It may be hard to imagine, but once upon a time, dinosaurs didn’t dominate their world. When they first originated, they were just small, two-legged carnivores 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 to compare their growth rates. 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, giant amphibians and sail-backed reptiles called therapsids thrived.

But within a blink of geologic time, in a span of about 60,000 years, scientists estimate 95% of all living things went extinct. Known as the Permian extinction or the Great Dying, it is the largest of the five known mass extinction events on Earth.

Most scientists agree this near total die-off 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 push global temperatures dramatically upward, eventually leading to ocean acidification, a loss of oxygen in ocean waters and a profound ecosystem collapse, 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 Triassic Period, 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 Archosauria, 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 split into two tribes – one group including modern crocodiles and their ancient relatives and a second including modern birds, along with their dinosaur ancestors.

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

Instead of directly competing with other archosaurs, it seems this group of dinosaur ancestors exploited different ecological niches – maybe by eating different foods or living in slightly different geographical areas. But early on, the dinosaurlike archosaurs were far less diverse 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 oldest known dinosaur fossils come from an area in Argentina now called Ischigualasto Provincial Park. Rocks there date back to roughly 230 million years ago.

The Ischigualasto dinosaurs include all three dinosaur groups: the meat-eating theropods, the ancestors of giant sauropods and the plant-eating ornithischians. They include Herrerasaurus, Sanjuansaurus, Eodromaeus, Eoraptor, Chromogisaurus, Panphagia and Pisanosaurus.

These early dinosaurs represent only a small fraction of animals found from that time period. In this ancient world, the crocodilelike archosaurs 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 late Triassic extinction event killed 75% of life on Earth. 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 less than 5% of animals on Earth to constituting more than 90%.

Bones tell the story of growth

My collaborators from 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 fast growth rates – 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 did indeed have elevated growth rates, 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 legs and bodies built for efficient, upright locomotion, potential changes in the way the earliest dinosaurs may have breathed 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 National Science Foundation and the David B. Jones Foundation.