New Dinosaurs Evolved After Dramatic Global Climate Changes Killed Competitors

The Dolomite Mountains in northeastern Italy are a paradise for geologists and paleontologists. Bob Strong/Reuters

The Dolomites, an Italian patch of the Alps known for their dramatic cliffs and narrow valleys, are a paradise for scientists who want to study the deep history of the Earth. Massimo Bernardi, a paleontologist at the Museum of Science, in Trento, in Italy, confesses he may be biased in calling it spectacular, since he was born in the region and lives in the range's foothills today. But he has a geologic rationale to back up his sentiment.

He describes climbing up steep slopes that reveal petrified signatures of changes in the region, with his feet planted on the remains of one era of the past and his hands stretching up to grasp rock that's a million or two years younger. That detailed geologic record has allowed scientists to pinpoint how old certain sections of rock are, which is usually a tricky feat.

"You see the different rock layers," he told Newsweek. "You can see time in front of your face."

More recently, Bernardi and his colleagues used ghostly footprints within those layers to track the rise of the earliest dinosaurs. In a new paper published in the journal Nature Communications, they tie the first wave of dinosaur diversification to a period of climatic upheaval known to scientists as the Carnian Pluvial Episode. During those 2 million years, the global climate flip-flopped dramatically from arid to humid to arid.

"The timeline is there," Stephen Brusatte, a paleontologist at the University of Edinburgh, in the U.K., who wasn't involved in the new research, told Newsweek. "Whether it was a cause-and-effect thing, that's a lot trickier to test."

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Geologists have recognized the strange rock signature of the Carnian Pluvial Episode in the Dolomites for more than a century. The region's rock is usually very pale, but it is interrupted by red, crumbly rock full of clay and sand. In the past few decades, scientists had linked that strange layer to worldwide ecosystem upheaval in which landscapes careened between being extremely dry and extremely wet.

"What was simply geological evidence turned out to be a signature of a global catastrophic event," Bernardi said of those climate changes and the rock changes they left behind. (The upheaval may have been spurred by a massive volcanic eruption in what's now Canada, although that hasn't been confirmed.)

In the new paper, Bernardi and his colleagues argue that climate change saw a dramatic change in fossil beds, from these remains containing just 5 or 10 percent dinosaurs surrounded by many other species to beds with 80 percent dinosaurs. These early dinosaurs settled in new territories and became somewhat larger, Bernardi says,from "kind of pet-size"—perhaps 5 to 8 feet long—to more like 13 feet. One of the best known examples of these post-crisis dinosaurs is the Eoraptor, a small carnivorous dinosaur from Argentina.

This is still very early in the history of dinosaurs. The Carnian Pluvial Episode and its miniature dinosaur boom happened about 30 million years before dinosaurs became a truly dominant group around the world. For their true reign to begin, dinosaurs needed to wait until another extinction crisis wiped out many reptiles, their key competitors. It was only after that second period of global upheaval that the huge, famous dinosaurs like the Tyrannosaurus rex evolved.

The new paper is based on evidence from skeletons found in South America and footprints from Bernardi's native Dolomites. Brusatte says that the footprints in particular are an underused way to learn about dinosaurs. "Oftentimes, we devote most of our attention to pretty fossils and to skeletons, to things that are complete or nearly complete," he said. But while an individual animal can only leave one skeleton, the same animal can leave thousands or even millions of footprints.

Bernardi says that his team's research shows that dinosaurs didn't thrive because they were somehow better species, as some decades-old theories had suggested. Instead, they were at the mercy of external forces, only developing a strong foothold when they survived a crisis other animals didn't. While we usually think of extinctions in terms of deaths, "it's only half of the story," he said. For survivors, those deaths aren't a loss, they're a huge opportunity.

Brusatte says there's not enough evidence to firmly dub the Carnian Pluvial Episode the cause of the dinosaurs' diversification but that it wouldn't be surprising to see the climate shaping the animals of the time. After all, climate change is what wiped out the dinosaurs themselves tens of millions of years later.

He added that when these early dinosaurs lived, they were at the mercy of weather patterns unfolding across the supercontinent Pangaea. Because it stretched so far north and south, there were so-called mega-monsoons and extreme temperature shifts. "It was a very dangerous place to call home," Brusatte said. "This was not a boring world."