It may be hard to imagine, but dinosaurs did not once dominate their world. When they first arose, they were just small, two-legged carnivores, dwarfed by a wide variety of other reptiles.
How did they come to power?
My colleagues and I recently studied the fossilized bones of the earliest known dinosaurs and their non-dinosaur rivals to compare their growth rates. We wanted to discover whether early dinosaurs were somehow special in the way they grew – and whether this might have given them an advantage in their rapidly changing world.
Before dinosaurs – the Great Dying
Life on Earth flourished 250 million years ago. Dinosaurs had yet to evolve. Instead, giant amphibians and sail-backed reptiles called therapsids flourished.
But within the blink of an eye of geological time, in a span of about 60,000 years, scientists estimate that 95% of all living things became 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 that this near-total extinction was caused by extensive volcanic activity in modern-day Siberia, which covered millions of square kilometers with lava. The resulting harmful gases and heat caused global temperatures to rise dramatically, ultimately 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 survived.
The survivors and their descendants
In the ecological vacuum following the mass extinction, on the scene of a healing Earth, the ancestors of dinosaurs evolved for the first time – along with the ancestors of today’s frogs, salamanders, lizards, turtles and mammals. It was the beginning of the Triassic, 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 out 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 included modern crocodiles and their ancient relatives, and the second group included modern birds, along with their dinosaur ancestors.
This second group walked on their toes and had large leg muscles. They also had extra connections between their spine and hip bones that allowed them to move efficiently in their new world.
Rather than competing directly with other archosaurs, it appears that this group of dinosaur ancestors exploited different ecological niches – perhaps eating different foods or living in slightly different geographic areas. But early on, the dinosaur-like archosaurs were far less diverse than the crocodile ancestors they coexisted with.
Slowly, the dinosaur lineage continued to evolve. It took tens of millions of years for dinosaurs to become abundant enough for their skeletons to appear in the fossil record.
The oldest known dinosaur fossils come from an area in Argentina now called Ischigualasto Provincial Park. The rocks there date from about 230 million years ago.
The Ischigualasto dinosaurs include all three dinosaur groups: the carnivorous theropods, the ancestors of giant sauropods, and the herbivorous ornithischians. They contain Herrerasaurus, Sanjuansaurus, Eodromaeus, Eoraptor, Chromogisaurus, Panphagia And Pisanosaurus.
These early dinosaurs represent only a small fraction of the animals found from that period. In this ancient world, the crocodilian archosaurs were at the top. They had a wider range of body shapes, sizes and lifestyles, easily surpassing the early dinosaurs in the diversity race.
It wasn’t until closer to the end of the Triassic, when another volcanism-induced mass extinction occurred, that the dinosaurs got their luck.
The Late Triassic extinction event killed 75% of life on Earth. It decimated the crocodilian archosaurs but left the early dinosaurs relatively untouched, paving the way for their rise to dominance.
Before long, dinosaurs represented less than 5% of the animals on Earth to more than 90%.
Bones tell the story of growth
My collaborators at the Universidad Nacional de San Juan, Argentina, and I wondered whether the rise of dinosaurs might have been partly aided by the speed at which they grew. Through microscopic examination of fossilized bones, we know that later dinosaurs had rapid growth rates – much faster than those of modern reptiles. But we didn’t know if that was true for the earliest dinosaurs.
We decided to examine the microscopic patterns preserved in the femurs of five of the earliest known dinosaur species and compare them with those of six non-dinosaur reptiles and one early mammalian relative. All the fossils we studied came from the two-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 have perforated the mineralized tissue. If we look at these features under a microscope, we can see how they are organized. The slower the growth occurs, the better organized microscopic features will be. As growth accelerates, the microscopic features of the bone appear more disorganized.
We discovered that early dinosaurs grew continuously and did not stop until they reached their full size. And they did indeed have higher growth rates, comparable to and sometimes even faster than those of their offspring. But so did many of their non-dinosaur contemporaries. It appears that most animals living in the Ischigualasto ecosystem grew rapidly, at a rate more similar to that of living mammals and birds than that of living reptiles.
Our data allowed us to see the subtle differences between closely related animals and those that occupy similar ecological niches. But most importantly, our data shows that rapid growth is a great survival strategy in the wake of mass destruction.
Scientists still don’t know exactly what allowed dinosaurs and their ancient ancestors to survive two of the most extensive extinctions Earth has ever experienced. We’re still studying this important interval, looking at details like legs and bodies built for efficient, upright locomotion, potential changes in the way the earliest dinosaurs breathed and the way they grew. We think it’s probably all of these factors, combined with luck, that ultimately allowed dinosaurs to rise and rule.
This article is republished from The Conversation, an independent nonprofit organization providing facts and trusted analysis to help you understand our complex world. It was written by: Kristi Curry Rogers, Macalester College
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Kristi Curry Rogers receives funding from the National Science Foundation and the David B. Jones Foundation.