal was alive. So we use microscopic structures like those which are really good proxies for relative speeds of growth in all kinds of backbone animals.

Chris - And how far back in time did you look?

Kristi - We looked back to 230 million years ago. And this is just at the very end of a period of geological time in which dinosaurs were just getting started.

Chris - Right. So they weren't the kingpins at that time. They were part of life's rich tapestry, but they certainly weren't the main part.

Kristi - They were really 'living in the darkness and screwing around' kind of animals at this time. There were other groups of reptiles closely related to dinosaurs. You can kind of think of them as almost like dinosaurs, distant cousins. The ancestors of today's living crocodiles. And those organisms were really ruling the world at this time period.

Chris - And did you find that these animals, this far back in time, appeared to be growing very quickly?

Kristi - Every early dinosaur that we could sample, all of them are growing at elevated growth rates, more similar to the growth rates of living mammals and birds than to modern reptiles. But the really interesting thing is that so were a lot of other reptiles that lived just alongside dinosaurs.

Chris - So do you think that this is just a reflection on what the environment was doing that selected for animals that grew really fast? Or were they growing really fast because everyone else was growing really fast and to keep up and to fend off the opposition they had to?

Kristi - Yeah, we have to think about it in the context of what was happening in the world when these early dinosaurs and their non-dinosaur sidekicks were evolving. The Earth had just experienced, 30 million years before, or 20 million years before the worst mass extinction that the Earth has ever seen. Earth's life almost went extinct. And so these organisms are the descendants of the things that recovered, that survived. And what we've learned is that in order to survive mass extinction, you're going to be more likely to get through an extinction filter if you're relatively small bodied, if you are an ecological generalist who can eat lots of different kinds of things, basically if you're not very specialised, you're more likely to make it through because you can adapt in that changing and fluctuating world. So we think that elevated growth rates work relatively common because the Earth was changing so much and so rapidly at this time. And in the times just before we find these dinosaurs and their non dinosaurian relatives.

Chris - I think it's amazing that you can see this much information in what isn't even the real bone. It's a fossilised impression of that bone from hundreds of millions of years ago.

Kristi - Yeah, it's one of the things that I think is, is such a fun part of palaeontology because we have the beautiful skeletons, the outsides of these bones, but when you crack them open and begin looking at the internal anatomy, the microscopic structures, you can learn so much more about how dinosaurs and all kinds of backboned animals lived. And that is such a fun part of my job.