Dinosaurs survived a mass extinction, but then their luck ran out

Dinosaur Mysteries delves into the secret side of “terrible lizards” and all the questions that keep paleontologists up at night.

WE ALL KNOW about the asteroid. About 66 million years ago, during the Northern Hemisphere spring, a 6-mile-wide chunk of space rock smashed into our planet, triggering the world’s fifth mass extinction. About 75 percent of all existing plant and animal species disappeared almost overnight, and our beloved dinosaurs were decimated. Of the herd of dinosaurs, only beaked birds made it to modern times.

But we’ve been so focused on how the age of the dinosaurs ended that their unexpected rise is often overlooked. Around 201 million years ago, at the dawn of the Jurassic Period, a different kind of mass extinction allowed dinosaurs to become the “terrible lizards” we love so much. Of the five mass extinctions recognized by paleontologists, it was the fourth that really laid the foundation.

The idea of ​​dinosaur “dominance” is so common that it’s strange to think that there was a time in the distant past when reptiles weren’t big and in charge. However, that is exactly what paleontologists have discovered. The oldest dinosaurs we know of today, from Triassic rocks dating back more than 230 million years, were relatively small, slender creatures that were odd compared to the other animals of the ancient landscape. The newly appointed Mbiresaurus raathi from Zimbabwe, for example, was a bipedal herbivorous dinosaur the size and weight of a German shepherd, far from the largest or most ferocious creature of its day.

The Triassic saw different families of reptiles prosper. A mass extinction early in the period, caused by huge volcanic eruptions in what is now Siberia, caused rapid global warming, changes in atmospheric oxygen levels and other ecological havoc that pushed the scaly creatures to evolve in new ways or go extinct. The first dinosaurs that evolved afterward were slender, omnivorous creatures about the size of a labradoodle. But the reptiles that left the biggest mark on the landscape were ancient cousins ​​of today’s crocodiles, a group called pseudosuchians.

In the past two decades, paleontologists have discovered multiple pseudosuchians that evolved dinosaur-like anatomies and behaviors long before real dinosaurs did. The square-headed carnivore postosuchus it looked so much like a tyrannosaurus rex which was initially mistaken for a t rex ancestor instead of the crocodile that he is. the little herbivore effigy okeefeae it ran on two legs and had a beak instead of teeth, resembling “ostrich mimicked” dinosaurs like Struthiomimus altus, a species that would evolve more than 100 million years later. The “armadillodiles” of thick sheet metal desmatosuchus They were also pseudosuchians, pioneering a pointed style that would later be reinvented by ankylosaurs. Prehistoric crocodiles came in all shapes and sizes in the Triassic, while dinosaurs were mostly small, slender, and not all that anatomically remarkable. Some were starting to grow late in the period, but they were nothing like the wonderful weirdos we saw in the Jurassic.

But in the 1950s, paleontologists noticed that many of the Triassic animal groups they had discovered had disappeared in the early days of the next period, the Jurassic. The vast majority of the various pseudosuchians disappeared, while the dinosaurs seem to persist through the Triassic-Jurassic boundary almost unscathed. Experts have proposed everything from changes in sea level to the impact of a previous asteroid to explain the reorganization of biodiversity, although the most likely culprit is another intense episode of volcanic eruptions in the sprawling Central Atlantic Magmatic Province. The eruptions occurred when the supercontinent Pangea was breaking up, but we can get an idea of ​​the affected areas by looking at the volcanic rocks preserved in the puzzle of today’s continents. Traces of the pulse have been found, which lasted for more than half a million years and increased global atmospheric carbon dioxide to levels 10 times higher than what we face today, from Nova Scotia to Brazil. And that’s not all. Geologists have also found evidence of sulfur dioxide in the rocks from this critical time, compounds that would have caused rapid cooling between warm periods created by greenhouse gases.

Why the protocrocs faded away while the dinosaurs ignored these changes is a mystery. On paper, you’d think a group of animals that evolved a wider variety of shapes, sizes, and behaviors would do better under pressure. While some crocodiles persisted, they were small, relatively widespread carnivores that went after insects and lizards rather than large, intricate ones. Small opportunists tend to do better in mass extinctions, as they can find enough food and habitat while larger, more specialized animals fight. But the fact that members of the three main groups of dinosaurs, the predecessors of the allosaurus, apatosaurusand stegosaurus genera—all survived seems strange when considering the fate of their distant crocodilian relatives.

The secret can be found within tissues and biological systems that are more difficult to preserve than bone. In 2020, paleontologists described a close relative of the common ancestor of dinosaurs and flying pterosaurs, a small reptile they named kongonaphon kely. Such a small animal would have benefited from its warm, fluffy protofeathers, a trait present in both dinosaurs and pterosaurs, and paleontologists suspect that early dinosaurs inherited the small size, warm blood, and an insulating coat. Those qualities would have allowed them to withstand a greater variety of habitats than their crocodilian cousins.

The backstory makes a bit more sense in light of the new research shared last year. Fossils of early dinosaurs are sometimes found in habitats that would have been frozen for at least parts of the year. Although prolific, pseudosuchians appear to have distributed only through warmer areas, while Triassic dinosaurs had a greater range, allowing them to persist through the changing climates created by the incredible eruptions 201 million years ago.

If those eruptions had not occurred or had been less intense, it is possible that the “Age of Dinosaurs” would have been the “Age of Crocodiles”. Dinosaur evolution would have been shaped by interactions with a broader cast of pseudosuchians, creating an alternate universe we’ll never get to see. A pair of mass extinctions cleared the ecological covers and allowed dinosaurs to venture to places where they could thrive and adapt to new ways, a melancholy contrast to their own disastrous moment many millions of years later.

We hope you enjoyed Riley Black’s column, Dinosaur Mysteries. Check back in PopSci+ in September for the next article.

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