Dinosaurs were not the obvious winners. Around 231 million years ago, in what is now the Ischigualasto Formation of northwestern Argentina, several kinds of early dinosaur were already living side by side. Most were small. Some were quick, two-legged meat eaters. One, Eoraptor, may have eaten plants as well as animals. They were a real part of the ecosystem, but they weren’t running it. Larger, crocodile-like reptiles held more of that territory, and for tens of millions of years a smart bet on the future of land life would not have been on the dinosaurs.
How they came to dominate anyway is the subject of “How Dinosaurs Took Over After Earth’s Worst Extinction,” Episode 8 of The Story of Life on Earth, an independent video series made as an homage to British author Martin Jenkins and his book Life: The First Four Billion Years. The episode picks up directly after the Great Dying and follows the survivors all the way to the long-necked giants of the Late Jurassic. The story it tells turns on luck as much as on anything the dinosaurs did.
A Slow, Uneven Recovery
The Permian extinction left the planet badly damaged. The episode puts the losses at roughly 70 percent of land vertebrate species and between 81 and 96 percent of marine species, and it describes a Triassic world that took somewhere between five and ten million years to properly recover, with different regions healing at different speeds.
The first animals to spread widely in that damaged world weren’t dinosaurs. They were other reptiles and the few surviving synapsids, the mammal relatives that had ruled before the extinction. Lystrosaurus, the tusked, pig-sized survivor of the Great Dying, was common everywhere in the early Triassic.
As the period went on, a different group moved up: the archosaurs, the family that includes both crocodiles and dinosaurs. The crocodile side of that family did especially well at first. Phytosaurs, long-snouted reptiles that looked a lot like modern crocodiles, became formidable predators in many Late Triassic rivers and lakes, though, as the episode points out, not in every waterway on the planet.
The First Dinosaurs Were Minor Players
When dinosaurs finally appear, the episode is clear that they didn’t arrive as conquerors. The earliest ones were often small and walked on two legs. Eoraptor was about the size of a dog.
The Ischigualasto Formation has produced a whole group of these early species, including Eoraptor, Herrerasaurus, Panphagia and Eodromaeus. When Ricardo Martinez, Paul Sereno and colleagues described Eodromaeus in 2011, Smithsonian magazine noted that it had a long, low skull filled with sharp, curved teeth, and that the same research moved Eoraptor from the meat-eating theropods over to the sauropodomorphs, the lineage that would eventually produce the giant long-necked plant eaters. Eoraptor’s leaf-shaped teeth suggested it ate at least some plants.
Not every early dinosaur was small. The episode gives Herrerasaurus a length of up to 6 meters and a weight of around 350 kilograms, large enough to be a serious predator in its setting. Plateosaurus, a plant eater from later in the Triassic, could grow to around 10 meters.
Size didn’t make them dominant, though. For millions of years, the episode says, dinosaurs were one experiment among many, often living in the shadow of their archosaur cousins.
The Numbers Favored the Crocodiles
That claim has solid research behind it. In 2008, Steve Brusatte and Mike Benton at the University of Bristol, with Marcello Ruta and Graeme Lloyd, compared Triassic dinosaurs with crurotarsans, the crocodile-line archosaurs, across nearly 500 skeletal features. They weren’t counting species. They were measuring disparity, the range of different body plans and ways of life a group had managed to evolve.
The crocodile relatives came out ahead. As ScienceDaily’s summary of the study put it, crurotarsans had a wider range of body plans, were more abundant and were at least as diverse as dinosaurs, while evolving just as fast. By any ordinary measure, they should have been the ones to inherit the Earth. The researchers’ explanation was two mass extinctions. As they put it, the dinosaurs “got lucky twice.”
A Second Catastrophe, 201 Million Years Ago
The second stroke of luck came at the end of the Triassic. As Pangaea began to pull apart, volcanic eruptions opened along the rift between what would become North America and Africa. Geologists call the result the Central Atlantic Magmatic Province, or CAMP. The episode describes the eruptions as massive and sustained. They pumped huge amounts of carbon dioxide into the air, and rapid warming and ocean acidification followed.
In 2013, a team led by Terrence Blackburn of the Carnegie Institution for Science, with Paul Olsen of Columbia’s Lamont-Doherty Earth Observatory and Paul Renne of the Berkeley Geochronology Center, used zircon crystals in the volcanic rock to tie the eruptions to the extinction in time. National Geographic reported that the dating placed the first eruptions just before the die-off, with a margin of error of only 20,000 to 30,000 years, and that at least half of all species on land and in the sea disappeared. “Zircon is a perfect time capsule for dating those rocks,” Blackburn told the magazine.
The losses fell unevenly. The episode says the extinction hit the dinosaurs’ main competitors hard. Many of the large crocodile-line archosaurs disappeared, along with most of the remaining big therapsids. The Bristol team’s work points the same way: nearly all crurotarsans died out except the lineage leading to crocodiles. Dinosaurs came through. Once again, a cleared world was waiting for whoever survived, and this time the dinosaurs were in position.
The Age of Giants
The Jurassic opened on a changed planet. Rivals were gone, and new environments kept appearing as Pangaea broke apart. Dinosaurs spread into those spaces and diversified, and above all they got big. The episode notes that the trend toward huge size had already started with large sauropodomorphs in the Late Triassic, the relatives of Plateosaurus. In the Jurassic it went much further.
The sauropods became the largest animals ever to walk on land. The episode highlights two from the Late Jurassic, about 150 to 154 million years ago. Brachiosaurus stood with its shoulders higher than its hips, giving it a towering, giraffe-like posture. Diplodocus went the other way, stretching out into an immensely long body. Their size and powerful digestive systems likely let them live on tough plants, such as the conifers that grew widely in the Jurassic climate.
Giant plant eaters meant giant predators. Much of what we know about this world comes from the Morrison Formation, a set of rock layers laid down across the western United States between roughly 157 and 150 million years ago, according to the National Park Service. The Morrison records rivers, floodplains and lakes, with ferns, cycads, conifers and ginkgoes but no flowering plants. Its sauropods include Apatosaurus, Diplodocus and Camarasaurus, alongside Stegosaurus and the predators that hunted them. The episode names Allosaurus as one of the top carnivores, a large theropod with serrated teeth and powerful claws. It shared its range with Ceratosaurus and Torvosaurus.
The Morrison has also been the site of some of paleontology’s most colorful history. Major finds there in 1877 set off the “Bone Wars,” the bitter rivalry between Edward Drinker Cope and Othniel Charles Marsh. Dinosaur National Monument, on the Colorado and Utah border, is home to the Carnegie Quarry, where a nearly complete Apatosaurus skeleton was found.
What the Book Gets Right, and One Thing It Gets Wrong
Dinosaurs are only one part of Life: The First Four Billion Years. Jenkins’s book covers the whole history of life, from the first cells to the first humans, and spends real time on the long stretch before any dinosaur existed. Rosemary Kiladitis, reviewing it for Mom Read It, called it a stunning book for the shelf and wrote that Jenkins makes the science of prehistory accessible to all ages. She recommended it for readers aged 10 to 14.
Dino Dad Reviews, a blog that reviews dinosaur books with an eye for scientific accuracy, was more mixed about this particular stretch. Its review praised the book’s handling of earlier life but found Grahame Baker-Smith’s Mesozoic reptiles looser than the rest, and it caught one sauropod labeled as Brachiosaurus that looks more like Apatosaurus. It’s a fair criticism. It’s also a mix-up between two animals that lived in the same Late Jurassic world, and in the reviewer’s view the book was still worth recommending.
Parents and teachers looking for more can browse Martin Jenkins titles in Amazon’s children’s dinosaur books category. On Goodreads, readers rate the US edition above four stars, and the artwork comes up again and again in their reviews.
Catastrophe, Then Opportunity
The episode’s central idea is simple. Extinction ends one world and clears ground for another. A group of reptiles that spent their first tens of millions of years on the sidelines survived two mass extinctions that their rivals did not, and inherited a planet with room to grow.
The Jurassic giants were only the start. The episode ends by pointing ahead to the next part of the dinosaur story, including Tyrannosaurus rex, which would not appear for more than 80 million years. The age of dinosaurs had only just begun.