ABOUT 250 million years ago, a creature raced along the edge of a lake in what is now Poland, leaving prints behind it in the mud. It was a meek and forgettable animal called Prorotodactylus, about the size of a pet cat and with slender limbs. But those prints weren’t the only legacy it left: its descendants somehow became the rulers of Earth.
Those descendants were the dinosaurs. The very word invokes majesty. These were among the most successful groups of animals ever, dominating the planet for more than 100 million years. They proliferated into creatures of all shapes and sizes, some even larger than a jet plane, and filled the land.
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Palaeontologists like me were long obsessed with understanding why these mighty animals were snuffed out 66 million years ago. We now know the answer: their days were ended by an enormous asteroid impact. Today, the greatest mystery of dinosaur evolution is how they rose to glory in the first place.
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The early descendants of Prorotodactylus would have stuck to the shadows, skulking away from much larger and more fearsome animals. So what was it that allowed them to take centre stage? Piecing together the answer is no easy matter. But over the past few years, a surprising new idea has gained traction. Perhaps the reason for the dinosaurs’ ascendency lies not in their teeth or claws or muscles. It may instead be thanks to a series of strange anatomical adaptations invisible from the outside – adaptations that allowed them to thrive in one of the most extreme periods of climate change the planet has seen up until now.
The world was almost unrecognisable back when Prorotodactylus made those lakeside prints. Our planet had just experienced one of the worst mass extinctions ever. A blast of global warming, fuelled by volcanic eruptions of unimaginable scale in Siberia, had caused more than 95 per cent of Earth’s species to die. From this catastrophe sprang the dinosaurs’ ancestors and closest cousins, including Prorotodactylus. Within 20 million years, they had : the meat-eating theropods, the long-necked, plant-guzzling sauropodomorphs and the beaked, herbivorous ornithischians. Much later, these lineages would spawn recognisable dinosaurs: Tyrannosaurus, Brontosaurus and Triceratops, respectively (see “An interrupted reign”).
Before this, in the Triassic period, most dinosaurs were horse-sized or smaller. And they weren’t alone. Proliferating alongside them were all sorts of other reptiles, including a particularly successful group called the pseudosuchians. This is the lineage to which modern crocodiles and alligators belong. They are a paltry bunch today, about 25 species all told, living in warm, semiaquatic environments. But back in the Triassic, there were scores of them, including armoured ones that ate plants, toothless omnivores that sprinted on their hind legs and apex predators called rauisuchians that were 9 metres from nose to tail and had teeth like steak knives.
See you later, alligator
If the pseudosuchians sound impressive, that’s because they were. So how did dinosaurs replace them as the dominant creatures on land? Back in the 1970s, some palaeontologists thought that early dinosaurs were unusually well-adapted to rapid running compared with their close relatives, says , an expert on animal muscles and locomotion at the Royal Veterinary College in London. They tended to walk on long, erect legs and were often bipedal. This view was articulated by leading dinosaur experts such as Robert Bakker, then at Harvard University, and Alan Charig at London’s Natural History Museum. Basically, dinosaurs were faster than the crocs and, over time, they outcompeted them.
It is an elegant story, but as a graduate student it didn’t sit well with me. And during the first decade of the 21st century, it began to seem increasingly untenable. A wealth of new Triassic pseudosuchian fossils had been discovered and many of them were dead ringers for dinosaurs, a sign that both groups were converging on the same diets and lifestyles. In 2008, I reviewed these new fossils to trace the evolution of both groups and came to a startling conclusion. During the entire Triassic, . They had more species, were more abundant in their ecosystems and had a greater variety of body plans, anatomical features and diets.
Earth unzipped
These findings helped reframe the question of how the dinosaurs became the world’s pre-eminent beasts. It must have happened, not gradually through the Triassic, but relatively quickly, towards the end of that period, when we know the pseudosuchians definitely did begin to decline. This actually makes a lot of sense because, at just this time, something epic happened to planet Earth.
In the days of Prorotodactylus, nearly all land was part of a supercontinent called Pangaea (see “Triassic world”). Then, 200 million years ago, Pangaea started to crack. The fracture began in the centre of the land mass and unzipped it down the middle. North America separated from Europe; South America from Africa. As they parted, Earth haemorrhaged lava from volcanoes in what is now the bottom of the Atlantic Ocean for more than 500,000 years.
Released with the lava were huge volumes of carbon dioxide and sulphur dioxide. These gases continually warmed the atmosphere, while bursts of ash from the eruptions shut out the sun’s light. This meant hot spells “alternated with intense volcanic winters that brought freezing for decades at a time”, says Paul Olsen at Columbia University in New York. At least 30 per cent of Earth’s species were killed as a result.
Yet the pattern in the fossil record unmistakably reveals that the dinosaurs sailed through this period. The pseudosuchians, on the other hand, were devastated. Nearly all of their rich Triassic diversity was extinguished, leaving only a few twigs on the family tree.
There are many hypotheses that attempt to explain this, all of which fall into one of two camps. One says that the dinosaurs really did have some advantage over the ancient crocs – be it speed, agility or intelligence – and, although this didn’t allow them to gradually outcompete them in the Triassic, it did finally give them the edge after Pangaea split. The other says that there is no single reason why the dinosaurs won. The rise in global temperatures was so quick and so brutal that animals survived mostly or only by chance.
Emma Schachner at Louisiana State University doesn’t think dinosaurs survived by mere chance. She has proposed an interesting idea that has been getting plenty of attention: that dinosaurs had a hidden superpower that helped them cope with the toxic atmospheres of the late Triassic.
To understand the idea, you need to know a little about how lungs work. In mammals, including us, muscles stretch the lungs out, which pulls air into them. This means the membrane of the lung can’t be too thin or else it would degrade as it moves and rubs against the ribs. But lungs work differently in some other animals, including birds, the direct descendants of dinosaurs. So let’s turn to bird lungs, and, as Schachner has put it, “it gets crazy, so hold on to your butts”.
In the chests of birds, the gas exchange portion of the respiratory system is like a dense sponge that doesn’t move. Because of this, its membrane can be extremely thin without rupturing, increasing the efficiency with which oxygen passes from the lungs to the blood. What’s more, several separate air sacs that aren’t part of the gas exchange system expand and contract to funnel air through the air exchange part of the lungs in one direction only. This also means that oxygen is drawn through the lungs during both inhalation and exhalation, so birds get more out of every breath.
In short, birds’ lungs are hyper-efficient, and Schachner has published arguing that , and that this helped them thrive.
Lungs are fleshy things that don’t tend to fossilise, but they can leave telltale signs behind. In birds, the air sacs often protrude into the vertebrae, creating indentations and sometimes hollows – so-called pneumaticity – in the bones. Do we see this in dinosaur bones? We sure do. Some sections of the back bones of Triassic dinosaurs are commonly pneumatised, indicating they probably had avian-style lungs. “This respiratory anatomy had the potential to give dinosaurs a major competitive advantage,” says Schachner.
In truth, the jury is out on whether the lungs alone made the difference for dinosaurs. In the past, we thought that air had much less oxygen during the Triassic than it does today, in which case more efficient lungs would have been obviously beneficial. But the latest thinking is that there was plenty of oxygen around in the Triassic.
“During the entire Triassic, pseudosuchians were completely outpacing the dinosaurs”
We also aren’t sure if the pseudosuchians had their own special lung adaptations. They certainly don’t have the same pneumaticity marks on their bones as early dinosaurs. But Richard Butler at the University of Birmingham, UK, has shown that they have . These might be signs of air sacs that were a tad different from those of modern birds.
Cecilia Apaldetti at the National University of San Juan in Argentina is contemplating an idea that takes pneumaticity to a whole new level. Over the past decade, her team has unearthed a bounty of new dinosaurs from the late Triassic rocks of the Marayes-El Carrizal basin in Argentina. Among these is a species she and her colleagues named Ingentia prima. This may be the oldest known dinosaur to get bigger than an elephant. And its skeleton is riddled with holes, suggesting the air sacs proliferated widely. Essentially, this animal’s lungs ran through its whole body. It is as weird as it sounds.
“These dinosaurs had an improved breathing system that provided them with numerous advantages,” says Apaldetti. With air sacs spread throughout most of their bodies, they were able to take in oxygen super-efficiently and circulate air through their innards, helping them keep cool. This, in turn, would have supported a fast metabolism and rapid growth. Their bones were also light. All of these factors together would have set dinosaurs up to get gigantic without running into problems, like getting too heavy to support themselves or overheating.
It is easy to imagine how any one of these things might have helped dinosaurs ride out a few hundred thousand years of global warming, foul atmospheres and ecosystem breakdown. Add them together, and they may have been almost indestructible.
“Essentially, this animal’s lungs ran through its whole body. It is as weird as it sounds”
Where does that leave Bakker and Charig’s hypothesis that dinosaurs were better runners than the crocs? Hutchinson’s group is revisiting this through . “Past ideas about locomotion were based almost solely on anatomy,” says Hutchinson. But that doesn’t necessarily tell you how fast an animal was. He and his team are instead using laser scans of fossils to build digital models of dinosaurs and pseudosuchians, which they put through gymnastics routines to test how the animals would have moved. The work won’t be finished for another year or two, but the team has already cast fresh light on how dinosaurs got around. One species seems to have .
There may never be a simple answer to the 200-million-year-old riddle of how the dinosaurs took the Jurassic throne. Our best guess is that they held a winning hand of adaptations: efficient lungs, high metabolism, fast growth and possibly other assets that we don’t yet understand. Together, they won the pot. But if the environmental conditions they faced had been just slightly different, the rules of the game would have been changed, and the age of the dinosaurs may never have come to pass. As it worked out, however, those footprints on the edge of the lake were the start of an epic journey to greatness.
Triassic world
In the days of the Triassic, the world would have been scarcely recognisable to us. All land was joined together into a supercontinent. “Pangaea extended from pole to pole and straddled the equator, shaped like the video-game icon Pac-Man,” says Jessica Whiteside at the University of Southampton, UK. Surrounding this was a vast ocean called Panthalassa.
You might think that animals would range freely across Pangaea, as there were no seas or major geographic barriers breaking up the land. But no. “Although an ambitious animal could walk across it in a single lifetime, there were climate zones that controlled where organisms lived,” says Whiteside. Her research has shown that the equatorial region of Pangaea was obscenely hot and muggy, while its deserts would routinely be hotter than 50°C.
Dinosaurs had trouble colonising these extreme regions and lived mostly in mid-latitude areas. Pseudosuchians, the successful group of Triassic lizards (see main story), also lived in these latitudes, but managed to spread to the interior deserts too. As animals go, they must have been seriously tough customers.
