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It may be hard to imagine, but once upon a time, dinosaurs didn’t dominate their world. When they first originated, they were just overshadowed by a diverse array of other reptiles.

How did they come to rule?

My colleagues and I recently studied the fossilized bones of the earliest known dinosaurs and their nondinosaur rivals . We wanted to find out whether early dinosaurs were somehow special in the way they grew – and if this may have given them a leg up in their rapidly changing world.

Before dinosaurs – the Great Dying

Life on Earth was flourishing 250 million years ago. Dinosaurs had yet to evolve. Instead, and thrived.

But within a blink of geologic time, in a span of about , scientists estimate 95% of all living things went extinct. Known as the or the Great Dying, it is the largest of the events on Earth.

Most scientists agree this was caused by extensive volcanic activity in modern-day Siberia, which covered millions of square miles with lava. The resulting noxious gases and heat combined to , eventually leading to , a and a , both on land and in the ocean.

Only a few lucky survivors made it through.

The survivors and their descendants

In the ecological vacuum after the mass extinction event, on the stage of a healing Earth, the ancestors of dinosaurs first evolved – along with the ancestors of today’s frogs, salamanders, lizards, turtles and mammals. It was the dawn of the , which lasted from 252 million years ago to 201 million years ago.

Collectively, the creatures that survived the Great Dying were not particularly remarkable. One animal group, known as , started off with relatively small and simple body plans. They were flexible eaters and could live in a wide variety of environmental conditions.

Archosaurs eventually – one group including modern crocodiles and their ancient relatives and a second including modern birds, along with their dinosaur ancestors.

This second group and had big leg muscles. They also had extra connections between their back bones and hip bones that allowed them to move efficiently .

Instead of directly competing with other archosaurs, it seems this group of dinosaur ancestors exploited – maybe by eating different foods or living in slightly different geographical areas. But early on, the dinosaurlike archosaurs were far than the crocodile ancestors they lived alongside.

Slowly, the dinosaur lineage continued to evolve. It took tens of millions of years before dinosaurs became abundant enough for their skeletons to show up in the fossil record.

The come from an area in Argentina now called . Rocks there date back to roughly .

The Ischigualasto dinosaurs include all three dinosaur groups: the meat-eating theropods, the ancestors of giant sauropods and the plant-eating ornithischians. They include , , , , , and .

These represent only a small fraction of animals found from that time period. In this ancient world, the were on top. They had a wider array of body shapes, sizes and lifestyles, easily outpacing early dinosaurs in the diversity race.

It wouldn’t be until closer to the end of the Triassic Period, when another volcanism-induced mass extinction event occurred, that dinosaurs got their lucky break.

The event . It decimated the crocodilelike archosaurs but left early dinosaurs relatively untouched, paving the way for their rise to dominance.

Before long, dinosaurs went from representing .

Bones tell the story of growth

the Universidad Nacional de San Juan, Argentina, and I wondered whether the rise of dinosaurs may have been underpinned, in part, by how fast they grew. We know, through microscopic study of fossilized bones, that later dinosaurs had – much faster than that of modern-day reptiles. But we didn’t know whether that was true for the earliest dinosaurs.

We decided to examine the microscopic patterns preserved in thigh bones from five of the earliest known dinosaur species and compare them with those of six nondinosaur reptiles and one early relative of mammals. All the fossils we studied came from the 2-million-year interval within the Ischigualasto Formation of Argentina.

Bones are an archive of growth history because, even in fossils, we can see the spaces where blood vessels and cells perforated the mineralized tissue. When we look at these features under a microscope, we can see how they are organized. The more slowly growth occurs, the more organized microscopic features will be. With quicker growth, the more disorganized the microscopic features of the bone look.

We discovered early dinosaurs grew continuously, not stopping until they reached full size. And they , on par with and, at times, even faster than those of their descendants. But so did many of their nondinosaur contemporaries. It appears most animals living in the Ischigualasto ecosystem grew quickly, at rates that are more like those of living mammals and birds than those of living reptiles.

Our data allowed us to see the subtle differences between closely related animals and those occupying similar ecological niches. But most of all, our data shows that fast growth is a great survival strategy in the aftermath of mass destruction.

Scientist still don’t know exactly what made it possible for dinosaurs and their ancient ancestors to survive two of the most extensive extinctions Earth has ever undergone. We are still studying this important interval, looking at details such as built for efficient, upright locomotion, potential changes in the and the way they grew. We think it’s probably all these factors, combined with luck, that finally allowed dinosaurs to rise and rule.

Kristi Curry Rogers receives funding from The �Թ���վ Science Foundation and the David B. Jones Foundation.