Image 1: Ischigualasto Formation, exposed in the Ischigualasto Provincial Park in San Juan Province, Argentina. Image 2 (by Jordan Harris): Herrerasaurus ischigualastensis is an early saurischian dinosaur. It shared a bipedal, running anatomy common to large carnivorous dinosaurs that would evolve in the future, but this dinosaur lived at a time when dinosaurs were small-bodied and rare.

Famous dinosaurs, like the long-necked giant sauropods, duck-billed hadrosaurs, and fearsome carnivores like T. rex grew up fast, but all of these icons evolved millions of years after dinosaurs originated. When dinosaurs first evolved, they were far from the most numerous, most diverse, or most specialized in their world. In fact, that world was ruled by different groups of reptiles, many of which are more closely related to living crocodiles than to dinosaurs. Sometime near the end of the Triassic Period around 230 million years ago, the tables turned, and dinosaurs became more common. Scientists have long surmised that growth may have helped them rise to dominance. But exactly how fast the very first dinosaurs grew compared to other animals has remained a mystery. Until now. 

In a groundbreaking new study published on April 3 in the journal PLoS ONE, lead researcher Dr. Kristina Curry Rogers, DeWitt Wallace Professor of Biology and Geology, and her colleagues, including DeWitt Wallace Geology Professor Ray Rogers, offer a first-ever look at growth patterns among the earliest-known dinosaurs. Their results pave the way for paleontologists to explore new ground that will help us better understand how dinosaurs came to be the ultimate survivors of multiple mass extinctions, and eventually rose to rule the earth for hundreds of millions of years. 

What’s the top-line takeaway from this study? 

 We’ve always wanted to know something about how and when dinosaurs may have originated some of their most special characteristics, and which of their many traits might have been critical to their survival. One feature that is often thought to set dinosaurs apart is their rapid growth rate. Studies of man species reveal that dinosaurs, as a group, grew more like mammals and birds than like reptiles. But when did dinosaurs first evolve these rapid growth patterns? In our study, we took a look at growth in the oldest known dinosaurs, as well as a suite of reptiles living at the same time and in the same ecosystem. We wanted to know whether at this earliest moment of dinosaur evolution, the first dinosaurs grew any differently than their non-dino sidekicks. It turns out that, at the start, there was a lot of overlap in the way all of these animals grew! When dinosaurs first arrived on the scene, their growth rates really didn’t set them apart. 

How do these findings alter what was previously understood about early dinosaur size and growth rates? 

This is actually the first time that anyone’s been able to take a look at these earliest dinosaurs, especially in the context of other animals in their world. The first dinosaurs are incredibly rare — some of them are only known from a single, partial skeleton. Getting access to the insides of their bones, which is an inherently destructive sampling technique, has never been possible before, so this is literally the first time we’ve had a look at these earliest animals. It doesn’t compare precisely to anything that has come before, because we’ve never been able to sample in this comprehensive and standardized way. 

What were some of the major challenges you had to overcome to produce these results? 

First, the earliest dinosaur fossils are so incredibly rare. Second, bone histological research is destructive — you have to cut bones open, remove chunks from them, make thin-sections,  and eventually grind those slices down until they are so thin that light will pass through, allowing you to study the details under a microscope. It’s hard to get curators to allow you to do that, because they’re worried about losing the outside information that skeletons provide, especially for very rare dinosaurs and other animals. Just getting access to these kinds of specimens and being able to have a sample that was comparable is a huge feat, and all thanks to my collaborators in Argentina. They understand the value of information contained deep within bones, and were excited to work together to learn as much as we could about these 230 million year old reptiles. 

What unique approach did you take with this study?

Being able to look at the same area of the same bones in all of the animals we sampled was an important part of our study. Most of the times when we study growth rates in any fossil animal, our samples are opportunistic. Often this might mean that samples come from very fragmentary specimens that don’t hold a lot of anatomical value otherwise. You may only be able to sample a shin bone from one animal, a thigh bone from another, and a rib from yet another. But since each of these bones does a different job in an organism’s body, such a diverse sample might be telling you a slightly different story about that animal’s growth history. To be very specific about comparing growth patterns among different kinds of creatures,, it’s best to sample the same bone, in the same place, and from the same environment.

For this study, we sampled only the middles of the thigh bones, where we have the longest record of growth history preserved for each  animal we studied. We sampled only the largest known skeletons of these animals in hopes of getting as much of a complete growth record history that we could. We also constrained our sample to a very narrow time window in the ancient past, so that we weren’t dealing with a background paleoenvironmental change that might impact what we were seeing in the bone tissue. Those controls gave us a really unique perspective on comparability across this dinosaur and non-dinosaur group. 

This study focuses on one particular geological unit in Argentina called the Ischigualasto Formation. Describe this place. What makes it so fertile for doing research like this? 

The Ischigualasto Formation is within the Ischigualasto Provincial Park in San Juan Province, northwestern Argentina. The bizarre and rugged desert terrain that exists there now has a popular name: “The Valley of the Moon.”  The sandstones and mudstones there  record a volcanically active floodplain dominated by rivers and strong seasonality. We’re lucky, because during the Triassic period when all of these early dinosaurs were running around, the area  was a perfect place for animals to live, but also a great place to die and get buried. You end up getting a really great fossil record in places like this, and so the Ischigualasto is the oldest place we find these dinosaurs. It also includes a series of rocks that show us what was happening before dinosaurs evolved and also what happened after dinosaurs first showed up. 

What are the next questions you plan to explore in this line of research? 

This study gives us a first look at how growth strategies varied among the early dinosaurs and a whole ecosystem of their contemporaries. Our results indicate that, though these earliest dinosaurs grow fast, they aren’t alone!  Most of the Ischigualasto reptiles exhibit elevated growth rates relative to modern reptiles. These speedy rates may have been an important adaptation to survival in a turbulent world marked by two of the most serious extinctions the earth has ever seen.  The obvious next steps are comparisons to a similar diversity of animals from other ecosystems around the world. We have a couple of other places on the surface of the Earth where we have similarly diverse faunas. I think expanding this kind of project to other early dinosaur localities will  be really important. 

In addition, as new dinosaurs continue to be discovered, our gaps in the evolutionary tree of dinosaurs get filled in. Tracking the features of fast growth through the dinosaur (and non-dinosaur) family tree will help us understand the relationships between growth and other features that paved the way for dinosaurs to eventually take over. 

April 3 2024

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